Human diseases are characterized by abnormalities in cellular function. Discuss a human disease caused by defective function of an organelle.
73
The Cell As a City
© Kendall Hunt Publishing Company
3
EssEnTiAl
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Theta and Joules are in a clique – Sally is no
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accepted
The cell i
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Primitive cells absorb mitochondria-like organismsA cell with its or
ganelles
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74 Unit 1: That’s Life
ChECk in
From reading this chapter, you will be able to:
• Explain how differences caused by inherited organelles could have societal implications.
• Describe how the characteristics that are valued change from culture to culture and over time.
• Outline the cell theory, list and describe types of cells, and explain endosymbiosis.
• List and describe the organelles found in a cell, and explain their main functions.
• Explain the processes of diffusion, osmosis, facilitated diffusion, active transport, and bulk transport.
The Case of the Meddling houseguest:
A Friendship Divided
Theta and Joules liked their friend Sally, but when they entered college, they learned
that Sally was different. When they were all young, they played together on the block,
went to each other’s birthday parties, and had some great sleepovers. “We had a lot of
fun with Sally in sixth grade . . . I wish she could join our sorority,” said Theta. Aghast at
the thought, Joules replied, “Don’t even say it – you know what that would mean for us.
We shouldn’t even admit that we know her.”
“Why can I not hang out with people I like? . . . Am I not allowed to be Sally’s
friend because of some test?” thought Theta. “There is no law against me being friends
with Sally!” exclaimed Theta, after a long pause. Joules dismissed Theta smugly, “You
know you can’t do it. It will never happen.” They were expecting Sally to come into the
dorm any minute. Sally was expecting to hang out with them as usual. But on this day,
their friendship had to end. On this day, Joules and Theta were going to pledge their new
sorority . . . and Sally did not have the mark.
It was an advanced society, in 2113 with all of the comforts – space travel beyond
the solar system, teleporting, and no more diseases that the ancients had; instead there
were life spans approaching two centuries for the marked people. Humans had it better
than ever, and teens had the world in their hands. Everyone with parents that had any
sense had a mark on their children to denote their superior genetic lineage. People in
the line of descent from genetically modified mitochondria had an “M” on the inside
of their ears. Their life expectancy was much higher and their health much better than
those without the mark. Finding out about one’s mitochondrial DNA was easy, with tests
dating back over 100 years to trace the origin of one’s genes.
Mitochondria are organelles that make energy for a cell; they are inherited from
mother to children because they have their own genetic material and divide on their
own. Mitochondria are, in fact, separate structures existing within our cells. They were
absorbed some 2.5 billion years ago, with their own set of DNA, making them houseg-
uests in our bodies.
The genes in the mitochondria stay intact from generation to generation. “This is
why the mark was so important – the health benefits,” thought Theta. Mitochondrial
DNA with modified genes of a particular line of mitochondria made people much health-
ier, free of many diseases in the society of this story. Mitochondria are the meddling
houseguests in the title because defects in them cause a range of diseases. For example,
Mitochondria
Is the organelle that
makes energy for a
cell.
Organelle (subcel-
lular structure)
Structures that
function within cells in
a discrete manner
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Chapter 3: The Cell As a City 75
mitochondrial defects in the 21st century were responsible for many ailments, ranging
from heart disease and diabetes to chronic sweating, optic nerve disorders, and epilepsy.
Joules told Theta, “People without the mark are jealous of us because they die earlier
and have a worse life with more diseases. You know Sally would never understand us.
Sally’s genes are still from the 21st century.” But something still bothered Theta: She
liked Sally. Sally came into the dorm and Joules explained that they were leaving for the
sorority. Sally knew what that meant and said good-bye. Theta looked deeply at Sally,
realizing that their past was gone and that they would not see each other again as friends.
Sally and Theta both had a single tear in their eyes and they knew they were part of each
other’s youth . . . and that meant something.
But Theta looked back one last time and said thoughtfully to herself, “She’s not one
of us.”
Culture, Biology, and social stratification
Culture plays an important role in defining what is desirable and valued in society. Often
decisions on what it means to be “better” are based on cell biology. Our genetic material
makes each of us unique and guides the workings of our cells. We all have the same set
of cell structures or organelles, but, as in our story, genetic variations give each per-
son unique characteristics. While the opening story is science fiction, its possibilities
are real. Gene technology is improving human health and has the potential to “design”
human genes and organelles, possibly leading to social issues like those described in the
conflict faced by Sally, Joules, and Theta.
Biological differences may lead to social changes based on what a society values
at any one time. For example, research shows that certain biological features are used
to decide social value of people: symmetry of one’s face, body fat distribution in both
genders, and musculature in males; smooth skin, good teeth, and a uniform gait. These
are all biologically determined, based on how our cell structures work together. Much
as mitochondrial inheritance, described in the story dictates health and organismal func-
tioning, all cell structures give living systems their characteristics.
Historically, all cultures have used biology to classify people. Humans are suscepti-
ble to group messages, such as the one that influenced Theta’s and Joules’ final decision
to abandon their friendship with Sally. The average American is exposed to about 3,000
marketing messages per day. This sets up a value system that requires us to reflect on
how biology and society can affect our thinking.
ChECk Up sECTion
The exclusion of people in our futuristic science fiction story reflects a theme in human society and
history. As a result of cell differences between Theta and Sally, their friendship ended – each possessed
a different type of mitochondrion.
Choose a particular situation in which a social stratification (layering) system is set up in a society,
in which one group thinks it is better than another. You may choose a present system or one of the
past. Is the stratification system reasonable? Is the system based on cell biology? What are the system’s
benefits? What are its drawbacks?
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76 Unit 1: That’s Life
BOdy Art And Skin BiOlOgy in SOciety
Body alterations in the quest for physical beauty are as old as history. Egyptians
used cosmetics in their First Dynasty (3100–2907 BC). Hairstyles, corsets, body-
weight goals, and body piercing and tattooing trends have changed through human
history. Scars have been viewed as masculine and a mark of courage, and tattoos
were drawn and carved in ancient European, Egyptian, and Japanese worlds.
Body art was popular in modern western society among the upper classes in
the early 19th century. It lost favor due to stories of disease spreading because
of unsanitary tattoo practices. Only the lower classes adopted body art to show
group affiliation. Tattooing has recently gained popularity; but body art has been
used as a symbol of self-expression and as a social-stratification mechanism in
many cultures: Indian tattoos mark caste; Polynesians used marks for showing mar-
ital status; the Nazis marked groups from their elite SS to concentration camp pris-
oners; and U.S. gangs use it to show group membership. Tattooing has been firmly
established in societies and continues to grow in popularity in the United States.
The canvas for tattoos is skin, which is part of the integumentary system
and has a variety of functions in humans (Figure 3.1). It
• maintains temperature;
• stores blood and fat; and
• provides a protective layer.
We will discuss this important system in a later chapter.
In this chapter, we will look at the structure and function of the eukaryotic cell. We
will see that, while there are marked differences between plant and animal cells, the
basic processes carried out at the cellular level are remarkably the same, as are those of
simple, unicellular organisms. We will compare the organelles (structures) of the cell
to functions of a city to emphasize that all parts are needed. Each organelle has its own
duties, and the parts work together to make an efficient machine. We begin by looking at
the development of the microscope, without which our understanding of cells and how
they function would be incomplete.
Figure 3.1 Tattoos and body art. Dyes penetrate into the skin cells of a tattoo.
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Chapter 3: The Cell As a City 77
Exploring the Cell
The Microscope
The human body is composed of over 10 trillion cells, and there are over 200 different
types of cells in a typical animal body, with an amazing variety in sizes (see Figure 3.2).
Despite the variety in size, all of these cells and the structures within them are too small
Figure 3.2 Biological size and cell diversity. When comparing the relatives’ sizes of
cells, we use multiples of 10 to show differences. The largest human cell, the female
egg, is 100 µm, while the smallest bacterial cell is 1000 times smaller at 100 nm. Most
cells are able to be seen with the light microscope. The smallest object a human eye can
see is about 1 mm, the size of a human egg cell (or a grain of sand). From Introductory
Plant Science, by Cynthia McKenney et al.
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78 Unit 1: That’s Life
to be visible to our naked eyes and can only be identified by using microscopes to
magnify them.
There are several types of microscopes; perhaps the one with which you are already
familiar is the compound light microscope. The compound light microscope uses two
lenses: an ocular and an objective lens. Each of these is a convex lens, meaning that its
center is thicker than its ends. Convex lenses bring light to a central, converging point to
magnify the specimen. A microscope’s parts are seen in Figure 3.3.
The purpose of a microscope is to magnify subcellular parts. What is magnifica-
tion? Magnification is the amount by which an image size is larger than the object’s size.
If a hair cell’s image is 10 times bigger than its original object, the magnification is 10
times. If it is 100 times bigger, then the magnification is 100 times. The microscope uses
two lenses to magnify the specimen: an ocular (eyepiece), which generally magnifies
between 10 and 20 times, and a series of objective lenses (each with higher magnifica-
tions). The total magnification of a specimen is equal to the ocular (in this example let’s
use10 times) times the magnification of one of the objective lenses.
Most animal cells are only 10–30 µm in width. It would take over 20 cells to span the
width of a single millimeter. Recall that a millimeter is only as wide as the wire used to
make a paper clip. See Table 3.1 for measurements used for looking at living structures.
How were cells and their smaller components discovered using the microscope?
Anton van Leeuwenhoek and Marcello Malpighi built microscopes in the late 1600s. At
this time, those instruments were very rudimentary. They consisted of a lens or a com-
bination of lenses to magnify smaller objects, including cells. Both scientists used their
instruments to observe blood, plants, single-celled animals, and even sperm. Van Leeu-
wenhoek’s microscope is shown in Figure 3.4. At about the same time that van Leeuwen-
hoek and Malpighi were making their observations, Robert Hooke (1635–1703) coined
the term cell, as he peered through a primitive microscope of his own construction.
When he viewed tissues of a cork plant, Hooke saw what seemed to be small cavities
separated by walls, similar to rooms or “cells” in a monastery (see Figure 3.4). These
cells are defined as functioning units separated from the nonliving world.
Although it has progressed in design, materials, and technology, the compound light
microscope is based on the same principle as in the 17th century: light bends as it passes
through the specimen to create a magnified image. Some amount of light always bends
compound light
microscope
Microscope that uses
two sets of lenses
(an ocular and an
objective lens).
Magnification
Is the amount by
which an image size is
larger than the object’s
size.
Figure 3.3 Compound light microscope – its parts and internal lens system.
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Chapter 3: The Cell As a City 79
when hitting the edges of the lens, causing scattering in a random way. The random
scattering of light, called diffraction is bad for getting a clear focus on the image. Dif-
fraction also limits the resolution of the image. Resolution is defined as the ability to see
two close objects as separate. (Think about looking at two lines on a chalkboard that is
very far away; chances are they blur together and look like one messy line.) In fact, the
human eye has a resolving power of about 100 µm or 1/10th of a millimeter for close-up
images. In other words, two lines on a paper closer than 1/10th of a millimeter apart look
blurry to us. The light microscope is limited in the same way by diffraction because the
diffracted rays create blurry images.
diffraction
The random scattering
of light.
resolution
Is the ability to see
two close objects as
separate.
Figure 3.4 Hooke’s microscope from the 1600s and van Leeuwenhoek with his
microscope. These simple microscopes led to the first descriptions of cells. Van
Leeuwenhoek’s microscope consisted of a small sphere of glass in a holder.
1 centimeter (cm) =
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1 micrometer (µm) =
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1 nanometer (nm) =
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table 3.1 Measurements Used for Microscopy. The units of measurement used in the study of molecules
and cells correspond with methods by which we are able to detect their presences.
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80 Unit 1: That’s Life
Higher magnification under the microscope leads to greater diffraction. This is the
reason a compound light microscope can magnify only up to 1000–1500 times (under
oil immersion), after which there is too much diffraction for a clear image to be formed.
To overcome the effect of diffraction and achieve clarity at higher magnifications, oil is
placed on the slide. However, even with oil immersion, only the large nucleus of a cell
can be seen; other organelles appear as small dots or not at all.
So how did the more complex world of even smaller structures within cells get dis-
covered? The 1930s saw the development of the electron microscope that allowed for
magnifications of over 200,000 times greater than that of the human eye. There are two
types of electron microscopes: transmission electron microscope (TEM) and scanning
electron microscope (SEM). Transmission electron microscopy allows a resolving power
of roughly 0.5 nm (see Table 3.1) that visualizes structures as small as five times the
diameter of a hydrogen atom. Electron microscopes use electrons instead of light, which
limits diffraction and increases resolution. Magnets instead of lenses focus electrons to
create the image. The electrons pass through very thin slices of the specimen and form
an image.
A SEM looks at the surfaces of objects in detail, while a TEM magnifies structures
within a cell. The SEM has a resolving power slightly less than the TEM, at 10 nm. (A
depiction of an electron microscope is shown in Figure 3.5.) Electron microscopy has
led to many scientific developments, uncovering subcellular structures to help us under-
stand cell biology. Seeing a mitochondrion enables us to better understand diseases and
perhaps, if our opening story becomes reality, improve societal health through its use.
Cell Theory
Fairly recent advances in microscopy have allowed scientists to learn about the structure
and function of even the tiniest components of cells, but the cell theory, which states key
ideas about cells, developed a long time ago. We have seen that scientists began study-
ing cells in the early 1700s. About a century later, in 1838, a German botanist named
Matthias Schleiden (1804–1881) concluded that all plants he observed were composed
of cells. In the next year, Theodor Schwann (1810–1882) extended Schleiden’s ideas,
transmission elec-
tron microscope
(teM)
A type of electron
microscope that
magnifies structures
within a cell.
Scanning electron
microscope (SeM)
An electron
microscope that
looks at the surfaces
of objects in detail
by focusing a beam
of electrons on the
surface of the object.
Figure 3.5 a. A researcher sits at a modern electron microscope. b. Apple tree pollen grains on cells, an
electron micrograph.
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Chapter 3: The Cell As a City 81
observing that all animals are also made of cells. But how did these cells come to survive
generation after generation? The celebrated pathologist Rudolf Virchow (1821–1902)
concluded in 1858 that all cells come from preexisting cells (He wrote this in Latin:
“Cellula e cellula”). These scientists contributed, together, to the postulates of the cell
theory. The cell theory is a unifying theory in biology that places the cell as the center of
life and unifies the many branches of biology under its umbrella. The cell theory states
that:
1) All living organisms are composed of cells.
2) The chemical reactions that occur within cells are separate from their
environment.
3) All cells arise from other cells.
4) Cells contain within them hereditary information that is passed down from par-
ent cell to offspring cell.
The cell theory showed not only that cells are the basic unit of life, but that there is
continuity from generation to generation. Genetic material is inherited in what we refer
today as the cell.
Types of Cells
Microscopes allowed researchers to examine differences between organisms that had
previously been impossible to determine. A current classification of organisms defines
five kingdoms, with organisms in those kingdoms having similar types of cells (There
is some debate arguing inclusion of Archaea bacteria as a separate kingdom, and a six-
system classification scheme is thus also accepted). Cells of organisms in the five king-
doms each have many internal differences, as summarized in Table 3.2. Images of some
organisms of each kingdom are given in Figure 3.17 as examples.
Prokaryotes (bacteria) are composed of cells containing no membrane-bound
nucleus and no compartments or membranous organelles. They are much smaller than
eukaryotes, by almost 10 times. Prokaryotic genetic material is “naked,” without the
protection of a membrane and nucleus. They are composed of very few cell parts: a
membrane, cytoplasm, and only protein-producing units called ribosomes. Even without
most structures found in other organisms, prokaryotes contain genetic material to repro-
duce and direct the functions of the chemical reactions occurring within its cytoplasm.
group domain cell type cell number cell Wall component energy Acquisition
Bacteria Bacteria Prokaryotic Unicellular Peptidoglycan Mostly heterotrophic,
some are autotrophic
Protists Eukarya Eukaryotic Mostly unicellular,
some are simple
multicellular
Cellulose, silica; some have
no cell wall
Autotrophic,
heterotrophic
Plants Eukarya Eukaryotic Multicellular Cellulose Autotrophic
Animals Eukarya Eukaryotic Multicellular No cell wall Heterotrophic
Fungi Eukarya Eukaryotic Mostly multicellular Chitin Heterotrophic
From Introductory Plant Science by Cynthia McKenney et al. Copyright © 2014 by Kendall Hunt Publishing Company. Reprinted by
permission.
table 3.2 Differences in Cell Structure within the Five Kingdoms: Plants, Animals and Prokaryotes.
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82 Unit 1: That’s Life
Prokaryotes have a simple set-up, but all of the needed equipment to carry out life func-
tions. Bacteria have a rapid rate of cell division and a faster metabolism than eukaryotes.
Most organisms on Earth, in terms of sheer number, are prokaryotes.
• As indicated in Chapter 1, prokaryotes include organisms in the Bacteria and
Archae domains. These organisms will be discussed further in Chapter 8.
All other organisms (plants, animals, fungi, and protists) are eukaryotes. Cells of
eukaryotes are complex, containing a membrane-bound nucleus that houses genetic
material. Eukaryotic cells comprise compartments that form a variety of smaller internal
structures, or organelles. Eukaryotic cells are the focus of this chapter, which will give
an overview of the primary organelles and their functions (Figure 3.6).
Eukaryotes may be examined by dividing into its four groups: plants, animals, fungi,
and protists. Plants contain cells that are surrounded by a cell wall, a rigid structure giv-
ing its organisms support. Plant cells contain chloroplasts, which enable plants to carry
out photosynthesis, using energy from sunlight to make food.
• Plant cell walls contain cellulose, which gives structure to plants as discussed
in Chapter 2. The process of photosynthesis, producing food for plants, will be
further discussed in Chapter 4.
Plants also have large vacuoles or storage compartments to hold water and minerals for a
plant’s functions. While both plants and animals have a cell membrane, animal cells are
Photosynthesis
The process by which
green plants use
sunlight to synthesize
nutrients from water
and carbon dioxide.
Figure 3.6 a. Differences between prokaryotes and eukaryotes. Prokaryotes have a
generally simple structure (see top cell in figure above), while eukaryotes (the lower
cell in figure above) have multiple organelles and membranes forming complex com-
partmentalization. From Biological Perspectives, 3rd ed by BSCS. b. Differences between
plants and animals. Plant and animal cells perform different functions, and their subcel-
lular structures are also different. Plant cells have chloroplasts to produce sugar and a
cell wall to give added strength. The animal cell shown has no cell wall or chloroplasts
but possesses centrioles. From Biological Perspectives, 3rd ed by BSCS.
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Chapter 3: The Cell As a City 83
Figure 3.6 (Continued)
(b) ©
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84 Unit 1: That’s Life
less rigid, surrounded only by a cell membrane and lacking a cell wall for support. Both
plants and animals contain membrane-bound organelles, but animals also contain a set
of small structures called centrioles, which serve in cell division. Animal cells are also
quite complex, as we will see. While lacking certain organelles, such as cell walls and
chloroplasts, they have flexible strategies to perform many functions.
Fungi have cell walls but no chloroplasts. They are not able to make their own food
and, instead live off of dead and decomposing matter as well as other living organisms,
centriole
Minute cylindrical
organelles found in
animal cells, which
serve in cell division
(not given in bold in
text).
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Figure 3.6 (Continued)
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Chapter 3: The Cell As a City 85
to obtain energy. Mushrooms and yeasts are familiar types of fungi, which will be dis-
cussed in Chapter 7.
Some species of protists are a bit animal-like in that they are able to move; other
species are a bit plant-like in that they have chloroplasts. Protists such as Amoeba in
Figure 3.7 have varied environments. Amoeba live in freshwater and, in a rare infectious
disease, grow and destroy human brain cells. We will discuss protists in more detail in
a later chapter.
Figure 3.7 Cells of the five kingdoms. While the cells of organisms in all of the kingdoms perform similar
life functions, their individual structures enable differing functions unique to each kingdom. From Biological
Perspectives, 3rd ed by BSCS.
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86 Unit 1: That’s Life
The Role of inheritance
The stratification system depicted in our opening story is based on the inheritance of
cellular components. We know that organelles are structures that carry out functions
within a cell. In fact, organelles work in concert with one another, coming together to
Figure 3.7 (Continued)
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Chapter 3: The Cell As a City 87
form a complex, dynamic cell. Mitochondria, so important in the society in our story, are
the powerhouses of the cell, providing energy for a cell’s functions.
All organelles are built and controlled by inherited genetic material. Thus, the way
a cell works is based upon its genetics. But some organelles are inherited separately
from the others. There are three ways to inherit organelles, including mitochondria: 1)
maternal inheritance, in which organelles are inherited from mothers; 2) paternal inher-
itance, in which organelles are inherited from fathers; and 3) bi-parental inheritance,
in which organelles are inherited from both mothers and fathers. The inheritance type
varies among species and for different organelles. For example, chloroplasts are pater-
nally inherited in the giant redwood Sequoia but maternally inherited in the sunflower
Helianthus. Most animal species have maternal transmission of mitochondria, as seen in
the story, because female eggs hold most of the mitochondria in their large cytoplasmic
cells. Sperm contributes very little cytoplasm or organelles in human species, although
there are exceptions among other organisms; for example, green algae Chlamydomonas
has paternal transmission of mitochondria.
Endosymbiosis
Eukaryotes appeared in Earth’s history about 1.5 billion years after prokaryotes. In their
2.0 billion years on Earth, eukaryotes have evolved into living systems that range from
butterflies to beavers, crocodiles to humans. As you progress through this text and the
course, you will learn about how this amazing diversity evolved.
Eukaryotes have two types of organelles – those that evolved as membranes and those
from other, simpler organisms as precursors, called endosymbionts. Endosymbionts
endosymbionts
Any organism living in
the body or cells of
another organism.
the Oxygen revOlutiOn
Have you ever tried to imagine the Earth in its early stages? After millions of
years during which the Earth was a mass of molten gases, those gases began
to cool into layers that became landmasses, while water vapor formed seas.
There were as yet no animals and only a few prokaryotic forms living in the
waters. One form of bacteria, known as cyanobacteria, is believed to have
been among the first organisms to photosynthesize (convert light energy into
chemical energy to drive cellular activities). Since a by-product of many forms
of photosynthesis is oxygen, over several million years, more and more oxygen
was released into the atmosphere – the oxygen revolution.
The oxygen revolution occurred, according to geologists, about 2.5 billion
years ago. It led to an availability of oxygen that could be used by cells that
evolved to use it. The advantage of using oxygen to yield larger amounts of
energy led to an increase in the number of aerobic cells. Aerobic cells, or those
cells that use oxygen as the fuel for obtaining energy, contain mitochondria to
provide large amounts of energy production. Over the course of many millions
of years, single-celled eukaryotes, with more complex cellular processes than
prokaryotes, evolved, then multicellular eukaryotes, and eventually organisms
became larger and more complex. It is important to realize that oxygen is a
key player in the development of organisms since it can be used to produce
fast, plentiful energy.
Oxygen revolution
The biologically
induced appearance
of dioxygen in Earth’s
atmosphere 2.5
billions of years ago.
Aerobic
Occurring in the
presence of oxygen or
require oxygen to live.
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88 Unit 1: That’s Life
have their own genetic material and are semi-independent within the cells of eukary-
otes. Endosymbionts include two types of organelles – mitochondria and chloroplasts.
Mitochondria are the energy producers of animal cells, and chloroplasts are solar power
transformers of plant cells. Mitochondria divide independently and have an internal
environment that is different from that in the rest of the cell.
Evidence tells us that endosymbionts were once independent prokaryotes that were
somehow incorporated into eukaryotic cells. Lynn Margulis, a well-known evolution-
ary biologist, formulated the endosymbiotic theory, which states that some organelles
in eukaryotes were descendants of ancient bacteria that were absorbed by larger cells.
(Symbiosis refers to a mutually beneficial relationship of organisms living together;
endo means within.) The larger cell gave ancient bacteria absorbed by larger cells a
“home.” The bacteria received protection and in return gave their unique set of chemical
reactions to the larger host cell.
Analysis of endosymbionts have particular uses in today’s society: Crime scene
investigations can test for mitochondrial DNA; ancestry can be traced using mitochon-
drial DNA; and research on diseases inherited due to faulty mitochondrial genetic mate-
rial may yield medical treatments. In our story, Joules and Theta had a different form of
mitochondria than Sally. Joules, Theta, and Sally all inherited their mother’s mitochon-
dria, but Sally’s inheritance included a predisposition for a number of diseases.
We have said that mitochondria can be thought of as the power plants of the cell. We
say this because mitochondria carry out the series of energy-producing reactions that
convert food energy into ATP (defined in Chapter 2; the form of energy used to drive
cell functions). This set of reactions is called cell respiration.
Chloroplasts may have originated from cyanobacteria, which were able to transform
light energy into usable sugar for energy in the process called photosynthesis – the
making of food (glucose) from sunlight, carbon dioxide, and water. Thus, precursor
mitochondria provided instant ATP energy for animal host cells, and ancient chloro-
plasts made stored glucose available for longer-term use in plants. Modern chloroplasts
use sunlight to rearrange carbon to form food in the form of sugars, for cell usage. They
are the solar power plants of cells because they trap sunlight and generate ATP energy.
• These energy-obtaining processes, cell respiration, and photosynthesis will be
discussed in greater detail in Chapter 4.
Evidence for mitochondria and chloroplasts as endosymbionts is considerable:
1) Mitochondria and chloroplasts are similar in size and shape to bacteria, roughly
7 µm in length.
2) Both contain their own genetic materials and divide in the same way as
prokaryotes, through binary fission (splitting in half).
3) Both contain the same type of 70S ribosomes (described below; small organ-
elles that make protein) as bacteria, whereas eukaryotes contain 80S ribosomes.
4) Mitochondrial and chloroplast DNA are more related to bacterial than to eukary-
otic DNA. See the proposed process on endosymbiosis in Figure 3.8.
We will now look at each of the structures of the cell, using the analogy of a city,
with the organelles being structures critical to the smooth functioning of the “city.”
Organelles work in concert with each other to carry out life functions. Chloroplasts
and mitochondria carry out key energy-producing and releasing functions to drive cell
activities. However, there is an intricacy to cell biology akin to the workings of a large
and dynamic city.
endosymbiotic
theory
The theory that states
that some organelles
in eukaryotes were
descendants of ancient
bacteria that were
absorbed by larger
cells.
cell respiration
A series of energy-
producing reactions
that convert food
energy into ATP.
chloroplast
A part of plant that
contains chlorophyll
and conducts
photosynthesis.
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Chapter 3: The Cell As a City 89
Figure 3.8 This model shows how mitochondria and chloroplasts wound up in
eukaryotic cells. Evidence for endosymbiosis. Chloroplasts and mitochondria are simi-
lar in size and shape to bacteria. The ribosomes in bacteria and mitochondria and chlo-
roplasts are “70S.” Most bacteria have a size of roughly 7–10 µm and 70S ribosomes,
both characteristics are similar to mitochondria and chloroplasts.
©
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Proteobacterium
engulfed by a
heterotrophic
eukaryote
Loss of bacterial cell wall
and transfer of some genes
to the nucleus-endosymbiont
becomes mitochondrium
Loss of bacterial cell wall and
transfer of some genes to the
nucleus-endosymbiont
becomes chloroplasts
Heterotrophic
eukaryotes: animals,
fungi and some protists
Autotrophic eukaryotes:
plants and algae
Cyanobacterium
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90 Unit 1: That’s Life
Cell Architecture: The Cell As a City
The organelles of a cell work independently but in concert, with its components inter-
acting actively, having many thousands of chemical reactions occurring at any one time.
In an analogous way, the components of a city work independently but cooperatively to
ensure its smooth functioning. The structures in the cell are shown in the cartoon image
depicting the cell as a city in Figure 3.9.
A cell membrane or plasma membrane surrounds each cell, which is a wrapping that
allows some materials across it. While all cells contain a plasma membrane, some cells
have structures surrounding the membrane for protection and support. For example,
plant cells have cell walls surrounding their membranes, and fungi have chitin barriers,
which are protective polysaccharides.
Plasma membranes are important to living systems because they control the mate-
rials entering and leaving them. Plasma membranes are selectively permeable. Selective
permeability means that the membrane allows some materials to pass through cells but
not others. Within the cell is the cytoplasm, a semisolid liquid that holds organelles
suspended within it. Cytoplasm is roughly 60–80% water by volume, and chemical reac-
tions occur within its medium. The other 20–40% of cytoplasm is composed of pro-
teins and dissolved ions. Cytoplasm is all of the cell material found between the plasma
Plasma (cell)
membrane
A biological membrane
that separates the
cell’s interior from the
outside environment.
Selectively
permeable
A condition in which
the membrane allows
some materials to pass
through cells but not
others.
cytoplasm
A semisolid liquid
that holds organelles
suspended within it.
Figure 3.9 A cell is like a city. Its parts work together to perform a cell’s function.
A
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E
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. M
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A. Nuclear envelope City Hall
Nuclear pores
Nucleoplasm
Outer and inner
membranes
F. Plasma membrane
the Border Patrol
B. Endoplasmic reticulum The Subway
C. Golgi body
Processing Plant
D. Centrioles MoversE. Mitochondrion Power Plant
Proteins
Phospholipid
bilayer
Channel (allows ions
and water to move in
and out of cell)
Inner and outer
membranes
Cristae
Microtubules
Maturing
face Secretory
vesicles
Forming
face
Rough E.R.
Smooth E.R.
Fixed
ribosomes
Cisternae
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Chapter 3: The Cell As a City 91
membrane and the nucleus. A nucleus, or control organelle, serves as the city hall of the
cell, usually centered within cytoplasm. It contains the genetic material that produces
and controls the cell’s parts.
A cell’s architecture is very complex, with continual activity within many com-
partments of each cell. Compartmentalization of a cell is accomplished by a series of
membranes throughout its cytoplasm. These membranes allow for a surface to per-
form chemical reactions such as those carried out by enzymes. The layers of mem-
branes also separate different chambers of the cell so that conditions may be different
within each chamber. An acidic pH in one compartment, for example, may be suitable
for cell functioning in one section, while a basic pH might be needed in another section.
Throughout the cell, ropes (such as collagen and elastin) and membranes maintain a
cell’s organization. Figure 3.9 gives you an idea of the complex architecture of the cell.
plasma Membrane: The
“Flexible” Border patrol
A border surrounds all cells: in eukaryotic animal cells, the border is the plasma or cell
membrane. This dynamic covering is very complex, with parts that continually move to
allow certain materials into the cell and keep other substances out. The plasma mem-
brane is composed of a phospholipid bilayer in and around which are membrane pro-
teins (see Figure 3.10).
• Recall from Chapter 2 that phospholipids are molecules with a phosphate
(hydrophilic) head and two tails made of carbon and hydrogen atoms.
The phospholipids are arranged as a bilayer, with the heads facing to the outside and
tails to the interior of the cell. The membrane proteins are part of the plasma membrane
border, moving in between cholesterol molecules and phospholipids. Although it may
seem that they are arranged randomly, in fact, they form a pattern. The membrane is
often referred to as a fluid mosaic (see Figure 3.11) because it is made of different pieces
that form a pattern and seem to float and move in the watery environment. Note the
phospholipid bilayer in Figure 3.11.
There are two types of membrane proteins suspended within the phospholipid
bilayer: integral proteins, which span the entire lipid bilayer; and peripheral proteins,
which station either inside or outside of the membrane. Integral proteins are also known
as transmembrane proteins. These membrane proteins serve to anchor cells to each
nucleus
The central and the
most important part
of a cell and contains
the genetic material.
compartmental-
ization
The formation of
cellular compartments.
Fluid mosaic model
A model that
describes the
structure of cell
membranes.
integral protein
(transmembrane or
carrier protein)
A type of membrane
protein permanently
embedded within the
biological membrane
(not given in bold in
text).
Peripheral protein
Is a protein that
adheres only
temporarily to the
biological membrane
with which it is
associated.
Figure 3.10 Every living cell is surrounded by a cell membrane. From Biological
Perspectives, 3rd ed by BSCS.
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92 Unit 1: That’s Life
other, move materials, much like a ferry, across the membrane, receive chemicals such
as hormones, and transport ions through pores within them. These proteins orient them-
selves according to the bonds they make with surrounding chemicals.
Integral proteins in the fluid mosaic model serve four functions:
1) As receptors, allowing chemicals to bind to cell surfaces. Insulin, a hormone-
regulating blood sugar, binds to cells to increase the absorption of sugar from
blood into cells. Insulin’s special shape matches to the shape on integral pro-
teins. The docking of the two elicits chemical reactions within the cell to main-
tain sugar balance;
2) As recognition proteins, giving the immune system a code that informs it that
a cell is its own and not a foreign substance. Cells with the wrong recognition
proteins are rejected, as occurs in cases of organ transplants whose codes do not
match up with those of the recipient;
3) As enzyme surfaces, facilitating various chemical reactions occurring on the
surface of a cell. Enzymes assist in the digestion of certain nutrients, the first
step in allowing nutrients to be absorbed; and
4) As transport proteins, moving material across a membrane.
Transport proteins act as a sort of flexible border-patrol system, which allows some
materials to pass through the membrane while keeping others out. This border-patrol
system is an important part of the cell as a city. The size, shape, and chemistry of sub-
stances attempting to move into and out of cells determine which materials pass through
the plasma membrane. Fatty materials pass through the membrane easily. For example,
ethanol in alcoholic drinks easily passes through membranes because it also dissolves
in fats. It is absorbed through the phospholipid bilayer and quickly giving a “high” to a
person after drinking. The interior of the lipid bilayer is hydrophobic, so it avoids water
and dissolves other substances that avoid water. Thus, only other hydrophobic, usually
fatty or fat-soluble, materials may pass easily through the bilayer.
Integral proteins of the membrane allow certain nonfatty materials, including smaller
charged or polar particles to move through the lipid bilayer directly. These include chem-
icals such as oxygen (O2), carbon dioxide (CO2), and ammonia (NH3). The quick move-
ment of these materials is essential for life functions. For example, NH3, a nitrogenous
waste that builds up within all cells, needs to be rapidly removed.
Thus, integral membrane proteins serve to facilitate movement of materials that can-
not easily pass across the membrane. Integral proteins move nonfatty materials across
the membrane, such as sodium ions Na+ and potassium ions K+. Larger polar molecules,
such as amino acids and glucose, also move through polar (or charged) channels within
integral proteins. Channels that are polar are thus hydrophilic, allowing materials with
a charge to pass through. Some substances use carrier proteins as pumps for transport,
as is the case for the very important ions sodium (Na+), potassium (K+), and calcium
(Ca+2).In the case of the movement of water (H2 O), the most abundant chemical in living
systems, it was recently determined from studies on aquaporins, or integral proteins
with channels within them, that water presses its way through the bilayer using integral
proteins. Sometimes water moves directly across the lipid bilayer by “wiggling” type
motion to press itself through. Physical forces drive this movement.
As shown in Figure 3.11, short carbohydrate chains jut out from the plasma mem-
brane. These serve as a recognition code for the immune system of animals. Embedded
within the phospholipid bilayer of animal cells, cholesterol binds together molecules,
helping to maintain the flexibility and motion of the membrane.
Consider for a moment, how many different pieces make up the plasma mem-
brane, and you can see why it is called a mosaic. It is called a fluid mosaic because it
receptor
A structure of the
cell’s surface that
selectively receives
and binds a specific
substance.
recognition protein
A protein type
functioning as binding
site for hormones.
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Chapter 3: The Cell As a City 93
is constantly moving, docking proteins, transporting materials, and changing its shape
with the movements of a cell. Membranes are also found throughout the cell’s structure,
with transport occurring continually across different areas. The types of transport will
be discussed later in the chapter. First, let’s explore the structure surrounding plasma
membranes in some organisms.
Walls of the City
What other borders exist around cells? Cell walls are found outside of plasma membranes
in plants, fungi, prokaryotes, and many protists, particularly in algae. For example, sin-
gle-celled algae known as dinoflagellates acquire their shape from their cell walls. Cell walls
give protection to the material within cells and add to their structure. Cellulose, composed
of large combinations of glucose units, polysaccharides, and lignin (a type of chemical
cement), makes up the stiffening agent of plant cell walls. While plants contain cellulose,
fungi cell walls contain chitin, also found in insect bodies and shells of some marine organ-
isms. Cell walls are not found in animal cells, which have no need for a stiff structure that
would prevent movement and flexibility. The cell walls of plants are shown in Figure 3.12.
Cytoskeleton: The City scaffolds
The shape of a cell is also determined by the skeleton within a cell’s cytoplasm, called
its cytoskeleton. You may be surprised to learn that the cell itself has a skeleton made up
of three types of fibers: microtubules, microfilaments, and intermediate fibers. All three
Microtubule
Is a larger filament
structure that helps
whole cells move.
Microfilament
A cytoskeletal fiber
used for muscle
movement.
intermediate fiber
The smallest fibers
of the cytoskeleton,
which circulate
materials within a cell.
Figure 3.11 Cell membrane structure. The cell membrane is composed of a phos-
pholipid bilayer with proteins embedded. Phospholipids are composed of two parts – a
phosphate head which attracts to water (hydrophilic) and fatty acid tails which repel
water (hydrophobic). Proteins are embedded within the lipid bilayer and each type
performs a specific function.
Plasma Membrane Structure
Extracellular fluid
Cytoplasm
Transmembrane
glycoprotein
Cholesterol Peripheral
protein
Transmembrane
protein
Channel protein
Glycolipid
Carbohydrates
Pore
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94 Unit 1: That’s Life
fibers are made up of chains of twisted proteins that serve to link and support parts of a
cell (see Figure 3.13). If the cell were a city, its cytoskeleton would be its infrastructure
or scaffolding.
Microtubules form larger structures – cilia and flagella – that help whole cells move.
Cilia are short extensions, and flagella are long, whip-like extensions on a cell’s surface.
Many types of organisms use cilia or flagella. A particular bacterium, Proteus mirabilis,
has numerous and extravagant flagella arrangements. Flagella are found in humans only
on sperm cells, to propel sperm toward their goal: the female egg.
Microfilaments, another cytoskeletal fiber, are used for muscle movement. Mus-
cles contract with the help of microfilaments moving in a sliding motion. Microfila-
ments also circulate materials within cells, in a process called intracellular transport.
Intracellular transport moves materials around the cell in a wavelike motion, adding
cilia
Are short extensions
that help cells move.
Flagella
Are long, whip-like
extensions on a cell’s
surface that help in the
movement of cells.
intracellular
transport
A process in which
microfilaments
circulate materials
within cells.
Figure 3.12 Plant cell wall. A rigid cell-wall composed of lignin and other stiffening
materials maintains structure in plants. Animal cells have only a thin cell membrane,
which gives almost no structure to the cell.
©
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animal cell plant cell
Figure 3.13 The internal skeleton of the cell. The cytoskeleton is composed of three
types of fibers: microtubules, microfilaments, and intermediate fibers. Cell parts are
held in place by the cytoskeleton and chemical reactions that occur on its scaffolding.
The cartoon image in the figure shows the cytoskeleton of an animal cell.
©
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Chapter 3: The Cell As a City 95
efficiency to internal transport. Microfilaments greatly aid how materials are moved
inside cells.
Intermediate fibers, the third type of cytoskeletal fiber, join cells together, especially
for muscles such as the heart, where there is a great deal of pressure. Intermediate fibers
are made of fibrous proteins, whereas, the other cytoskeleton fibers are composed of
globular proteins.
• Fibrous proteins are not easily disassembled as described in the protein section of
Chapter 2.
All three fibers anchor organelles in place, such as the mitochondria discussed in our
opening story, within the cytoplasm. The cytoskeleton gives a solid framework on which
chemical reactions take place. Some chemicals move along their fibers from one loca-
tion in a cell to another. But overall, the cytoskeleton is not a permanent structure, as
the name “skeleton” implies. Much like the plasma membrane, the cytoskeleton changes
and reforms with the needs of a cell.
nucleus: A City’s City hall
The most important membranous organelle is the nucleus, the control center of the cell.
In our analogy of the cell as city, the nucleus is City Hall. The source of its leadership is
DNA (deoxyribonucleic acid), the master planner for the organism. The nucleus is espe-
cially protected more than any of the other organelles, guarding its vital components.
Thus, it is composed of a double membrane, called the nuclear envelop, with special
pores or holes that have an octagonal shape and are highly selective. Only certain mate-
rials may pass through the nuclear membrane because the pores are very narrow.
In order to communicate with the cytoplasm and the outside world, DNA in the
nucleus sends its messenger chemical RNA (ribonucleic acid) out through nuclear pores.
RNA directs the synthesis of proteins, which in turn controls cell functions and the
building of cell structures. A nucleus is shown in Figure 3.14.
In most organisms, DNA is inherited from both father and mother through genetic
material stored and transmitted from nucleus to nucleus. Unlike in our story, which
deals with the maternal lineage of mitochondrial DNA, nuclear DNA is obtained from
the egg of one’s mother and the sperm of one’s father in sexually reproducing organisms.
When cells are not dividing, DNA is coiled around histone proteins, which are a group
of five small round proteins bound together with DNA in eukaryotes, together forming
chromatin (Figure 3.15).When a cell divides, its DNA coils more tightly to histones,
forming a shorter and thicker mass called chromosomes.
Ribosomes, the City’s Factory
The nucleus also contains large bodies called nucleoli (see Figure 3.16), which are spe-
cial regions of DNA that clump together and produce small, spherical organelles known
as the ribosomes. Ribosomes are made up of RNA. Ribosomal RNA directs the pro-
duction of proteins, guided by the RNA message sent out of the nucleus from DNA.
Ribosomes are the protein factories of the cell and thus you might think of them as the
manufacturing plants of the cell city. RNA carries a message that is translated on the
ribosome. This message gives the type of protein ordered by DNA in the nucleus. These
processes of protein manufacturing will be discussed further in a later chapter.
All organisms require proteins and thus all living systems contain ribosomes. Bacte-
rial ribosomes are lighter than eukaryotic ribosomes, but all ribosomes make proteins in
the form of strings of amino acids. Proteins comprise so many cell activities and struc-
tures that ribosomes are essential organelles found in all living organisms.
nuclear envelop
The double membrane
that protects the
nucleus.
chromatin
Is a complex of
macromolecules found
in cells and consist
of protein, RNA, and
DNA.
chromosome
A thread-like structure
formed when a cell
divides and its DNA
coils more tightly to
histones.
nucleoli
A small, dense round
structure found in the
nucleus of a cell.
ribosome
Small, spherical
organelle that is the
site protein synthesis.
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96 Unit 1: That’s Life
Endoplasmic Reticulum: A City’s subway
Ribosomes may be found freely within the cytoplasm or attached to membranes of other
organelles. One organelle that often has ribosomes on its surface is the endoplasmic
reticulum or ER for short. You can think of the ER as the subway system of the cell city.
endoplasmic reticu-
lum (er)
Is a system of
interconnected
membranes that form
canals or channels
throughout the
cytoplasm of a cell.
Figure 3.14 The cell nucleus and its components. The nucleus is the control center
of a cell. It communicates with the rest of the cell by directing the production of pro-
teins. Proteins serve many roles, particularly in chemical reactions. Vesicles transport
materials from the nucleus to other parts of the cell and to the exterior through its
endoplasmic reticulum.
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Nucleus
Nuclear
Pores
Rough Endoplasmic
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Ribosome
Chromatin
Nucleolus
Nuclear Envelope
Figure 3.15 DNA coils around histone proteins, with large amounts of genetic
materials becoming packaged into very small units.
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Chapter 3: The Cell As a City 97
The ER is a system of interconnected membranes that form canals or channels through-
out the cytoplasm of a cell. Materials are transported rapidly through the ER. Ribosomes
found on ER surfaces dump newly produced proteins into the channels, allowing them
to be transported to areas of a cell that need them. ER with ribosomes attached is called
rough ER because it is rough in appearance. ER without ribosomes attached is termed
smooth ER due to its undotted appearance. Figure 3.16 shows images of both smooth
and rough ER
Rough ER is found in cells that export large amounts of protein, such as pan-
creatic cells that produce the protein-based hormone insulin. Smooth ER is a main
component of organs that produce lipids, because it has enzymes that help in their
manufacture. Organs (both testes and ovaries) that produce lipid-based sex hormones
have smooth ER. Cells that manufacture and export lots of fatty material have elon-
gated smooth ER.
Golgi Apparatus: A City’s processing plant
When materials are transported out of the ER, they are often not fully processed and
need the addition or removal of parts. The Golgi apparatus is the processing plant of the
cell city that refines the materials passing through it. The Golgi apparatus consists of a
series of four to six flattened sacs, resembling a stack of pancakes. The Golgi apparatus
is shown in Figure 3.17.
As processing occurs, the Golgi apparatus rearranges bonds, adds carbohydrates, or
places a lipid on materials moving through. For example, the Golgi apparatus adds phos-
phate (glycerol) heads to lipid chains to form the phospholipids that make up plasma
membranes. At the end of Golgi processing, vesicles or sacs are pinched off the export
side. Vesicles are now ready for transport and contain needed materials for either within
or outside of the cell. Certain vesicles are also made into another organelle, called the
lysosome.
golgi apparatus
Is the processing plant
of the cell city that
refines the materials
passing through it.
Figure 3.16 Smooth and rough ER. Rough ER has ribosomes attached to it, and
smooth ER does not have ribosomes attached. Both act as the subway of the cell,
transporting materials.
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Ribosomes
Rough
Endoplasmic
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Smooth
Endoplasmic
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98 Unit 1: That’s Life
lysosomes: A City’s police officer
The lysosome, a small sac filled with digestive, hydrolytic enzymes, may be thought of
as the cell’s police officer.
• The term hydrolytic refers to hydrolysis, the breakdown of substances (described
in Chapter 2).
Lysosomes are found throughout the cytoplasm. They act as defenders of cells, just
like police officers do. When invading microbes enter cells, lysosomes fuse with the
“enemy” to digest it. Their contents break down materials within cells, and thus they are
responsible for intracellular digestion. Consider the Amoeba (Figure 3.18) that is taking
in food. Lysosomes act as its entire digestive system, fusing with a food particle and
breaking it down.
Thus lysosomes are the police officers of the cell city, continually breaking down
those substances that need to be removed and cooperating with the immune system to
defend against invaders.
Lysosomes also play a critical role in reorganizing material. For example, did you
know that our fingers and toes were fused together, or webbed, during our fetal develop-
ment? Lysosomes digest the tissue between our digits, allowing fingers and toes to form.
In roughly 1% of the population, lysosomes fail to fully digest this tissue, and fused toes
or fingers are the result.
Tadpoles lose their tails during metamorphosis to adult frogs due to lysosome action
that digests tails. Normal cell aging, or senescence, is a result of deterioration of lyso-
somes, according to one hypothesis. The deterioration of lysosomes releases their enzy-
matic contents, thereby digesting our cells from the inside out. Lysosomes normally
have a double membrane, similar to the nucleus, giving double protection that limits
such a disaster. The double membrane, however, cannot stand the effects of time, result-
ing in membrane failure and aging.
lysosome
A small sac filled with
digestive, hydrolytic
enzymes enclosed in a
membrane.
intracellular
digestion
The process of
breakdown of
substances within the
cytoplasm of a cell.
Figure 3.17 The Golgi apparatus processes materials such as a phospholipid as they
pass through its sacs. A phosphate is added to a lipid molecule to produce a phospho-
lipid for the plasma membrane. The golgi apparatus is the purifier of substances within
a cell.
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Chapter 3: The Cell As a City 99
There are 30 known inherited human diseases associated with the abnormal func-
tioning of lysosomes. These disorders are called lysosome-storage diseases because in
each case lysosomes lack a particular enzyme that breaks down substances that would
normally be removed. These waste products build up in lysosomes, accumulating in
cells. Tay–Sachs disease, commonly found in Ashkenazi Jewish populations, is a pro-
gressive and fatal disease in which fatty substances build up in tissues and nerve cells
of the brain. There is blindness, mental deficiency, and death at an early age due to
Tay–Sachs disease. Like most lysosome storage diseases, Tay–Sachs disease has few
effective treatments.
Vacuoles: A City’s Warehouse
Storage in cells is accomplished by vacuoles, which are single membrane structures that
hold materials in a cell; they are the warehouses of the cell city. There are three types of
vacuoles: food, water, and waste vacuoles, each holding what their name indicates. Food
vacuoles bring materials to and from the plasma membrane, depending upon a cell’s
need for nutrients. Adipose or fat cells in humans contain food (fat) vacuoles that com-
prise up to 80% of the cell, with enormous capabilities for energy storage. Food vacuoles
shrink or expand based on the amount of fat stored in adipose cells.
Waste vacuoles are temporary storage compartments that send materials directly to
the plasma membrane for export. Nitrogen-containing compounds are regularly stored
in waste vacuoles. Their accumulation is dangerous, which is a major reason untreated
kidney failure is often fatal after only a few days. One nitrogenous waste, ammonia
(NH3), is particularly poisonous, as you can attest if you have handled household ammo-
nia. Wastes are therefore exported from a cell as quickly as they accumulate.
Water vacuoles, which are storage vesicles for water, can also grow very large,
especially in plants. They serve to give shape to a plant cell, exerting pressure from
within the vacuole onto the plant cell wall. A special type of water vacuole, found in
simple-celled organisms such as the Paramecium is called the contractile vacuole. The
contractile vacuole has the unique ability to actively pump water out water of cells,
acting as a mini-kidney. Its shape is shown in Figure 3.19, with a star-like appearance
denoting its structure.
lysosome storage
disease
A group of 30 known
inherited human
diseases associated
with the abnormal
functioning of
lysosomes.
Figure 3.18 Amoeba digesting food. The amoeba surrounds its prey, and lysosomes
digest it.
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100 Unit 1: That’s Life
Figure 3.19 The Paramecium has many of the structures found in protists.
The contractile vacuole pumps excess water out of the paramecium.
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Paramecium
Cilia
Micronucleus
Macronucleus
Contractile
vacuole
Oral
groove
Food
vacuole
Anal pore
Cytoplasm
plastids: The Cell City’s paint shops
Plastids are organelles that store special substances. They are found only in plants and
algae. Colored substances called pigments are stored in some plastids. Chloroplasts,
which contain chlorophyll, are the most well-known plastid.
• Chlorophyll is a green-colored pigment that will be discussed in Chapter 4.
Chromoplasts store yellow and orange pigments that give colors to trees, flowers, fruits
and vegetables, such as carrots. Leukoplasts store starch and other food storage materi-
als. Leukoplasts are usually found in roots, as in turnips and potatoes. We eat the starch-
filled roots of these and several other foods.
Cell Junctions: The City’s Bridges
In the same way that cities have bridges to connect and support their parts, cells contain
cell junctions. Cell junctions serve a cell by providing areas of extra support, enabling
communication between cells and preventing leaking through their borders.
There are three different types of connections or bridges between cells, each playing
a needed role in particular cases. The first type of cell junction called desmosomes
are anchoring junctions or spot welds between cells. You can think of desmosomes/
anchoring junctions as the reinforcing rods embedded in concrete slab bridges of a city.
Within desmosomes, intermediate fibers entangle with linker proteins and attach to a
thickened region of the plasma membrane to give extra support in holding cells together.
Heart or cardiac cells contain many desmosomes to withstand the pressure of millions
of heartbeats.
The second type of cell junction, tight junctions, fuses areas together to prevent leak-
ing and act as sealants. Intestinal cells have tight junctions to prevent their contents from
leaking out of the bodies of animals.
The third type known as gap or communicating junctions are channels that run from
one cell into another to allow rapid transport. These bridges help cells communicate with
Plastid
Organelle, found only
in plants and algae,
which store special
substances.
chromoplast
An organelle that
contains any plant
pigment other than
chlorophyll.
desmosome
One of the three
types of connections
between cells, is a cell
structure specialized
for cell-to-cell
adhesion.
tight junction
A specialized cell
junction that fuses
areas together to
prevent leaking and act
as sealants.
gap (communicat-
ing) junction
Are channels that run
from one cell into
another to allow rapid
transport helping cells
communicate with
other cells.
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Chapter 3: The Cell As a City 101
other cells. Embryonic cells have gap junctions because, in the absence of a developed
vessel system for transport, an embryo compensates with gap junctions. The structures
of cell junctions are shown in Figure 3.20.
Cell shape and size
Most cells in the body are round or spherical, except when a cell wall shapes a cell to
conform to the forces around it. There are several reasons for the spherical shape, but
resisting torque is a main one. Round surfaces resist the damaging effects of torque
forces (circular forces). For example, consider a square cell, with edges that are likely to
break off or become damaged when torque forces are applied to such cells. Any shape
with a corner is more likely to be damaged than something that is spherical.
In senescence studies, the wear-and-tear hypothesis contends that damage due to
forces placed upon living systems is a main reason for the aging of cells and the loss of
cellular function. As a cell ages, it is more and more likely to succumb to the dangers
and damages of the outside environment. Senescence is the loss of cell functions and in
our story of Uncle Hans in Chapter 1, his dementia-related symptoms were a result of
the loss of proper transmission of nerve signals within his brain. Senescence studies are
continually looking at the damages to cells in attempting to reverse or slow the aging
process.
There are many types of cells within multicellular organisms. As stated earlier,
humans are constructed of about 200 different kinds of somatic or body cells. While
they are each of very distinct types, with different characteristics, what is so surprising
is their similarity. As discussed earlier, every cell has a control center to maintain the
operations of the cell on a daily basis: a membrane that separates itself from the outside
environment; and a semi-solid plasma, called cytoplasm, in which cellular activities
occur. Most animal cells are between 10 and 30 µm in width, but can be as large as
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Figure 3.20 Cell junctions are connections between two different cities.
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102 Unit 1: That’s Life
100 µm (the size of a single grain of sand), as seen in the human egg or up to 1,500 µm
as in frog’s eggs. Eggs tend to be very large because they support a growing organism.
The small size of cells is explained by the surface-to-volume hypothesis, which
states that the surface area (surface that is in contact with the outside environment)
decreases rapidly in proportion to the volume of the cell. Thus, if the volume of a cell is
too great, it cannot exchange enough of the materials (for example, oxygen) to support
its size. The surface area of a variety of cubes that the little boy is hammering is shown
in Figure 3.21. Note that the surface area of the single large cube is so much smaller than
that of the aggregate of the many smaller cubes.
Another reason for limits on a cell’s size and shape is its ability to control a large cell
from a nucleus. Just as a long distance relationship is difficult due to distance, a cell too
large has a more difficult time regulating activities of its structures from farther away.
There are some organisms that have developed ways to adapt to these problems. A slime
mold, shown in Figure 3.22 is, in fact, one giant, thin cell with thousands of nuclei strewn
throughout its cytoplasm. In this way, it avoids the constant-volume problem by remaining
thin for exchanges to occur and has control centers in enough areas to keep control effi-
cient. Human muscle cells also are multinucleated and very large to perform their motility
functions.
The Moving Crew: Rules and procedures are City law
The description of the cell’s organelles and their functions provides a glimpse at how
complex their structure is and how many functions they carry out. Physical laws govern
how they interact to transport materials. Let’s look now at a few of the basic laws of the
city. The different transport types will be discussed according to their requirement for
energy: 1) No energy needed: simple diffusion, facilitated diffusion, osmosis; 2) Energy
needed: active transport, bulk transport (phagocytosis, pinocytosis, receptor-mediated
endocytosis, and exocytosis).
Figure 3.21 Surface area of large cube vs. several smaller cubes of equal size. The
surface area of the many smaller cubes is much greater than that of a single cube with
a comparable volume.
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Chapter 3: The Cell As a City 103
Passive Transport
Passive transport involves the movement of materials without the use of cellular energy.
Recall that the cell is bathed in water. Most atoms and molecules therefore exist as
ions. Ions move constantly, jiggling, hitting one another, and forcing themselves farther
and farther apart to fill the available space. The motion of each ion is driven by kinetic
energy, or energy of motion. This kind of movement is called passive transport because
it occurs naturally and spontaneously and again, requires no cellular energy.
Substances move apart during passive transport until they spread evenly through
an area. This even level of dispersion is known as equilibrium. You have experienced
this action of molecules if you have ever been in a bus or subway car with someone
whose deodorant has failed on a hot day. The underarm odor is first detected by nearby
passengers, but the smell can travel until it reaches even those people at the other end
of the vehicle. Sweat odor molecules may travel in any direction at the same time.
However, there is a net movement of smell molecules from an area of higher concen-
tration to an area of lower concentration. The movement from a higher concentration
to a lower concentration is called diffusion. In living systems, substances spontaneously
diffuse from higher to lower areas of pressure, electrical charge, or concentration with-
out the use of cellular energy. Figure 3.23 shows the process of diffusion in this subway
situation.
The difference between higher and lower concentration areas is known as a gradient.
Many examples of diffusion are seen throughout this text. Oxygen, carbon dioxide, alco-
hol, and vitamins move through a cell via diffusion. The kidneys work by regulating
body levels of ions using diffusion principles. Simple cutting of an onion often makes
us cry – not because we care about an onion’s rights to live – but because sulfuric acid,
an irritant, diffuses from the cut onion toward our eye membranes. Passive transport
processes within cells can describe movement for many of the chemicals discussed in
the previous chapter.
The mitochondria in our opening story use gradients to generate energy for a cell to
use. Joules and Theta were fortunate to have more efficiently developed mitochondria
to help their physiological functions. Movement of substances within the mitochondria
Passive transport
The movement of
substances across cell
membranes without
the need of energy
expenditure by the cell.
equilibrium
The even level
of dispersion of
substances.
concentration,
higher and lower
The presence of a
large amount of a
specific substance in a
solution or mixture is
higher concentration.
Any solution
containing fewer
dissolved particles is
lower concentration.
diffusion
The net movement of
molecules from higher
concentration to a
lower concentration.
gradient
The difference
between higher and
lower concentration
areas.
Figure 3.22 Slime molds. These organisms have multiple nuclei to control their large
sizes. During some phases of their life cycles, they appear as gelatinous slime, giving
them their infamous name.
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104 Unit 1: That’s Life
Figure 3.23 Diffusion in a subway. Odor moves from a higher to a lower concen-
tration in a subway car. Molecules bounce off of each other, moving farther and farther
apart in diffusion.
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High
concentration
will be discussed in greater detail in the next chapter, energetics. Next, we will look at a
special case of passive diffusion of water across membranes, osmosis.
Osmosis: A Special Case of Diffusion
Because water is critical for life, it must be able to move freely into and out of cells –
in other words, across membranes. Diffusion of water across a selectively permeable
membrane – one that allows the transfer of only some substances – is termed osmosis.
Osmosis occurs spontaneously, so it requires no cellular energy and is thus a special
type of passive transport. It occurs only when water moves across a membrane. Most
molecules within a cell cannot simply diffuse across a plasma membrane. If they could,
their contents would readily spill out and a separation from the outside world would not
be possible.
While the integrity of the cell is critical, it is also critical that materials other than
water move across the membrane. A unique property of water – its ability to dissolve
other substances – makes this possible. The cell’s cytoplasm is roughly 1% dissolved
materials, and the right amount of dissolved material in the cell is required for proper
cell functioning.
If a cell has a higher concentration of solute as compared with its surrounding envi-
ronment it is hypertonic; if it has less solute, it is hypotonic. Osmosis is the mechanism
by which the internal and external environments equalize. An example of what can hap-
pen when conditions are abnormal occurs when an athlete drinks a large amount of
water in a short period of time following exercise and suffers from hypernatremia, or ion
imbalance, which can be fatal (Figure 3.24).
The term hypertonic can be remembered because “hyper” refers to a large amount
of something, as in a hyperactive child who has too much energy. For example, if a cell
with a concentration of 0.9% NaCl were placed in a beaker with a concentration of .01%
NaCl, the cell would have the greater amount of solute. The cell, with more solute, has
less water, and water would flow from the fluid outside into the cell. When that happens,
the cell swells and could even lyse (break open) if too much water enters it.
Hypotonic solutions are those that have less solute dissolved than their external
environment. The term hypotonic can be remembered because “hypo” rhymes with H2O,
which implies that there is more plentiful water in a hypotonic situation. To illustrate,
Osmosis
The process
of diffusion of
water through a
semipermeable
membrane that allows
the transfer of only
some substances.
hypertonic
A cell having a higher
concentration of
solute as compared
with its surrounding
environment.
hypotonic
A cell having a lower
concentration of
solute as compared
with its surrounding
environment.
lyse
Breakdown of cell
membrane.
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Chapter 3: The Cell As a City 105
when a cell with a concentration of 0.9% NaCl is placed in a beaker with a concentration
of 10% NaCl, the cell has a lower amount of solute than the solution outside. The cell
in this case is hypotonic, but the outside is hypertonic. Thus, water will flow out of the
hypotonic cell along its concentration gradient. A good way to remember where water
flows in cells is by the golden rule: “water follows solute.”
Of course, no net movement of substances occurs if a cell has the same concentra-
tion of solute as its environment. It is then said to be isotonic (iso = the same as) to its
outside. There is an even concentration of solute and water on either side of the plasma
membrane. A cell might have a 0.9% concentration of NaCl both inside and outside of
the cell. In this case, solute and water are both traveling in and out of the cell, but their
net or overall movement is the same toward either side.
Special Cases in Osmosis
Gradients drive the movement of solutes and water in each of the examples above.
Single-celled organisms, such as the Paramecium described earlier in this chapter, live
in hypotonic environments. Their surroundings contain a high concentration of water.
Water is therefore always attempting to enter the Paramecium because it contains more
dissolved solutes. This requires a contractile vacuole to act as a constant pump, moving
the excess water out of the cell. This would make sense, because Paramecium is a fresh-
water protist, which needs dissolved solutes within its cell to survive.
Plants also use gradients to maintain their structure and stand upright. My grand-
mother often served older lettuce, as she was quite frugal. It wilted in the refrigerator,
but after she placed the lettuce in a bowl of water, it perked up and looked as good as
isotonic
Even concentration
of solute and water
on either side of the
plasma membrane.
Figure 3.24 Red blood cells in hypotonic, hypertonic, and isotonic solutions. Red blood cells shrivel
up in a hypertonic environment and blow up in a hypotonic environment. They are normal in an isotonic
situation. Note the changes in a red blood cell’s shape in each of the conditions.
Isotonic solutions Hypertonic solutions Hypotonic solutions
(b)(a) (c)
Cells retain their normal size and shape
in isotonic solutions (same solute/water
concentration as inside cells; water
moves in and out).
Cells lose water by osmosis and shrink
in a hypertonic solution (contains a
higher concentration of solutes than are
present inside the cells).
Cells take on water by osmosis until they
become bloated and burst (lyse) in a
hypotonic solution (contains a lower
concentration of solutes than are present
in cells).
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106 Unit 1: That’s Life
new. Lettuce cells were hypertonic to the outside, which was hypotonic to normal tap
water. Water thus rushed into the cells of the lettuce leaves along their concentration gra-
dient. Water entered water vacuoles within the plant cell, creating what is called turgor
pressure, pressure exerted against the walls of a plant cell. Turgor is any pressure on a cell
wall that gives the wall rigidity. My grandmother used the principles of osmosis to save
money. When a plant cell is placed in a hypertonic environment, what do you predict will
happen? It will lose water to its outside, causing it to shrink. Water’s movement across a
membrane not allowing solutes, as in our lettuce example, is shown in Figure 3.25.
Passive Transport with a Helper
Some substances require the help of carrier proteins to move across the plasma
membrane. This final type of passive transport is called facilitated diffusion. Substances
move along their concentration gradient through the channels of carrier proteins, which
facilitate their movement. Sugars such as glucose, and amino acids, as components of
proteins, both move into and out of cells through facilitated diffusion. Because they
have charges on them they cannot cross the membrane on their own. Polar channels in
carrier proteins enable these ions to be carried across the plasma membrane. Facilitated
diffusion requires no cellular energy and it occurs spontaneously, moving from a higher
to lower concentration.
turgor pressure
The pressure exerted
against the walls of a
plant cell when water
enters the water
vacuoles of plant cell.
Vacuole: Single
membrane structures
that hold materials in
a cell.
Facilitated diffusion
A type of passive
transport requiring a
carrier protein.
Figure 3.25 Water moves across the membrane between two sides of the beaker
but does not permit the movement of dissolved substances. This leads to a column of
water rising as water follows solute into the right side of the beaker. Lettuce, in our
example, has increased turgor pressure as water rushes into its cells.
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High concentration
of H2O molecules
Low concentration
of solute (NaCl)
molecules
Higher concentration of
solute (NaCl) molecules
results in fewer H2O
molecules on right side
of membrane
Membrane is permeable to
water, but not to solutes
H2O molecules move
through membrane to
create equilibrium of
solute concentrations,
resulting in higher
volume on right side
(a)
(b)
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Chapter 3: The Cell As a City 107
Our story discussed mitochondria, which have much movement of ions within its
inner membranes to make ATP energy. Next we will look at how ATP energy generated
by mitochondria drives another form of movement: active transport.
Active Transport
Some forms of movement occur with the addition of energy to drive its actions. Active
transport is the movement of substances against a concentration gradient, from a lower
concentration to a higher concentration, which requires cellular energy. Active transport
does not occur spontaneously; it requires ATP energy to overcome the nonspontaneous
movement of materials against a concentration gradient. Consider our subway example:
Would it not be strange if all of the smell molecules aggregated toward the armpit of
an unsuspecting person on the subway, making him or her smell terribly? Could smell
molecules throughout the subway car attack this person? It cannot happen unless there is
an active transport mechanism at work.
In order for cells to carry out some processes, they must use active transport. For
example, the sodium–potassium pump is an integral protein that uses ATP energy to
move sodium ions (Na+) out of the cell and bring potassium ions (K+) into the cell. Keep-
ing the right concentrations of Na+ and K+ is vital in nerve and muscle activity.
A concentration gradient forms as a result of three Na+ pumped out of the cell for
every two K+ pumped into the cell. This specific movement results in an electrical and
concentration gradient across the membrane. More sodium winds up outside of the cell
and more potassium inside. The environment outside the cell becomes more positive
than inside it because there are more positive ions being pumped out. Potential or stored
energy develops and drives many cellular activities much as a dam on a river stores
energy that is converted to electrical power for our use. Figure 3.26 shows the workings
of the sodium–potassium pump.
Secondary active transport is the movement of substances using stored energy. The
sodium–potassium pump creates a store of electrical and potential energy for later use
by cell processes. Instant energy for moving one’s arm or riding a bicycle is obtained
through secondary active transport by using stored ATP. Let’s give a look in the next
section to the active movement of bulk materials in and out of the cell.
Bulk Transport: A Bigger Moving Van
At times, cells as a city require transport of larger materials – sometimes, whole organ-
isms – to carry out their life functions. Bulk transport is the movement of large amounts
of material across the plasma membrane. It is a form of active transport obtaining energy
from ATP. Movement of single ions or molecules is often not enough for the many cell
reactions occurring at any one time. Cell activities often require removal or inputs of
large resources.
The moving van of the cell is the vesicle, containing the bulk materials. Bulk trans-
port can occur by moving materials into the cell (called endocytosis) or by moving mate-
rial out of a cell (exocytosis). (Endo- sounds like “in” and exo- sounds like “exit,” which
should help you recall the terms.) There are three types of endocytosis: phagocytosis,
pinocytosis, and receptor-mediated endocytosis.
Phagocytosis is defined as the movement of solid particles into a cell. It is
accomplished by a cell’s cytoplasm projecting around the material to be dissolved. This
action appears much like a blob, engulfing its prey. The substance may be a whole organ-
ism or a particle of organic matter. Immune cells, particularly macrophages, a special
kind of white blood cell, attack foreign invaders by phagocytosis. Once the material is
Active transport
Is the movement of
substances against
a concentration
gradient, from a lower
concentration to a
higher concentration,
which requires cellular
energy.
Sodium– potassium
pump
An integral protein
that uses ATP energy
to move sodium ions
out of the cell and
bring potassium ions
into the cell.
Secondary active
transport
The movement of
substances using
stored energy.
Bulk transport
Is the movement
of large amounts of
material across the
plasma membrane.
endocytosis
The process of moving
materials into the cell.
exocytosis
The process of moving
materials outside the
cell.
Phagocytosis
The movement of
solid particles into a
cell.
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108 Unit 1: That’s Life
fully engulfed through this process, lysosomes fuse with the newly formed vesicle con-
taining the material. Lysosome’s hydrolytic enzymes digest the particle for use by a cell
or release it as a waste product.
Cells also move large amounts of liquid, along with their dissolved solutes, into their
cytoplasm through pinocytosis. Cell drinking or pinocytosis requires a furrow, or invagi-
nation, in the plasma membrane of a cell to input liquids. Many animal cells obtain their
nutrients through pinocytosis, with large human egg cells developing multiple furrows
to obtain nutrients from the surrounding cells.
A form of endocytosis that requires a specific binding of a receptor protein to cell
membrane is known as receptor-mediated endocytosis. In this case, bulk materials
move into a cell by docking with a specifically shaped receptor. The receptor is an inte-
gral protein jutting out of a plasma membrane. It has a specific shape onto which a
substance matching its shape may bind. A signal stimulated by the joining of receptor to
substance starts to bring the plasma membrane inward. The membrane forms a vesicle
filled with both the substance bound to the receptor and the materials dissolved, as the
membrane moves inward. Hormones, nutrients, and neurotransmitters, such as acetyl-
choline (described in Chapter 1), move in and out of a cell through this process.
Because receptors are proteins, they are genetically determined. For example, in
cases of hyperlipidemia, high amounts of cholesterol build up in the blood. Recep-
tors that dock with vesicles carrying cholesterol are either malformed or available in
Pinocytosis
The mechanism by
which cells ingest
extracellular fluid and
its contents.
invagination
The process of being
folded back on itself
to form a pouch (not
given in bold in text).
receptor- mediated
endocytosis
A form of endocytosis
that requires a specific
binding of a receptor
protein to cell
membrane.
Figure 3.26 The workings of the sodium–potassium pump. With each turn, the pump moves three sodium
ions out of its cell and two potassium ions into its cell. A gradient is set up with more sodium ions outside
than inside a cell. The inside of the cell becomes relatively more negative than the outside because more
positive charges are pumped to the outside of the cell.
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Chapter 3: The Cell As a City 109
insufficient amounts in hyperlipidemia. Lipids cannot be removed from the blood as
easily, reducing rates of receptor-mediated endocytosis and leading to buildup of fats in
the blood. It is a dangerous disease due to its close link with heart attacks and stroke.
Many other diseases are associated with improperly formed receptors, including diabe-
tes, depression, and susceptibility to viruses.
All bulk movement out of a cell is accomplished by exocytosis. Any material to be
exported – wastes, hormones, mucous or enzymes – is removed from a cell by exocy-
tosis. Exocytosis removes either waste products or materials for export from the cell.
Vesicles containing these materials fuse with the plasma membrane and are expelled.
Figure 3.27 shows the types of bulk transport.
All of the forms of bulk transport are used by mitochondria described in our opening
story. For a mitochondrion to function, much like any other organelle, it communicates
with the nucleus, cell membrane, and other cell parts to perform its role in helping the cell
to function. Joules and Theta probably did not realize the complexity of cell movement
within their mitochondria that contribute to its survival. Perhaps if they had, they would
have accepted their friend Sally. Knowing that mitochondrial DNA is not the only contrib-
utor to an individual cell’s survival would have opened their eyes to the many unknowns
in making each of us unique. Cell biology might have made a difference in the outcome
of our story and in the friendship of Joules, Theta, and Sally.
Figure 3.27 Endocytosis and exocytosis.
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it’S All ABOut eve: everyOne hAS the SAMe MOther in
Our hiStOry.
Our story opens with the importance of ancestral mitochondrial DNA in
social organization. However, evidence shows that all humans have the same
maternal mitochondria, derived from the same ancestral mother.
A group of genetic researchers published a study in the journal Nature
revealing a maternal ancestor to all living humans called “mitochondrial Eve.”
Researchers concluded that every human on Earth now could trace lineage
back to this single common female ancestor who lived around 200,000 years
ago.
Mitochondrial DNA was taken from 147 people across continents and
ethnic groups. It was determined that people alive today have lineage on one of
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110 Unit 1: That’s Life
two branches in the human family tree. One branch consists of African lineage
only and the other of all other groups including African. The scientists con-
cluded that Africa is the place where this woman lived. Note that this maternal
ancestor was not the first woman, but her descendants survive to present
day while other women of her time had female descendants with no children,
halting the passing on mitochondrial DNA.
Thus, Joules, Theta, and Sally were of one lineage, bound by an ancestor
long ago. While society broke them apart, their biology had more in common
than Joules would know or care to admit. Their commonalities should have
held them together, with the humanness of a shared heritage. Perhaps their
ancestor is one of us now . . .
summary
Chemistry is the language of structure and movement within living systems. Physi-
cal and chemical principles determine the behavior of cell structures. Developments in
microscopy led to the discovery of the detailed workings of cells. While most organ-
elles evolved through invaginations of membranes, mitochondria and eukaryotes devel-
oped by becoming absorbed into larger cells. Together, organelle interactions with one
another and the outside environment comprise a living organism. The fluid mosaic
model shows how cells transport materials across their many membranes. Transport
systems accomplish varied goals from moving small particles directly through cells
to moving materials in bulk. All of the needs of cells are accomplished by organelles
communicating through use of the different transport systems. Our story showed that
cell parts not only dictate only life processes, but influence human health and the social
structure of our society.
ChECk oUT
summary: key points
• Cell biology affects our lives in many ways, from diseases such as lysosome storage disorders to the
way society is structured based on human differences.
• Organelles have specific roles within cells, communicating with each other and the outside environ-
ment through membranes.
• The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells
based on chemical and biological evidence.
• The development of microscopy led to the discovery of cells and cell parts.
• The plasma membrane has a unique structure that enables transport across a cell.
• Cells of the five kingdoms diverge based on just a few key differences.
• Passive and active transport mechanisms are used to move materials within a cell.
• Different chemicals use transport mechanisms differently to move across a cell.
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Chapter 3: The Cell As a City 111
active transport
aerobic
bulk transport
cell respiration
centriole
chloroplast
chromatin
chromosome
chromoplast
cilia
compartmentalization
compound light microscope
concentration, higher and lower
cristae
cytoplasm
desmosome
diffraction
diffusion
endocytosis
endoplasmic reticulum (ER)
endosymbionts
endosymbiotic theory
equilibrium
exocytosis
facilitated diffusion
flagella
fluid mosaic model
gap (communicating) junction
golgi apparatus
gradient
hypertonic
hypotonic
integral protein (transmembrane or
carrier protein)
intermediate fiber
intracellular digestion
intracellular transport
invagination
isotonic
leucoplast
lyse
lysosome
lysosome storage disease
magnification
microfilament
micrometer
microtubule
mitochondria
nanometer
nuclear envelop
nucleoli
nucleus
organelle (subcellular structure)
osmosis
oxygen revolution
passive transport
peripheral protein
phagocytosis
photosynthesis
pinocytosis
plasma (cell) membrane
plasmolysis
plastid
receptor
receptor-mediated endocytosis
recognition protein
resolution
ribosome
scanning electron microscope (SEM)
secondary active transport
selectively permeable
sodium–potassium pump
stoma
thylakoid membrane
tight junction
transmission electron microscope (TEM)
turgor pressure
vacuole
key terMS
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Chapter 3: The Cell As a City 113
Multiple Choice Questions
1. How might a chloroplast disease, preventing its key processes, affect human society?
a. There would be no more food.
b. There would be no more light.
c. There would be no more water.
d. There would be no more carbon dioxide.
2. If a plant cell absorbs water and becomes stiff, it becomes __________ through the
process of __________.
a. crenated; passive transport
b. turgid; osmosis
c. rigid; active transport
d. reactive; facilitated diffusion
3. Which sentence BEST describes a ribosome?
a. It is a large organelle that makes protein.
b. It is a small organelle that makes protein.
c. It is a large organelle that breaks down protein.
d. It is a small organelle that breaks down protein.
4. Which type of microscope can best view the cristae of a mitochondrion?
a. compound light microscope
b. phase contrast microscope
c. scanning electron microscope
d. transmission electron microscope
5. Which term best describes the plasma membrane?
a. static
b. dynamic
c. organized
d. rigid
6. Which term best differentiates between active transport and passive transport?
a. polarity
b. facilitation
c. concentration
d. energy
7. Which is NOT a passive transport mechanism across a plant cell?
a. turgor pressure
b. diffusion
c. osmosis
d. receptor-mediated endocytosis
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114 Unit 1: That’s Life
8. When a cell builds a new Golgi apparatus, new materials need to be brought into the
cell, sometimes against a concentration gradient. Which process best accomplishes
the task of building a new Golgi apparatus?
a. diffusion
b. osmosis
c. exocytosis
d. active transport
9. In question #8 above, which chemical drives the process for the correct answer?
a. sodium
b. water
c. ATP
d. adenine
10. An experiment places a skin cell within a petri dish containing pure, distilled water.
Predict what will happen to the skin cell within minutes:
a. The skin cell will shrink as water leaves the cell.
b. The skin cell will lyse as water leaves the cell.
c. The skin cell will shrink as water enters the cell.
d. The skin cell will lyse as water enters the cell.
short Answers
1. Describe how senescence is linked to lysosomes. Explain how lysosomes are both
linked to human diseases and to our proper functioning. Give an example of each.
2. Define the following terms: passive transport, active transport, and receptor-medi-
ated endocytosis. List one way each of the terms differs from the others.
3. Compare the two types of electron microscopes, explaining how their images are
similar and how they are different. Use a drawing to make the distinctions clear.
Show your art work. Which was more important in discovering subcellular structure?
4. There are several ways chemicals move within cells. At times they use passive trans-
port and at times they use active transport. List the types of passive transport and
explain how each works. Be sure to include the following terms in your explanation:
gradient, concentration, and ATP energy?
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Chapter 3: The Cell As a City 115
5. For question #4 above, list and draw an example of an active transport mechanism
within a cell. Include in the picture the electron arrangement around the atoms.
6. Name two differences and two similarities between cells found in the matched
kingdoms:
a. fungi–protist
b. animal–plant
c. plant–protist
d. bacteria–animal
7. ATP plays a major role in life’s processes. How does ATP energy function in sec-
ondary active transport processes? Explain how the sodium–potassium pump plays
a role in secondary active transport.
8. Draw the fluid mosaic model of the cell membrane. Describe how and why a charged
molecule such as sugar that needed by every cell enters through the fluid mosaic.
9. Explain the differences between passive and active transport. Explain the difference
using the following terms: ATP, spontaneous, gradient, concentration, ions.
10. A battle between an amoeba and a single-celled protist, chilomonas ensues. The
amoeba wins, but the large piece of chilomonas is too big to pass through the plasma
membrane of the amoeba to eat. Describe the process by which the amoeba absorbs
and digests the chilomonas.
Biology and society Corner: Discussion Questions
1. Many scientists argue that our most pressing resource crisis is not food shortage or
climate change, but water scarcity. How is water important for a cell? Predict how
the role of a limited water supply will affect society. Explain how the water crisis is
already impacting policy changes, the economy and overall human impacts on the
environment.
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116 Unit 1: That’s Life
2. How would van Leeuwenhoek be surprised by discoveries since his viewing of
cells? What role do you think microscopy will play in future scientific develop-
ments? How did microscopy affect society in the 20th century? Use either the scan-
ning electron microscope or transmission electron microscope to frame your thesis.
3. If a person is stranded on a desert island, without freshwater, what are the dangers
of drinking sea water. Explain the transport mechanism at work, which leads to
problems for a human cell in contact with seawater.
4. Mitochondrial diseases are attributed to human health problems, from sweating and
epilepsy to diabetes and heart disease. Do you support biotechnology research to
help people with these diseases? Do you support its use in potentially making “bet-
ter” mitochondria to help people live better lives, as seen in our story. What restric-
tions would you place, if any, on such developments?
5. Plasma membranes are delicate and can weaken in some cases causing human
health disorders. Research the effects of plasma membrane problems on the health
of humans and animals. Choose one of the organelles discussed in this chapter.
Describe what happens to the organelle and human or animal health as a result of
the delicate nature of plasma membranes.
Figure – Concept Map of Chapter 3 Big Ideas
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73
The Cell As a Cit
y
© Kendall Hunt Publishing Company
3
EssEnTiAl
s
Theta and Joules are in a clique – Sally is no
t
accepted
The cell i
s like a city
Primitive cells absorb mitochondria-like organismsA cell with its or
ganelles
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Microvilli
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Nucleus
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Chromatin (Threads
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Nuclear Envelope
Rough Endoplasmic Reticulum
(R.E.R.)
Flagellum
Phospholipid Bilayer
Smooth Endoplasmic Reticulum
(S.E.R.)
Golgi Apparatus
Microtubules
Ribosomes
Mitochondri
Cytosol (Cytoplasmic fluid)
Cytoplasm (Cell contents outside nucleu
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Vesicle
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74 Unit 1: That’s Life
ChECk in
From reading this chapter, you will be able to:
• Explain how differences caused by inherited organelles could have societal implications.
• Describe how the characteristics that are valued change from culture to culture and over time.
• Outline the cell theory, list and describe types of cells, and explain endosymbiosis.
• List and describe the organelles found in a cell, and explain their main functions.
• Explain the processes of diffusion, osmosis, facilitated diffusion, active transport, and bulk transport.
The Case of the Meddling houseguest:
A Friendship Divided
Theta and Joules liked their friend Sally, but when they entered college, they learned
that Sally was different. When they were all young, they played together on the block,
went to each other’s birthday parties, and had some great sleepovers. “We had a lot of
fun with Sally in sixth grade . . . I wish she could join our sorority,” said Theta. Aghast at
the thought, Joules replied, “Don’t even say it – you know what that would mean for us.
We shouldn’t even admit that we know her.”
“Why can I not hang out with people I like? . . . Am I not allowed to be Sally’s
friend because of some test?” thought Theta. “There is no law against me being friends
with Sally!” exclaimed Theta, after a long pause. Joules dismissed Theta smugly, “You
know you can’t do it. It will never happen.” They were expecting Sally to come into the
dorm any minute. Sally was expecting to hang out with them as usual. But on this day,
their friendship had to end. On this day, Joules and Theta were going to pledge their new
sorority . . . and Sally did not have the mark.
It was an advanced society, in 2113 with all of the comforts – space travel beyond
the solar system, teleporting, and no more diseases that the ancients had; instead there
were life spans approaching two centuries for the marked people. Humans had it better
than ever, and teens had the world in their hands. Everyone with parents that had any
sense had a mark on their children to denote their superior genetic lineage. People in
the line of descent from genetically modified mitochondria had an “M” on the inside
of their ears. Their life expectancy was much higher and their health much better than
those without the mark. Finding out about one’s mitochondrial DNA was easy, with tests
dating back over 100 years to trace the origin of one’s genes.
Mitochondria are organelles that make energy for a cell; they are inherited from
mother to children because they have their own genetic material and divide on their
own. Mitochondria are, in fact, separate structures existing within our cells. They were
absorbed some 2.5 billion years ago, with their own set of DNA, making them houseg-
uests in our bodies.
The genes in the mitochondria stay intact from generation to generation. “This is
why the mark was so important – the health benefits,” thought Theta. Mitochondrial
DNA with modified genes of a particular line of mitochondria made people much health-
ier, free of many diseases in the society of this story. Mitochondria are the meddling
houseguests in the title because defects in them cause a range of diseases. For example,
Mitochondria
Is the organelle that
makes energy for a
cell.
Organelle (subcel-
lular structure)
Structures that
function within cells in
a discrete manner
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Chapter 3: The Cell As a City 75
mitochondrial defects in the 21st century were responsible for many ailments, ranging
from heart disease and diabetes to chronic sweating, optic nerve disorders, and epilepsy.
Joules told Theta, “People without the mark are jealous of us because they die earlier
and have a worse life with more diseases. You know Sally would never understand us.
Sally’s genes are still from the 21st century.” But something still bothered Theta: She
liked Sally. Sally came into the dorm and Joules explained that they were leaving for the
sorority. Sally knew what that meant and said good-bye. Theta looked deeply at Sally,
realizing that their past was gone and that they would not see each other again as friends.
Sally and Theta both had a single tear in their eyes and they knew they were part of each
other’s youth . . . and that meant something.
But Theta looked back one last time and said thoughtfully to herself, “She’s not one
of us.”
Culture, Biology, and social stratification
Culture plays an important role in defining what is desirable and valued in society. Often
decisions on what it means to be “better” are based on cell biology. Our genetic material
makes each of us unique and guides the workings of our cells. We all have the same set
of cell structures or organelles, but, as in our story, genetic variations give each per-
son unique characteristics. While the opening story is science fiction, its possibilities
are real. Gene technology is improving human health and has the potential to “design”
human genes and organelles, possibly leading to social issues like those described in the
conflict faced by Sally, Joules, and Theta.
Biological differences may lead to social changes based on what a society values
at any one time. For example, research shows that certain biological features are used
to decide social value of people: symmetry of one’s face, body fat distribution in both
genders, and musculature in males; smooth skin, good teeth, and a uniform gait. These
are all biologically determined, based on how our cell structures work together. Much
as mitochondrial inheritance, described in the story dictates health and organismal func-
tioning, all cell structures give living systems their characteristics.
Historically, all cultures have used biology to classify people. Humans are suscepti-
ble to group messages, such as the one that influenced Theta’s and Joules’ final decision
to abandon their friendship with Sally. The average American is exposed to about 3,000
marketing messages per day. This sets up a value system that requires us to reflect on
how biology and society can affect our thinking.
ChECk Up sECTion
The exclusion of people in our futuristic science fiction story reflects a theme in human society and
history. As a result of cell differences between Theta and Sally, their friendship ended – each possessed
a different type of mitochondrion.
Choose a particular situation in which a social stratification (layering) system is set up in a society,
in which one group thinks it is better than another. You may choose a present system or one of the
past. Is the stratification system reasonable? Is the system based on cell biology? What are the system’s
benefits? What are its drawbacks?
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76 Unit 1: That’s Life
BOdy Art And Skin BiOlOgy in SOciety
Body alterations in the quest for physical beauty are as old as history. Egyptians
used cosmetics in their First Dynasty (3100–2907 BC). Hairstyles, corsets, body-
weight goals, and body piercing and tattooing trends have changed through human
history. Scars have been viewed as masculine and a mark of courage, and tattoos
were drawn and carved in ancient European, Egyptian, and Japanese worlds.
Body art was popular in modern western society among the upper classes in
the early 19th century. It lost favor due to stories of disease spreading because
of unsanitary tattoo practices. Only the lower classes adopted body art to show
group affiliation. Tattooing has recently gained popularity; but body art has been
used as a symbol of self-expression and as a social-stratification mechanism in
many cultures: Indian tattoos mark caste; Polynesians used marks for showing mar-
ital status; the Nazis marked groups from their elite SS to concentration camp pris-
oners; and U.S. gangs use it to show group membership. Tattooing has been firmly
established in societies and continues to grow in popularity in the United States.
The canvas for tattoos is skin, which is part of the integumentary system
and has a variety of functions in humans (Figure 3.1). It
• maintains temperature;
• stores blood and fat; and
• provides a protective layer.
We will discuss this important system in a later chapter.
In this chapter, we will look at the structure and function of the eukaryotic cell. We
will see that, while there are marked differences between plant and animal cells, the
basic processes carried out at the cellular level are remarkably the same, as are those of
simple, unicellular organisms. We will compare the organelles (structures) of the cell
to functions of a city to emphasize that all parts are needed. Each organelle has its own
duties, and the parts work together to make an efficient machine. We begin by looking at
the development of the microscope, without which our understanding of cells and how
they function would be incomplete.
Figure 3.1 Tattoos and body art. Dyes penetrate into the skin cells of a tattoo.
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Chapter 3: The Cell As a City 77
Exploring the Cell
The Microscope
The human body is composed of over 10 trillion cells, and there are over 200 different
types of cells in a typical animal body, with an amazing variety in sizes (see Figure 3.2).
Despite the variety in size, all of these cells and the structures within them are too small
Figure 3.2 Biological size and cell diversity. When comparing the relatives’ sizes of
cells, we use multiples of 10 to show differences. The largest human cell, the female
egg, is 100 µm, while the smallest bacterial cell is 1000 times smaller at 100 nm. Most
cells are able to be seen with the light microscope. The smallest object a human eye can
see is about 1 mm, the size of a human egg cell (or a grain of sand). From Introductory
Plant Science, by Cynthia McKenney et al.
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78 Unit 1: That’s Life
to be visible to our naked eyes and can only be identified by using microscopes to
magnify them.
There are several types of microscopes; perhaps the one with which you are already
familiar is the compound light microscope. The compound light microscope uses two
lenses: an ocular and an objective lens. Each of these is a convex lens, meaning that its
center is thicker than its ends. Convex lenses bring light to a central, converging point to
magnify the specimen. A microscope’s parts are seen in Figure 3.3.
The purpose of a microscope is to magnify subcellular parts. What is magnifica-
tion? Magnification is the amount by which an image size is larger than the object’s size.
If a hair cell’s image is 10 times bigger than its original object, the magnification is 10
times. If it is 100 times bigger, then the magnification is 100 times. The microscope uses
two lenses to magnify the specimen: an ocular (eyepiece), which generally magnifies
between 10 and 20 times, and a series of objective lenses (each with higher magnifica-
tions). The total magnification of a specimen is equal to the ocular (in this example let’s
use10 times) times the magnification of one of the objective lenses.
Most animal cells are only 10–30 µm in width. It would take over 20 cells to span the
width of a single millimeter. Recall that a millimeter is only as wide as the wire used to
make a paper clip. See Table 3.1 for measurements used for looking at living structures.
How were cells and their smaller components discovered using the microscope?
Anton van Leeuwenhoek and Marcello Malpighi built microscopes in the late 1600s. At
this time, those instruments were very rudimentary. They consisted of a lens or a com-
bination of lenses to magnify smaller objects, including cells. Both scientists used their
instruments to observe blood, plants, single-celled animals, and even sperm. Van Leeu-
wenhoek’s microscope is shown in Figure 3.4. At about the same time that van Leeuwen-
hoek and Malpighi were making their observations, Robert Hooke (1635–1703) coined
the term cell, as he peered through a primitive microscope of his own construction.
When he viewed tissues of a cork plant, Hooke saw what seemed to be small cavities
separated by walls, similar to rooms or “cells” in a monastery (see Figure 3.4). These
cells are defined as functioning units separated from the nonliving world.
Although it has progressed in design, materials, and technology, the compound light
microscope is based on the same principle as in the 17th century: light bends as it passes
through the specimen to create a magnified image. Some amount of light always bends
compound light
microscope
Microscope that uses
two sets of lenses
(an ocular and an
objective lens).
Magnification
Is the amount by
which an image size is
larger than the object’s
size.
Figure 3.3 Compound light microscope – its parts and internal lens system.
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Chapter 3: The Cell As a City 79
when hitting the edges of the lens, causing scattering in a random way. The random
scattering of light, called diffraction is bad for getting a clear focus on the image. Dif-
fraction also limits the resolution of the image. Resolution is defined as the ability to see
two close objects as separate. (Think about looking at two lines on a chalkboard that is
very far away; chances are they blur together and look like one messy line.) In fact, the
human eye has a resolving power of about 100 µm or 1/10th of a millimeter for close-up
images. In other words, two lines on a paper closer than 1/10th of a millimeter apart look
blurry to us. The light microscope is limited in the same way by diffraction because the
diffracted rays create blurry images.
diffraction
The random scattering
of light.
resolution
Is the ability to see
two close objects as
separate.
Figure 3.4 Hooke’s microscope from the 1600s and van Leeuwenhoek with his
microscope. These simple microscopes led to the first descriptions of cells. Van
Leeuwenhoek’s microscope consisted of a small sphere of glass in a holder.
1 centimeter (cm) =
1/100 meter or 0.4 inch
3 cm
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1 mm
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1 meter = 102 cm = 103 mm = 106 µm =109 nm
Unaided human eye
1 millimeter (mm) =
1/1,000 meter
1 micrometer (µm) =
1/1,000,000 meter
1 nanometer (nm) =
1/1,000,000 meter
100 µm
Hu
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Light microscopes
Electron microscopes
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table 3.1 Measurements Used for Microscopy. The units of measurement used in the study of molecules
and cells correspond with methods by which we are able to detect their presences.
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80 Unit 1: That’s Life
Higher magnification under the microscope leads to greater diffraction. This is the
reason a compound light microscope can magnify only up to 1000–1500 times (under
oil immersion), after which there is too much diffraction for a clear image to be formed.
To overcome the effect of diffraction and achieve clarity at higher magnifications, oil is
placed on the slide. However, even with oil immersion, only the large nucleus of a cell
can be seen; other organelles appear as small dots or not at all.
So how did the more complex world of even smaller structures within cells get dis-
covered? The 1930s saw the development of the electron microscope that allowed for
magnifications of over 200,000 times greater than that of the human eye. There are two
types of electron microscopes: transmission electron microscope (TEM) and scanning
electron microscope (SEM). Transmission electron microscopy allows a resolving power
of roughly 0.5 nm (see Table 3.1) that visualizes structures as small as five times the
diameter of a hydrogen atom. Electron microscopes use electrons instead of light, which
limits diffraction and increases resolution. Magnets instead of lenses focus electrons to
create the image. The electrons pass through very thin slices of the specimen and form
an image.
A SEM looks at the surfaces of objects in detail, while a TEM magnifies structures
within a cell. The SEM has a resolving power slightly less than the TEM, at 10 nm. (A
depiction of an electron microscope is shown in Figure 3.5.) Electron microscopy has
led to many scientific developments, uncovering subcellular structures to help us under-
stand cell biology. Seeing a mitochondrion enables us to better understand diseases and
perhaps, if our opening story becomes reality, improve societal health through its use.
Cell Theory
Fairly recent advances in microscopy have allowed scientists to learn about the structure
and function of even the tiniest components of cells, but the cell theory, which states key
ideas about cells, developed a long time ago. We have seen that scientists began study-
ing cells in the early 1700s. About a century later, in 1838, a German botanist named
Matthias Schleiden (1804–1881) concluded that all plants he observed were composed
of cells. In the next year, Theodor Schwann (1810–1882) extended Schleiden’s ideas,
transmission elec-
tron microscope
(teM)
A type of electron
microscope that
magnifies structures
within a cell.
Scanning electron
microscope (SeM)
An electron
microscope that
looks at the surfaces
of objects in detail
by focusing a beam
of electrons on the
surface of the object.
Figure 3.5 a. A researcher sits at a modern electron microscope. b. Apple tree pollen grains on cells, an
electron micrograph.
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Chapter 3: The Cell As a City 81
observing that all animals are also made of cells. But how did these cells come to survive
generation after generation? The celebrated pathologist Rudolf Virchow (1821–1902)
concluded in 1858 that all cells come from preexisting cells (He wrote this in Latin:
“Cellula e cellula”). These scientists contributed, together, to the postulates of the cell
theory. The cell theory is a unifying theory in biology that places the cell as the center of
life and unifies the many branches of biology under its umbrella. The cell theory states
that:
1) All living organisms are composed of cells.
2) The chemical reactions that occur within cells are separate from their
environment.
3) All cells arise from other cells.
4) Cells contain within them hereditary information that is passed down from par-
ent cell to offspring cell.
The cell theory showed not only that cells are the basic unit of life, but that there is
continuity from generation to generation. Genetic material is inherited in what we refer
today as the cell.
Types of Cells
Microscopes allowed researchers to examine differences between organisms that had
previously been impossible to determine. A current classification of organisms defines
five kingdoms, with organisms in those kingdoms having similar types of cells (There
is some debate arguing inclusion of Archaea bacteria as a separate kingdom, and a six-
system classification scheme is thus also accepted). Cells of organisms in the five king-
doms each have many internal differences, as summarized in Table 3.2. Images of some
organisms of each kingdom are given in Figure 3.17 as examples.
Prokaryotes (bacteria) are composed of cells containing no membrane-bound
nucleus and no compartments or membranous organelles. They are much smaller than
eukaryotes, by almost 10 times. Prokaryotic genetic material is “naked,” without the
protection of a membrane and nucleus. They are composed of very few cell parts: a
membrane, cytoplasm, and only protein-producing units called ribosomes. Even without
most structures found in other organisms, prokaryotes contain genetic material to repro-
duce and direct the functions of the chemical reactions occurring within its cytoplasm.
group domain cell type cell number cell Wall component energy Acquisition
Bacteria Bacteria Prokaryotic Unicellular Peptidoglycan Mostly heterotrophic,
some are autotrophic
Protists Eukarya Eukaryotic Mostly unicellular,
some are simple
multicellular
Cellulose, silica; some have
no cell wall
Autotrophic,
heterotrophic
Plants Eukarya Eukaryotic Multicellular Cellulose Autotrophic
Animals Eukarya Eukaryotic Multicellular No cell wall Heterotrophic
Fungi Eukarya Eukaryotic Mostly multicellular Chitin Heterotrophic
From Introductory Plant Science by Cynthia McKenney et al. Copyright © 2014 by Kendall Hunt Publishing Company. Reprinted by
permission.
table 3.2 Differences in Cell Structure within the Five Kingdoms: Plants, Animals and Prokaryotes.
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82 Unit 1: That’s Life
Prokaryotes have a simple set-up, but all of the needed equipment to carry out life func-
tions. Bacteria have a rapid rate of cell division and a faster metabolism than eukaryotes.
Most organisms on Earth, in terms of sheer number, are prokaryotes.
• As indicated in Chapter 1, prokaryotes include organisms in the Bacteria and
Archae domains. These organisms will be discussed further in Chapter 8.
All other organisms (plants, animals, fungi, and protists) are eukaryotes. Cells of
eukaryotes are complex, containing a membrane-bound nucleus that houses genetic
material. Eukaryotic cells comprise compartments that form a variety of smaller internal
structures, or organelles. Eukaryotic cells are the focus of this chapter, which will give
an overview of the primary organelles and their functions (Figure 3.6).
Eukaryotes may be examined by dividing into its four groups: plants, animals, fungi,
and protists. Plants contain cells that are surrounded by a cell wall, a rigid structure giv-
ing its organisms support. Plant cells contain chloroplasts, which enable plants to carry
out photosynthesis, using energy from sunlight to make food.
• Plant cell walls contain cellulose, which gives structure to plants as discussed
in Chapter 2. The process of photosynthesis, producing food for plants, will be
further discussed in Chapter 4.
Plants also have large vacuoles or storage compartments to hold water and minerals for a
plant’s functions. While both plants and animals have a cell membrane, animal cells are
Photosynthesis
The process by which
green plants use
sunlight to synthesize
nutrients from water
and carbon dioxide.
Figure 3.6 a. Differences between prokaryotes and eukaryotes. Prokaryotes have a
generally simple structure (see top cell in figure above), while eukaryotes (the lower
cell in figure above) have multiple organelles and membranes forming complex com-
partmentalization. From Biological Perspectives, 3rd ed by BSCS. b. Differences between
plants and animals. Plant and animal cells perform different functions, and their subcel-
lular structures are also different. Plant cells have chloroplasts to produce sugar and a
cell wall to give added strength. The animal cell shown has no cell wall or chloroplasts
but possesses centrioles. From Biological Perspectives, 3rd ed by BSCS.
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Figure 3.6 (Continued)
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less rigid, surrounded only by a cell membrane and lacking a cell wall for support. Both
plants and animals contain membrane-bound organelles, but animals also contain a set
of small structures called centrioles, which serve in cell division. Animal cells are also
quite complex, as we will see. While lacking certain organelles, such as cell walls and
chloroplasts, they have flexible strategies to perform many functions.
Fungi have cell walls but no chloroplasts. They are not able to make their own food
and, instead live off of dead and decomposing matter as well as other living organisms,
centriole
Minute cylindrical
organelles found in
animal cells, which
serve in cell division
(not given in bold in
text).
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Figure 3.6 (Continued)
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Chapter 3: The Cell As a City 85
to obtain energy. Mushrooms and yeasts are familiar types of fungi, which will be dis-
cussed in Chapter 7.
Some species of protists are a bit animal-like in that they are able to move; other
species are a bit plant-like in that they have chloroplasts. Protists such as Amoeba in
Figure 3.7 have varied environments. Amoeba live in freshwater and, in a rare infectious
disease, grow and destroy human brain cells. We will discuss protists in more detail in
a later chapter.
Figure 3.7 Cells of the five kingdoms. While the cells of organisms in all of the kingdoms perform similar
life functions, their individual structures enable differing functions unique to each kingdom. From Biological
Perspectives, 3rd ed by BSCS.
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86 Unit 1: That’s Life
The Role of inheritance
The stratification system depicted in our opening story is based on the inheritance of
cellular components. We know that organelles are structures that carry out functions
within a cell. In fact, organelles work in concert with one another, coming together to
Figure 3.7 (Continued)
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Chapter 3: The Cell As a City 87
form a complex, dynamic cell. Mitochondria, so important in the society in our story, are
the powerhouses of the cell, providing energy for a cell’s functions.
All organelles are built and controlled by inherited genetic material. Thus, the way
a cell works is based upon its genetics. But some organelles are inherited separately
from the others. There are three ways to inherit organelles, including mitochondria: 1)
maternal inheritance, in which organelles are inherited from mothers; 2) paternal inher-
itance, in which organelles are inherited from fathers; and 3) bi-parental inheritance,
in which organelles are inherited from both mothers and fathers. The inheritance type
varies among species and for different organelles. For example, chloroplasts are pater-
nally inherited in the giant redwood Sequoia but maternally inherited in the sunflower
Helianthus. Most animal species have maternal transmission of mitochondria, as seen in
the story, because female eggs hold most of the mitochondria in their large cytoplasmic
cells. Sperm contributes very little cytoplasm or organelles in human species, although
there are exceptions among other organisms; for example, green algae Chlamydomonas
has paternal transmission of mitochondria.
Endosymbiosis
Eukaryotes appeared in Earth’s history about 1.5 billion years after prokaryotes. In their
2.0 billion years on Earth, eukaryotes have evolved into living systems that range from
butterflies to beavers, crocodiles to humans. As you progress through this text and the
course, you will learn about how this amazing diversity evolved.
Eukaryotes have two types of organelles – those that evolved as membranes and those
from other, simpler organisms as precursors, called endosymbionts. Endosymbionts
endosymbionts
Any organism living in
the body or cells of
another organism.
the Oxygen revOlutiOn
Have you ever tried to imagine the Earth in its early stages? After millions of
years during which the Earth was a mass of molten gases, those gases began
to cool into layers that became landmasses, while water vapor formed seas.
There were as yet no animals and only a few prokaryotic forms living in the
waters. One form of bacteria, known as cyanobacteria, is believed to have
been among the first organisms to photosynthesize (convert light energy into
chemical energy to drive cellular activities). Since a by-product of many forms
of photosynthesis is oxygen, over several million years, more and more oxygen
was released into the atmosphere – the oxygen revolution.
The oxygen revolution occurred, according to geologists, about 2.5 billion
years ago. It led to an availability of oxygen that could be used by cells that
evolved to use it. The advantage of using oxygen to yield larger amounts of
energy led to an increase in the number of aerobic cells. Aerobic cells, or those
cells that use oxygen as the fuel for obtaining energy, contain mitochondria to
provide large amounts of energy production. Over the course of many millions
of years, single-celled eukaryotes, with more complex cellular processes than
prokaryotes, evolved, then multicellular eukaryotes, and eventually organisms
became larger and more complex. It is important to realize that oxygen is a
key player in the development of organisms since it can be used to produce
fast, plentiful energy.
Oxygen revolution
The biologically
induced appearance
of dioxygen in Earth’s
atmosphere 2.5
billions of years ago.
Aerobic
Occurring in the
presence of oxygen or
require oxygen to live.
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88 Unit 1: That’s Life
have their own genetic material and are semi-independent within the cells of eukary-
otes. Endosymbionts include two types of organelles – mitochondria and chloroplasts.
Mitochondria are the energy producers of animal cells, and chloroplasts are solar power
transformers of plant cells. Mitochondria divide independently and have an internal
environment that is different from that in the rest of the cell.
Evidence tells us that endosymbionts were once independent prokaryotes that were
somehow incorporated into eukaryotic cells. Lynn Margulis, a well-known evolution-
ary biologist, formulated the endosymbiotic theory, which states that some organelles
in eukaryotes were descendants of ancient bacteria that were absorbed by larger cells.
(Symbiosis refers to a mutually beneficial relationship of organisms living together;
endo means within.) The larger cell gave ancient bacteria absorbed by larger cells a
“home.” The bacteria received protection and in return gave their unique set of chemical
reactions to the larger host cell.
Analysis of endosymbionts have particular uses in today’s society: Crime scene
investigations can test for mitochondrial DNA; ancestry can be traced using mitochon-
drial DNA; and research on diseases inherited due to faulty mitochondrial genetic mate-
rial may yield medical treatments. In our story, Joules and Theta had a different form of
mitochondria than Sally. Joules, Theta, and Sally all inherited their mother’s mitochon-
dria, but Sally’s inheritance included a predisposition for a number of diseases.
We have said that mitochondria can be thought of as the power plants of the cell. We
say this because mitochondria carry out the series of energy-producing reactions that
convert food energy into ATP (defined in Chapter 2; the form of energy used to drive
cell functions). This set of reactions is called cell respiration.
Chloroplasts may have originated from cyanobacteria, which were able to transform
light energy into usable sugar for energy in the process called photosynthesis – the
making of food (glucose) from sunlight, carbon dioxide, and water. Thus, precursor
mitochondria provided instant ATP energy for animal host cells, and ancient chloro-
plasts made stored glucose available for longer-term use in plants. Modern chloroplasts
use sunlight to rearrange carbon to form food in the form of sugars, for cell usage. They
are the solar power plants of cells because they trap sunlight and generate ATP energy.
• These energy-obtaining processes, cell respiration, and photosynthesis will be
discussed in greater detail in Chapter 4.
Evidence for mitochondria and chloroplasts as endosymbionts is considerable:
1) Mitochondria and chloroplasts are similar in size and shape to bacteria, roughly
7 µm in length.
2) Both contain their own genetic materials and divide in the same way as
prokaryotes, through binary fission (splitting in half).
3) Both contain the same type of 70S ribosomes (described below; small organ-
elles that make protein) as bacteria, whereas eukaryotes contain 80S ribosomes.
4) Mitochondrial and chloroplast DNA are more related to bacterial than to eukary-
otic DNA. See the proposed process on endosymbiosis in Figure 3.8.
We will now look at each of the structures of the cell, using the analogy of a city,
with the organelles being structures critical to the smooth functioning of the “city.”
Organelles work in concert with each other to carry out life functions. Chloroplasts
and mitochondria carry out key energy-producing and releasing functions to drive cell
activities. However, there is an intricacy to cell biology akin to the workings of a large
and dynamic city.
endosymbiotic
theory
The theory that states
that some organelles
in eukaryotes were
descendants of ancient
bacteria that were
absorbed by larger
cells.
cell respiration
A series of energy-
producing reactions
that convert food
energy into ATP.
chloroplast
A part of plant that
contains chlorophyll
and conducts
photosynthesis.
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Chapter 3: The Cell As a City 89
Figure 3.8 This model shows how mitochondria and chloroplasts wound up in
eukaryotic cells. Evidence for endosymbiosis. Chloroplasts and mitochondria are simi-
lar in size and shape to bacteria. The ribosomes in bacteria and mitochondria and chlo-
roplasts are “70S.” Most bacteria have a size of roughly 7–10 µm and 70S ribosomes,
both characteristics are similar to mitochondria and chloroplasts.
©
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Proteobacterium
engulfed by a
heterotrophic
eukaryote
Loss of bacterial cell wall
and transfer of some genes
to the nucleus-endosymbiont
becomes mitochondrium
Loss of bacterial cell wall and
transfer of some genes to the
nucleus-endosymbiont
becomes chloroplasts
Heterotrophic
eukaryotes: animals,
fungi and some protists
Autotrophic eukaryotes:
plants and algae
Cyanobacterium
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90 Unit 1: That’s Life
Cell Architecture: The Cell As a City
The organelles of a cell work independently but in concert, with its components inter-
acting actively, having many thousands of chemical reactions occurring at any one time.
In an analogous way, the components of a city work independently but cooperatively to
ensure its smooth functioning. The structures in the cell are shown in the cartoon image
depicting the cell as a city in Figure 3.9.
A cell membrane or plasma membrane surrounds each cell, which is a wrapping that
allows some materials across it. While all cells contain a plasma membrane, some cells
have structures surrounding the membrane for protection and support. For example,
plant cells have cell walls surrounding their membranes, and fungi have chitin barriers,
which are protective polysaccharides.
Plasma membranes are important to living systems because they control the mate-
rials entering and leaving them. Plasma membranes are selectively permeable. Selective
permeability means that the membrane allows some materials to pass through cells but
not others. Within the cell is the cytoplasm, a semisolid liquid that holds organelles
suspended within it. Cytoplasm is roughly 60–80% water by volume, and chemical reac-
tions occur within its medium. The other 20–40% of cytoplasm is composed of pro-
teins and dissolved ions. Cytoplasm is all of the cell material found between the plasma
Plasma (cell)
membrane
A biological membrane
that separates the
cell’s interior from the
outside environment.
Selectively
permeable
A condition in which
the membrane allows
some materials to pass
through cells but not
others.
cytoplasm
A semisolid liquid
that holds organelles
suspended within it.
Figure 3.9 A cell is like a city. Its parts work together to perform a cell’s function.
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A. Nuclear envelope City Hall
Nuclear pores
Nucleoplasm
Outer and inner
membranes
F. Plasma membrane
the Border Patrol
B. Endoplasmic reticulum The Subway
C. Golgi body
Processing Plant
D. Centrioles MoversE. Mitochondrion Power Plant
Proteins
Phospholipid
bilayer
Channel (allows ions
and water to move in
and out of cell)
Inner and outer
membranes
Cristae
Microtubules
Maturing
face Secretory
vesicles
Forming
face
Rough E.R.
Smooth E.R.
Fixed
ribosomes
Cisternae
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Chapter 3: The Cell As a City 91
membrane and the nucleus. A nucleus, or control organelle, serves as the city hall of the
cell, usually centered within cytoplasm. It contains the genetic material that produces
and controls the cell’s parts.
A cell’s architecture is very complex, with continual activity within many com-
partments of each cell. Compartmentalization of a cell is accomplished by a series of
membranes throughout its cytoplasm. These membranes allow for a surface to per-
form chemical reactions such as those carried out by enzymes. The layers of mem-
branes also separate different chambers of the cell so that conditions may be different
within each chamber. An acidic pH in one compartment, for example, may be suitable
for cell functioning in one section, while a basic pH might be needed in another section.
Throughout the cell, ropes (such as collagen and elastin) and membranes maintain a
cell’s organization. Figure 3.9 gives you an idea of the complex architecture of the cell.
plasma Membrane: The
“Flexible” Border patrol
A border surrounds all cells: in eukaryotic animal cells, the border is the plasma or cell
membrane. This dynamic covering is very complex, with parts that continually move to
allow certain materials into the cell and keep other substances out. The plasma mem-
brane is composed of a phospholipid bilayer in and around which are membrane pro-
teins (see Figure 3.10).
• Recall from Chapter 2 that phospholipids are molecules with a phosphate
(hydrophilic) head and two tails made of carbon and hydrogen atoms.
The phospholipids are arranged as a bilayer, with the heads facing to the outside and
tails to the interior of the cell. The membrane proteins are part of the plasma membrane
border, moving in between cholesterol molecules and phospholipids. Although it may
seem that they are arranged randomly, in fact, they form a pattern. The membrane is
often referred to as a fluid mosaic (see Figure 3.11) because it is made of different pieces
that form a pattern and seem to float and move in the watery environment. Note the
phospholipid bilayer in Figure 3.11.
There are two types of membrane proteins suspended within the phospholipid
bilayer: integral proteins, which span the entire lipid bilayer; and peripheral proteins,
which station either inside or outside of the membrane. Integral proteins are also known
as transmembrane proteins. These membrane proteins serve to anchor cells to each
nucleus
The central and the
most important part
of a cell and contains
the genetic material.
compartmental-
ization
The formation of
cellular compartments.
Fluid mosaic model
A model that
describes the
structure of cell
membranes.
integral protein
(transmembrane or
carrier protein)
A type of membrane
protein permanently
embedded within the
biological membrane
(not given in bold in
text).
Peripheral protein
Is a protein that
adheres only
temporarily to the
biological membrane
with which it is
associated.
Figure 3.10 Every living cell is surrounded by a cell membrane. From Biological
Perspectives, 3rd ed by BSCS.
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92 Unit 1: That’s Life
other, move materials, much like a ferry, across the membrane, receive chemicals such
as hormones, and transport ions through pores within them. These proteins orient them-
selves according to the bonds they make with surrounding chemicals.
Integral proteins in the fluid mosaic model serve four functions:
1) As receptors, allowing chemicals to bind to cell surfaces. Insulin, a hormone-
regulating blood sugar, binds to cells to increase the absorption of sugar from
blood into cells. Insulin’s special shape matches to the shape on integral pro-
teins. The docking of the two elicits chemical reactions within the cell to main-
tain sugar balance;
2) As recognition proteins, giving the immune system a code that informs it that
a cell is its own and not a foreign substance. Cells with the wrong recognition
proteins are rejected, as occurs in cases of organ transplants whose codes do not
match up with those of the recipient;
3) As enzyme surfaces, facilitating various chemical reactions occurring on the
surface of a cell. Enzymes assist in the digestion of certain nutrients, the first
step in allowing nutrients to be absorbed; and
4) As transport proteins, moving material across a membrane.
Transport proteins act as a sort of flexible border-patrol system, which allows some
materials to pass through the membrane while keeping others out. This border-patrol
system is an important part of the cell as a city. The size, shape, and chemistry of sub-
stances attempting to move into and out of cells determine which materials pass through
the plasma membrane. Fatty materials pass through the membrane easily. For example,
ethanol in alcoholic drinks easily passes through membranes because it also dissolves
in fats. It is absorbed through the phospholipid bilayer and quickly giving a “high” to a
person after drinking. The interior of the lipid bilayer is hydrophobic, so it avoids water
and dissolves other substances that avoid water. Thus, only other hydrophobic, usually
fatty or fat-soluble, materials may pass easily through the bilayer.
Integral proteins of the membrane allow certain nonfatty materials, including smaller
charged or polar particles to move through the lipid bilayer directly. These include chem-
icals such as oxygen (O2), carbon dioxide (CO2), and ammonia (NH3). The quick move-
ment of these materials is essential for life functions. For example, NH3, a nitrogenous
waste that builds up within all cells, needs to be rapidly removed.
Thus, integral membrane proteins serve to facilitate movement of materials that can-
not easily pass across the membrane. Integral proteins move nonfatty materials across
the membrane, such as sodium ions Na+ and potassium ions K+. Larger polar molecules,
such as amino acids and glucose, also move through polar (or charged) channels within
integral proteins. Channels that are polar are thus hydrophilic, allowing materials with
a charge to pass through. Some substances use carrier proteins as pumps for transport,
as is the case for the very important ions sodium (Na+), potassium (K+), and calcium
(Ca+2).In the case of the movement of water (H2 O), the most abundant chemical in living
systems, it was recently determined from studies on aquaporins, or integral proteins
with channels within them, that water presses its way through the bilayer using integral
proteins. Sometimes water moves directly across the lipid bilayer by “wiggling” type
motion to press itself through. Physical forces drive this movement.
As shown in Figure 3.11, short carbohydrate chains jut out from the plasma mem-
brane. These serve as a recognition code for the immune system of animals. Embedded
within the phospholipid bilayer of animal cells, cholesterol binds together molecules,
helping to maintain the flexibility and motion of the membrane.
Consider for a moment, how many different pieces make up the plasma mem-
brane, and you can see why it is called a mosaic. It is called a fluid mosaic because it
receptor
A structure of the
cell’s surface that
selectively receives
and binds a specific
substance.
recognition protein
A protein type
functioning as binding
site for hormones.
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Chapter 3: The Cell As a City 93
is constantly moving, docking proteins, transporting materials, and changing its shape
with the movements of a cell. Membranes are also found throughout the cell’s structure,
with transport occurring continually across different areas. The types of transport will
be discussed later in the chapter. First, let’s explore the structure surrounding plasma
membranes in some organisms.
Walls of the City
What other borders exist around cells? Cell walls are found outside of plasma membranes
in plants, fungi, prokaryotes, and many protists, particularly in algae. For example, sin-
gle-celled algae known as dinoflagellates acquire their shape from their cell walls. Cell walls
give protection to the material within cells and add to their structure. Cellulose, composed
of large combinations of glucose units, polysaccharides, and lignin (a type of chemical
cement), makes up the stiffening agent of plant cell walls. While plants contain cellulose,
fungi cell walls contain chitin, also found in insect bodies and shells of some marine organ-
isms. Cell walls are not found in animal cells, which have no need for a stiff structure that
would prevent movement and flexibility. The cell walls of plants are shown in Figure 3.12.
Cytoskeleton: The City scaffolds
The shape of a cell is also determined by the skeleton within a cell’s cytoplasm, called
its cytoskeleton. You may be surprised to learn that the cell itself has a skeleton made up
of three types of fibers: microtubules, microfilaments, and intermediate fibers. All three
Microtubule
Is a larger filament
structure that helps
whole cells move.
Microfilament
A cytoskeletal fiber
used for muscle
movement.
intermediate fiber
The smallest fibers
of the cytoskeleton,
which circulate
materials within a cell.
Figure 3.11 Cell membrane structure. The cell membrane is composed of a phos-
pholipid bilayer with proteins embedded. Phospholipids are composed of two parts – a
phosphate head which attracts to water (hydrophilic) and fatty acid tails which repel
water (hydrophobic). Proteins are embedded within the lipid bilayer and each type
performs a specific function.
Plasma Membrane Structure
Extracellular fluid
Cytoplasm
Transmembrane
glycoprotein
Cholesterol Peripheral
protein
Transmembrane
protein
Channel protein
Glycolipid
Carbohydrates
Pore
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94 Unit 1: That’s Life
fibers are made up of chains of twisted proteins that serve to link and support parts of a
cell (see Figure 3.13). If the cell were a city, its cytoskeleton would be its infrastructure
or scaffolding.
Microtubules form larger structures – cilia and flagella – that help whole cells move.
Cilia are short extensions, and flagella are long, whip-like extensions on a cell’s surface.
Many types of organisms use cilia or flagella. A particular bacterium, Proteus mirabilis,
has numerous and extravagant flagella arrangements. Flagella are found in humans only
on sperm cells, to propel sperm toward their goal: the female egg.
Microfilaments, another cytoskeletal fiber, are used for muscle movement. Mus-
cles contract with the help of microfilaments moving in a sliding motion. Microfila-
ments also circulate materials within cells, in a process called intracellular transport.
Intracellular transport moves materials around the cell in a wavelike motion, adding
cilia
Are short extensions
that help cells move.
Flagella
Are long, whip-like
extensions on a cell’s
surface that help in the
movement of cells.
intracellular
transport
A process in which
microfilaments
circulate materials
within cells.
Figure 3.12 Plant cell wall. A rigid cell-wall composed of lignin and other stiffening
materials maintains structure in plants. Animal cells have only a thin cell membrane,
which gives almost no structure to the cell.
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animal cell plant cell
Figure 3.13 The internal skeleton of the cell. The cytoskeleton is composed of three
types of fibers: microtubules, microfilaments, and intermediate fibers. Cell parts are
held in place by the cytoskeleton and chemical reactions that occur on its scaffolding.
The cartoon image in the figure shows the cytoskeleton of an animal cell.
©
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Chapter 3: The Cell As a City 95
efficiency to internal transport. Microfilaments greatly aid how materials are moved
inside cells.
Intermediate fibers, the third type of cytoskeletal fiber, join cells together, especially
for muscles such as the heart, where there is a great deal of pressure. Intermediate fibers
are made of fibrous proteins, whereas, the other cytoskeleton fibers are composed of
globular proteins.
• Fibrous proteins are not easily disassembled as described in the protein section of
Chapter 2.
All three fibers anchor organelles in place, such as the mitochondria discussed in our
opening story, within the cytoplasm. The cytoskeleton gives a solid framework on which
chemical reactions take place. Some chemicals move along their fibers from one loca-
tion in a cell to another. But overall, the cytoskeleton is not a permanent structure, as
the name “skeleton” implies. Much like the plasma membrane, the cytoskeleton changes
and reforms with the needs of a cell.
nucleus: A City’s City hall
The most important membranous organelle is the nucleus, the control center of the cell.
In our analogy of the cell as city, the nucleus is City Hall. The source of its leadership is
DNA (deoxyribonucleic acid), the master planner for the organism. The nucleus is espe-
cially protected more than any of the other organelles, guarding its vital components.
Thus, it is composed of a double membrane, called the nuclear envelop, with special
pores or holes that have an octagonal shape and are highly selective. Only certain mate-
rials may pass through the nuclear membrane because the pores are very narrow.
In order to communicate with the cytoplasm and the outside world, DNA in the
nucleus sends its messenger chemical RNA (ribonucleic acid) out through nuclear pores.
RNA directs the synthesis of proteins, which in turn controls cell functions and the
building of cell structures. A nucleus is shown in Figure 3.14.
In most organisms, DNA is inherited from both father and mother through genetic
material stored and transmitted from nucleus to nucleus. Unlike in our story, which
deals with the maternal lineage of mitochondrial DNA, nuclear DNA is obtained from
the egg of one’s mother and the sperm of one’s father in sexually reproducing organisms.
When cells are not dividing, DNA is coiled around histone proteins, which are a group
of five small round proteins bound together with DNA in eukaryotes, together forming
chromatin (Figure 3.15).When a cell divides, its DNA coils more tightly to histones,
forming a shorter and thicker mass called chromosomes.
Ribosomes, the City’s Factory
The nucleus also contains large bodies called nucleoli (see Figure 3.16), which are spe-
cial regions of DNA that clump together and produce small, spherical organelles known
as the ribosomes. Ribosomes are made up of RNA. Ribosomal RNA directs the pro-
duction of proteins, guided by the RNA message sent out of the nucleus from DNA.
Ribosomes are the protein factories of the cell and thus you might think of them as the
manufacturing plants of the cell city. RNA carries a message that is translated on the
ribosome. This message gives the type of protein ordered by DNA in the nucleus. These
processes of protein manufacturing will be discussed further in a later chapter.
All organisms require proteins and thus all living systems contain ribosomes. Bacte-
rial ribosomes are lighter than eukaryotic ribosomes, but all ribosomes make proteins in
the form of strings of amino acids. Proteins comprise so many cell activities and struc-
tures that ribosomes are essential organelles found in all living organisms.
nuclear envelop
The double membrane
that protects the
nucleus.
chromatin
Is a complex of
macromolecules found
in cells and consist
of protein, RNA, and
DNA.
chromosome
A thread-like structure
formed when a cell
divides and its DNA
coils more tightly to
histones.
nucleoli
A small, dense round
structure found in the
nucleus of a cell.
ribosome
Small, spherical
organelle that is the
site protein synthesis.
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96 Unit 1: That’s Life
Endoplasmic Reticulum: A City’s subway
Ribosomes may be found freely within the cytoplasm or attached to membranes of other
organelles. One organelle that often has ribosomes on its surface is the endoplasmic
reticulum or ER for short. You can think of the ER as the subway system of the cell city.
endoplasmic reticu-
lum (er)
Is a system of
interconnected
membranes that form
canals or channels
throughout the
cytoplasm of a cell.
Figure 3.14 The cell nucleus and its components. The nucleus is the control center
of a cell. It communicates with the rest of the cell by directing the production of pro-
teins. Proteins serve many roles, particularly in chemical reactions. Vesicles transport
materials from the nucleus to other parts of the cell and to the exterior through its
endoplasmic reticulum.
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Nucleus
Nuclear
Pores
Rough Endoplasmic
Reticulum
Ribosome
Chromatin
Nucleolus
Nuclear Envelope
Figure 3.15 DNA coils around histone proteins, with large amounts of genetic
materials becoming packaged into very small units.
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Chapter 3: The Cell As a City 97
The ER is a system of interconnected membranes that form canals or channels through-
out the cytoplasm of a cell. Materials are transported rapidly through the ER. Ribosomes
found on ER surfaces dump newly produced proteins into the channels, allowing them
to be transported to areas of a cell that need them. ER with ribosomes attached is called
rough ER because it is rough in appearance. ER without ribosomes attached is termed
smooth ER due to its undotted appearance. Figure 3.16 shows images of both smooth
and rough ER
Rough ER is found in cells that export large amounts of protein, such as pan-
creatic cells that produce the protein-based hormone insulin. Smooth ER is a main
component of organs that produce lipids, because it has enzymes that help in their
manufacture. Organs (both testes and ovaries) that produce lipid-based sex hormones
have smooth ER. Cells that manufacture and export lots of fatty material have elon-
gated smooth ER.
Golgi Apparatus: A City’s processing plant
When materials are transported out of the ER, they are often not fully processed and
need the addition or removal of parts. The Golgi apparatus is the processing plant of the
cell city that refines the materials passing through it. The Golgi apparatus consists of a
series of four to six flattened sacs, resembling a stack of pancakes. The Golgi apparatus
is shown in Figure 3.17.
As processing occurs, the Golgi apparatus rearranges bonds, adds carbohydrates, or
places a lipid on materials moving through. For example, the Golgi apparatus adds phos-
phate (glycerol) heads to lipid chains to form the phospholipids that make up plasma
membranes. At the end of Golgi processing, vesicles or sacs are pinched off the export
side. Vesicles are now ready for transport and contain needed materials for either within
or outside of the cell. Certain vesicles are also made into another organelle, called the
lysosome.
golgi apparatus
Is the processing plant
of the cell city that
refines the materials
passing through it.
Figure 3.16 Smooth and rough ER. Rough ER has ribosomes attached to it, and
smooth ER does not have ribosomes attached. Both act as the subway of the cell,
transporting materials.
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98 Unit 1: That’s Life
lysosomes: A City’s police officer
The lysosome, a small sac filled with digestive, hydrolytic enzymes, may be thought of
as the cell’s police officer.
• The term hydrolytic refers to hydrolysis, the breakdown of substances (described
in Chapter 2).
Lysosomes are found throughout the cytoplasm. They act as defenders of cells, just
like police officers do. When invading microbes enter cells, lysosomes fuse with the
“enemy” to digest it. Their contents break down materials within cells, and thus they are
responsible for intracellular digestion. Consider the Amoeba (Figure 3.18) that is taking
in food. Lysosomes act as its entire digestive system, fusing with a food particle and
breaking it down.
Thus lysosomes are the police officers of the cell city, continually breaking down
those substances that need to be removed and cooperating with the immune system to
defend against invaders.
Lysosomes also play a critical role in reorganizing material. For example, did you
know that our fingers and toes were fused together, or webbed, during our fetal develop-
ment? Lysosomes digest the tissue between our digits, allowing fingers and toes to form.
In roughly 1% of the population, lysosomes fail to fully digest this tissue, and fused toes
or fingers are the result.
Tadpoles lose their tails during metamorphosis to adult frogs due to lysosome action
that digests tails. Normal cell aging, or senescence, is a result of deterioration of lyso-
somes, according to one hypothesis. The deterioration of lysosomes releases their enzy-
matic contents, thereby digesting our cells from the inside out. Lysosomes normally
have a double membrane, similar to the nucleus, giving double protection that limits
such a disaster. The double membrane, however, cannot stand the effects of time, result-
ing in membrane failure and aging.
lysosome
A small sac filled with
digestive, hydrolytic
enzymes enclosed in a
membrane.
intracellular
digestion
The process of
breakdown of
substances within the
cytoplasm of a cell.
Figure 3.17 The Golgi apparatus processes materials such as a phospholipid as they
pass through its sacs. A phosphate is added to a lipid molecule to produce a phospho-
lipid for the plasma membrane. The golgi apparatus is the purifier of substances within
a cell.
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Chapter 3: The Cell As a City 99
There are 30 known inherited human diseases associated with the abnormal func-
tioning of lysosomes. These disorders are called lysosome-storage diseases because in
each case lysosomes lack a particular enzyme that breaks down substances that would
normally be removed. These waste products build up in lysosomes, accumulating in
cells. Tay–Sachs disease, commonly found in Ashkenazi Jewish populations, is a pro-
gressive and fatal disease in which fatty substances build up in tissues and nerve cells
of the brain. There is blindness, mental deficiency, and death at an early age due to
Tay–Sachs disease. Like most lysosome storage diseases, Tay–Sachs disease has few
effective treatments.
Vacuoles: A City’s Warehouse
Storage in cells is accomplished by vacuoles, which are single membrane structures that
hold materials in a cell; they are the warehouses of the cell city. There are three types of
vacuoles: food, water, and waste vacuoles, each holding what their name indicates. Food
vacuoles bring materials to and from the plasma membrane, depending upon a cell’s
need for nutrients. Adipose or fat cells in humans contain food (fat) vacuoles that com-
prise up to 80% of the cell, with enormous capabilities for energy storage. Food vacuoles
shrink or expand based on the amount of fat stored in adipose cells.
Waste vacuoles are temporary storage compartments that send materials directly to
the plasma membrane for export. Nitrogen-containing compounds are regularly stored
in waste vacuoles. Their accumulation is dangerous, which is a major reason untreated
kidney failure is often fatal after only a few days. One nitrogenous waste, ammonia
(NH3), is particularly poisonous, as you can attest if you have handled household ammo-
nia. Wastes are therefore exported from a cell as quickly as they accumulate.
Water vacuoles, which are storage vesicles for water, can also grow very large,
especially in plants. They serve to give shape to a plant cell, exerting pressure from
within the vacuole onto the plant cell wall. A special type of water vacuole, found in
simple-celled organisms such as the Paramecium is called the contractile vacuole. The
contractile vacuole has the unique ability to actively pump water out water of cells,
acting as a mini-kidney. Its shape is shown in Figure 3.19, with a star-like appearance
denoting its structure.
lysosome storage
disease
A group of 30 known
inherited human
diseases associated
with the abnormal
functioning of
lysosomes.
Figure 3.18 Amoeba digesting food. The amoeba surrounds its prey, and lysosomes
digest it.
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100 Unit 1: That’s Life
Figure 3.19 The Paramecium has many of the structures found in protists.
The contractile vacuole pumps excess water out of the paramecium.
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Paramecium
Cilia
Micronucleus
Macronucleus
Contractile
vacuole
Oral
groove
Food
vacuole
Anal pore
Cytoplasm
plastids: The Cell City’s paint shops
Plastids are organelles that store special substances. They are found only in plants and
algae. Colored substances called pigments are stored in some plastids. Chloroplasts,
which contain chlorophyll, are the most well-known plastid.
• Chlorophyll is a green-colored pigment that will be discussed in Chapter 4.
Chromoplasts store yellow and orange pigments that give colors to trees, flowers, fruits
and vegetables, such as carrots. Leukoplasts store starch and other food storage materi-
als. Leukoplasts are usually found in roots, as in turnips and potatoes. We eat the starch-
filled roots of these and several other foods.
Cell Junctions: The City’s Bridges
In the same way that cities have bridges to connect and support their parts, cells contain
cell junctions. Cell junctions serve a cell by providing areas of extra support, enabling
communication between cells and preventing leaking through their borders.
There are three different types of connections or bridges between cells, each playing
a needed role in particular cases. The first type of cell junction called desmosomes
are anchoring junctions or spot welds between cells. You can think of desmosomes/
anchoring junctions as the reinforcing rods embedded in concrete slab bridges of a city.
Within desmosomes, intermediate fibers entangle with linker proteins and attach to a
thickened region of the plasma membrane to give extra support in holding cells together.
Heart or cardiac cells contain many desmosomes to withstand the pressure of millions
of heartbeats.
The second type of cell junction, tight junctions, fuses areas together to prevent leak-
ing and act as sealants. Intestinal cells have tight junctions to prevent their contents from
leaking out of the bodies of animals.
The third type known as gap or communicating junctions are channels that run from
one cell into another to allow rapid transport. These bridges help cells communicate with
Plastid
Organelle, found only
in plants and algae,
which store special
substances.
chromoplast
An organelle that
contains any plant
pigment other than
chlorophyll.
desmosome
One of the three
types of connections
between cells, is a cell
structure specialized
for cell-to-cell
adhesion.
tight junction
A specialized cell
junction that fuses
areas together to
prevent leaking and act
as sealants.
gap (communicat-
ing) junction
Are channels that run
from one cell into
another to allow rapid
transport helping cells
communicate with
other cells.
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Chapter 3: The Cell As a City 101
other cells. Embryonic cells have gap junctions because, in the absence of a developed
vessel system for transport, an embryo compensates with gap junctions. The structures
of cell junctions are shown in Figure 3.20.
Cell shape and size
Most cells in the body are round or spherical, except when a cell wall shapes a cell to
conform to the forces around it. There are several reasons for the spherical shape, but
resisting torque is a main one. Round surfaces resist the damaging effects of torque
forces (circular forces). For example, consider a square cell, with edges that are likely to
break off or become damaged when torque forces are applied to such cells. Any shape
with a corner is more likely to be damaged than something that is spherical.
In senescence studies, the wear-and-tear hypothesis contends that damage due to
forces placed upon living systems is a main reason for the aging of cells and the loss of
cellular function. As a cell ages, it is more and more likely to succumb to the dangers
and damages of the outside environment. Senescence is the loss of cell functions and in
our story of Uncle Hans in Chapter 1, his dementia-related symptoms were a result of
the loss of proper transmission of nerve signals within his brain. Senescence studies are
continually looking at the damages to cells in attempting to reverse or slow the aging
process.
There are many types of cells within multicellular organisms. As stated earlier,
humans are constructed of about 200 different kinds of somatic or body cells. While
they are each of very distinct types, with different characteristics, what is so surprising
is their similarity. As discussed earlier, every cell has a control center to maintain the
operations of the cell on a daily basis: a membrane that separates itself from the outside
environment; and a semi-solid plasma, called cytoplasm, in which cellular activities
occur. Most animal cells are between 10 and 30 µm in width, but can be as large as
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Figure 3.20 Cell junctions are connections between two different cities.
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102 Unit 1: That’s Life
100 µm (the size of a single grain of sand), as seen in the human egg or up to 1,500 µm
as in frog’s eggs. Eggs tend to be very large because they support a growing organism.
The small size of cells is explained by the surface-to-volume hypothesis, which
states that the surface area (surface that is in contact with the outside environment)
decreases rapidly in proportion to the volume of the cell. Thus, if the volume of a cell is
too great, it cannot exchange enough of the materials (for example, oxygen) to support
its size. The surface area of a variety of cubes that the little boy is hammering is shown
in Figure 3.21. Note that the surface area of the single large cube is so much smaller than
that of the aggregate of the many smaller cubes.
Another reason for limits on a cell’s size and shape is its ability to control a large cell
from a nucleus. Just as a long distance relationship is difficult due to distance, a cell too
large has a more difficult time regulating activities of its structures from farther away.
There are some organisms that have developed ways to adapt to these problems. A slime
mold, shown in Figure 3.22 is, in fact, one giant, thin cell with thousands of nuclei strewn
throughout its cytoplasm. In this way, it avoids the constant-volume problem by remaining
thin for exchanges to occur and has control centers in enough areas to keep control effi-
cient. Human muscle cells also are multinucleated and very large to perform their motility
functions.
The Moving Crew: Rules and procedures are City law
The description of the cell’s organelles and their functions provides a glimpse at how
complex their structure is and how many functions they carry out. Physical laws govern
how they interact to transport materials. Let’s look now at a few of the basic laws of the
city. The different transport types will be discussed according to their requirement for
energy: 1) No energy needed: simple diffusion, facilitated diffusion, osmosis; 2) Energy
needed: active transport, bulk transport (phagocytosis, pinocytosis, receptor-mediated
endocytosis, and exocytosis).
Figure 3.21 Surface area of large cube vs. several smaller cubes of equal size. The
surface area of the many smaller cubes is much greater than that of a single cube with
a comparable volume.
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Chapter 3: The Cell As a City 103
Passive Transport
Passive transport involves the movement of materials without the use of cellular energy.
Recall that the cell is bathed in water. Most atoms and molecules therefore exist as
ions. Ions move constantly, jiggling, hitting one another, and forcing themselves farther
and farther apart to fill the available space. The motion of each ion is driven by kinetic
energy, or energy of motion. This kind of movement is called passive transport because
it occurs naturally and spontaneously and again, requires no cellular energy.
Substances move apart during passive transport until they spread evenly through
an area. This even level of dispersion is known as equilibrium. You have experienced
this action of molecules if you have ever been in a bus or subway car with someone
whose deodorant has failed on a hot day. The underarm odor is first detected by nearby
passengers, but the smell can travel until it reaches even those people at the other end
of the vehicle. Sweat odor molecules may travel in any direction at the same time.
However, there is a net movement of smell molecules from an area of higher concen-
tration to an area of lower concentration. The movement from a higher concentration
to a lower concentration is called diffusion. In living systems, substances spontaneously
diffuse from higher to lower areas of pressure, electrical charge, or concentration with-
out the use of cellular energy. Figure 3.23 shows the process of diffusion in this subway
situation.
The difference between higher and lower concentration areas is known as a gradient.
Many examples of diffusion are seen throughout this text. Oxygen, carbon dioxide, alco-
hol, and vitamins move through a cell via diffusion. The kidneys work by regulating
body levels of ions using diffusion principles. Simple cutting of an onion often makes
us cry – not because we care about an onion’s rights to live – but because sulfuric acid,
an irritant, diffuses from the cut onion toward our eye membranes. Passive transport
processes within cells can describe movement for many of the chemicals discussed in
the previous chapter.
The mitochondria in our opening story use gradients to generate energy for a cell to
use. Joules and Theta were fortunate to have more efficiently developed mitochondria
to help their physiological functions. Movement of substances within the mitochondria
Passive transport
The movement of
substances across cell
membranes without
the need of energy
expenditure by the cell.
equilibrium
The even level
of dispersion of
substances.
concentration,
higher and lower
The presence of a
large amount of a
specific substance in a
solution or mixture is
higher concentration.
Any solution
containing fewer
dissolved particles is
lower concentration.
diffusion
The net movement of
molecules from higher
concentration to a
lower concentration.
gradient
The difference
between higher and
lower concentration
areas.
Figure 3.22 Slime molds. These organisms have multiple nuclei to control their large
sizes. During some phases of their life cycles, they appear as gelatinous slime, giving
them their infamous name.
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104 Unit 1: That’s Life
Figure 3.23 Diffusion in a subway. Odor moves from a higher to a lower concen-
tration in a subway car. Molecules bounce off of each other, moving farther and farther
apart in diffusion.
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High
concentration
will be discussed in greater detail in the next chapter, energetics. Next, we will look at a
special case of passive diffusion of water across membranes, osmosis.
Osmosis: A Special Case of Diffusion
Because water is critical for life, it must be able to move freely into and out of cells –
in other words, across membranes. Diffusion of water across a selectively permeable
membrane – one that allows the transfer of only some substances – is termed osmosis.
Osmosis occurs spontaneously, so it requires no cellular energy and is thus a special
type of passive transport. It occurs only when water moves across a membrane. Most
molecules within a cell cannot simply diffuse across a plasma membrane. If they could,
their contents would readily spill out and a separation from the outside world would not
be possible.
While the integrity of the cell is critical, it is also critical that materials other than
water move across the membrane. A unique property of water – its ability to dissolve
other substances – makes this possible. The cell’s cytoplasm is roughly 1% dissolved
materials, and the right amount of dissolved material in the cell is required for proper
cell functioning.
If a cell has a higher concentration of solute as compared with its surrounding envi-
ronment it is hypertonic; if it has less solute, it is hypotonic. Osmosis is the mechanism
by which the internal and external environments equalize. An example of what can hap-
pen when conditions are abnormal occurs when an athlete drinks a large amount of
water in a short period of time following exercise and suffers from hypernatremia, or ion
imbalance, which can be fatal (Figure 3.24).
The term hypertonic can be remembered because “hyper” refers to a large amount
of something, as in a hyperactive child who has too much energy. For example, if a cell
with a concentration of 0.9% NaCl were placed in a beaker with a concentration of .01%
NaCl, the cell would have the greater amount of solute. The cell, with more solute, has
less water, and water would flow from the fluid outside into the cell. When that happens,
the cell swells and could even lyse (break open) if too much water enters it.
Hypotonic solutions are those that have less solute dissolved than their external
environment. The term hypotonic can be remembered because “hypo” rhymes with H2O,
which implies that there is more plentiful water in a hypotonic situation. To illustrate,
Osmosis
The process
of diffusion of
water through a
semipermeable
membrane that allows
the transfer of only
some substances.
hypertonic
A cell having a higher
concentration of
solute as compared
with its surrounding
environment.
hypotonic
A cell having a lower
concentration of
solute as compared
with its surrounding
environment.
lyse
Breakdown of cell
membrane.
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Chapter 3: The Cell As a City 105
when a cell with a concentration of 0.9% NaCl is placed in a beaker with a concentration
of 10% NaCl, the cell has a lower amount of solute than the solution outside. The cell
in this case is hypotonic, but the outside is hypertonic. Thus, water will flow out of the
hypotonic cell along its concentration gradient. A good way to remember where water
flows in cells is by the golden rule: “water follows solute.”
Of course, no net movement of substances occurs if a cell has the same concentra-
tion of solute as its environment. It is then said to be isotonic (iso = the same as) to its
outside. There is an even concentration of solute and water on either side of the plasma
membrane. A cell might have a 0.9% concentration of NaCl both inside and outside of
the cell. In this case, solute and water are both traveling in and out of the cell, but their
net or overall movement is the same toward either side.
Special Cases in Osmosis
Gradients drive the movement of solutes and water in each of the examples above.
Single-celled organisms, such as the Paramecium described earlier in this chapter, live
in hypotonic environments. Their surroundings contain a high concentration of water.
Water is therefore always attempting to enter the Paramecium because it contains more
dissolved solutes. This requires a contractile vacuole to act as a constant pump, moving
the excess water out of the cell. This would make sense, because Paramecium is a fresh-
water protist, which needs dissolved solutes within its cell to survive.
Plants also use gradients to maintain their structure and stand upright. My grand-
mother often served older lettuce, as she was quite frugal. It wilted in the refrigerator,
but after she placed the lettuce in a bowl of water, it perked up and looked as good as
isotonic
Even concentration
of solute and water
on either side of the
plasma membrane.
Figure 3.24 Red blood cells in hypotonic, hypertonic, and isotonic solutions. Red blood cells shrivel
up in a hypertonic environment and blow up in a hypotonic environment. They are normal in an isotonic
situation. Note the changes in a red blood cell’s shape in each of the conditions.
Isotonic solutions Hypertonic solutions Hypotonic solutions
(b)(a) (c)
Cells retain their normal size and shape
in isotonic solutions (same solute/water
concentration as inside cells; water
moves in and out).
Cells lose water by osmosis and shrink
in a hypertonic solution (contains a
higher concentration of solutes than are
present inside the cells).
Cells take on water by osmosis until they
become bloated and burst (lyse) in a
hypotonic solution (contains a lower
concentration of solutes than are present
in cells).
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106 Unit 1: That’s Life
new. Lettuce cells were hypertonic to the outside, which was hypotonic to normal tap
water. Water thus rushed into the cells of the lettuce leaves along their concentration gra-
dient. Water entered water vacuoles within the plant cell, creating what is called turgor
pressure, pressure exerted against the walls of a plant cell. Turgor is any pressure on a cell
wall that gives the wall rigidity. My grandmother used the principles of osmosis to save
money. When a plant cell is placed in a hypertonic environment, what do you predict will
happen? It will lose water to its outside, causing it to shrink. Water’s movement across a
membrane not allowing solutes, as in our lettuce example, is shown in Figure 3.25.
Passive Transport with a Helper
Some substances require the help of carrier proteins to move across the plasma
membrane. This final type of passive transport is called facilitated diffusion. Substances
move along their concentration gradient through the channels of carrier proteins, which
facilitate their movement. Sugars such as glucose, and amino acids, as components of
proteins, both move into and out of cells through facilitated diffusion. Because they
have charges on them they cannot cross the membrane on their own. Polar channels in
carrier proteins enable these ions to be carried across the plasma membrane. Facilitated
diffusion requires no cellular energy and it occurs spontaneously, moving from a higher
to lower concentration.
turgor pressure
The pressure exerted
against the walls of a
plant cell when water
enters the water
vacuoles of plant cell.
Vacuole: Single
membrane structures
that hold materials in
a cell.
Facilitated diffusion
A type of passive
transport requiring a
carrier protein.
Figure 3.25 Water moves across the membrane between two sides of the beaker
but does not permit the movement of dissolved substances. This leads to a column of
water rising as water follows solute into the right side of the beaker. Lettuce, in our
example, has increased turgor pressure as water rushes into its cells.
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a
n
y
High concentration
of H2O molecules
Low concentration
of solute (NaCl)
molecules
Higher concentration of
solute (NaCl) molecules
results in fewer H2O
molecules on right side
of membrane
Membrane is permeable to
water, but not to solutes
H2O molecules move
through membrane to
create equilibrium of
solute concentrations,
resulting in higher
volume on right side
(a)
(b)
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Chapter 3: The Cell As a City 107
Our story discussed mitochondria, which have much movement of ions within its
inner membranes to make ATP energy. Next we will look at how ATP energy generated
by mitochondria drives another form of movement: active transport.
Active Transport
Some forms of movement occur with the addition of energy to drive its actions. Active
transport is the movement of substances against a concentration gradient, from a lower
concentration to a higher concentration, which requires cellular energy. Active transport
does not occur spontaneously; it requires ATP energy to overcome the nonspontaneous
movement of materials against a concentration gradient. Consider our subway example:
Would it not be strange if all of the smell molecules aggregated toward the armpit of
an unsuspecting person on the subway, making him or her smell terribly? Could smell
molecules throughout the subway car attack this person? It cannot happen unless there is
an active transport mechanism at work.
In order for cells to carry out some processes, they must use active transport. For
example, the sodium–potassium pump is an integral protein that uses ATP energy to
move sodium ions (Na+) out of the cell and bring potassium ions (K+) into the cell. Keep-
ing the right concentrations of Na+ and K+ is vital in nerve and muscle activity.
A concentration gradient forms as a result of three Na+ pumped out of the cell for
every two K+ pumped into the cell. This specific movement results in an electrical and
concentration gradient across the membrane. More sodium winds up outside of the cell
and more potassium inside. The environment outside the cell becomes more positive
than inside it because there are more positive ions being pumped out. Potential or stored
energy develops and drives many cellular activities much as a dam on a river stores
energy that is converted to electrical power for our use. Figure 3.26 shows the workings
of the sodium–potassium pump.
Secondary active transport is the movement of substances using stored energy. The
sodium–potassium pump creates a store of electrical and potential energy for later use
by cell processes. Instant energy for moving one’s arm or riding a bicycle is obtained
through secondary active transport by using stored ATP. Let’s give a look in the next
section to the active movement of bulk materials in and out of the cell.
Bulk Transport: A Bigger Moving Van
At times, cells as a city require transport of larger materials – sometimes, whole organ-
isms – to carry out their life functions. Bulk transport is the movement of large amounts
of material across the plasma membrane. It is a form of active transport obtaining energy
from ATP. Movement of single ions or molecules is often not enough for the many cell
reactions occurring at any one time. Cell activities often require removal or inputs of
large resources.
The moving van of the cell is the vesicle, containing the bulk materials. Bulk trans-
port can occur by moving materials into the cell (called endocytosis) or by moving mate-
rial out of a cell (exocytosis). (Endo- sounds like “in” and exo- sounds like “exit,” which
should help you recall the terms.) There are three types of endocytosis: phagocytosis,
pinocytosis, and receptor-mediated endocytosis.
Phagocytosis is defined as the movement of solid particles into a cell. It is
accomplished by a cell’s cytoplasm projecting around the material to be dissolved. This
action appears much like a blob, engulfing its prey. The substance may be a whole organ-
ism or a particle of organic matter. Immune cells, particularly macrophages, a special
kind of white blood cell, attack foreign invaders by phagocytosis. Once the material is
Active transport
Is the movement of
substances against
a concentration
gradient, from a lower
concentration to a
higher concentration,
which requires cellular
energy.
Sodium– potassium
pump
An integral protein
that uses ATP energy
to move sodium ions
out of the cell and
bring potassium ions
into the cell.
Secondary active
transport
The movement of
substances using
stored energy.
Bulk transport
Is the movement
of large amounts of
material across the
plasma membrane.
endocytosis
The process of moving
materials into the cell.
exocytosis
The process of moving
materials outside the
cell.
Phagocytosis
The movement of
solid particles into a
cell.
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108 Unit 1: That’s Life
fully engulfed through this process, lysosomes fuse with the newly formed vesicle con-
taining the material. Lysosome’s hydrolytic enzymes digest the particle for use by a cell
or release it as a waste product.
Cells also move large amounts of liquid, along with their dissolved solutes, into their
cytoplasm through pinocytosis. Cell drinking or pinocytosis requires a furrow, or invagi-
nation, in the plasma membrane of a cell to input liquids. Many animal cells obtain their
nutrients through pinocytosis, with large human egg cells developing multiple furrows
to obtain nutrients from the surrounding cells.
A form of endocytosis that requires a specific binding of a receptor protein to cell
membrane is known as receptor-mediated endocytosis. In this case, bulk materials
move into a cell by docking with a specifically shaped receptor. The receptor is an inte-
gral protein jutting out of a plasma membrane. It has a specific shape onto which a
substance matching its shape may bind. A signal stimulated by the joining of receptor to
substance starts to bring the plasma membrane inward. The membrane forms a vesicle
filled with both the substance bound to the receptor and the materials dissolved, as the
membrane moves inward. Hormones, nutrients, and neurotransmitters, such as acetyl-
choline (described in Chapter 1), move in and out of a cell through this process.
Because receptors are proteins, they are genetically determined. For example, in
cases of hyperlipidemia, high amounts of cholesterol build up in the blood. Recep-
tors that dock with vesicles carrying cholesterol are either malformed or available in
Pinocytosis
The mechanism by
which cells ingest
extracellular fluid and
its contents.
invagination
The process of being
folded back on itself
to form a pouch (not
given in bold in text).
receptor- mediated
endocytosis
A form of endocytosis
that requires a specific
binding of a receptor
protein to cell
membrane.
Figure 3.26 The workings of the sodium–potassium pump. With each turn, the pump moves three sodium
ions out of its cell and two potassium ions into its cell. A gradient is set up with more sodium ions outside
than inside a cell. The inside of the cell becomes relatively more negative than the outside because more
positive charges are pumped to the outside of the cell.
©
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ig
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/S
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1 2 3
4 5 6
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Chapter 3: The Cell As a City 109
insufficient amounts in hyperlipidemia. Lipids cannot be removed from the blood as
easily, reducing rates of receptor-mediated endocytosis and leading to buildup of fats in
the blood. It is a dangerous disease due to its close link with heart attacks and stroke.
Many other diseases are associated with improperly formed receptors, including diabe-
tes, depression, and susceptibility to viruses.
All bulk movement out of a cell is accomplished by exocytosis. Any material to be
exported – wastes, hormones, mucous or enzymes – is removed from a cell by exocy-
tosis. Exocytosis removes either waste products or materials for export from the cell.
Vesicles containing these materials fuse with the plasma membrane and are expelled.
Figure 3.27 shows the types of bulk transport.
All of the forms of bulk transport are used by mitochondria described in our opening
story. For a mitochondrion to function, much like any other organelle, it communicates
with the nucleus, cell membrane, and other cell parts to perform its role in helping the cell
to function. Joules and Theta probably did not realize the complexity of cell movement
within their mitochondria that contribute to its survival. Perhaps if they had, they would
have accepted their friend Sally. Knowing that mitochondrial DNA is not the only contrib-
utor to an individual cell’s survival would have opened their eyes to the many unknowns
in making each of us unique. Cell biology might have made a difference in the outcome
of our story and in the friendship of Joules, Theta, and Sally.
Figure 3.27 Endocytosis and exocytosis.
©
K
e
n
d
a
ll
H
u
n
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u
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lis
h
in
g
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y
it’S All ABOut eve: everyOne hAS the SAMe MOther in
Our hiStOry.
Our story opens with the importance of ancestral mitochondrial DNA in
social organization. However, evidence shows that all humans have the same
maternal mitochondria, derived from the same ancestral mother.
A group of genetic researchers published a study in the journal Nature
revealing a maternal ancestor to all living humans called “mitochondrial Eve.”
Researchers concluded that every human on Earth now could trace lineage
back to this single common female ancestor who lived around 200,000 years
ago.
Mitochondrial DNA was taken from 147 people across continents and
ethnic groups. It was determined that people alive today have lineage on one of
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110 Unit 1: That’s Life
two branches in the human family tree. One branch consists of African lineage
only and the other of all other groups including African. The scientists con-
cluded that Africa is the place where this woman lived. Note that this maternal
ancestor was not the first woman, but her descendants survive to present
day while other women of her time had female descendants with no children,
halting the passing on mitochondrial DNA.
Thus, Joules, Theta, and Sally were of one lineage, bound by an ancestor
long ago. While society broke them apart, their biology had more in common
than Joules would know or care to admit. Their commonalities should have
held them together, with the humanness of a shared heritage. Perhaps their
ancestor is one of us now . . .
summary
Chemistry is the language of structure and movement within living systems. Physi-
cal and chemical principles determine the behavior of cell structures. Developments in
microscopy led to the discovery of the detailed workings of cells. While most organ-
elles evolved through invaginations of membranes, mitochondria and eukaryotes devel-
oped by becoming absorbed into larger cells. Together, organelle interactions with one
another and the outside environment comprise a living organism. The fluid mosaic
model shows how cells transport materials across their many membranes. Transport
systems accomplish varied goals from moving small particles directly through cells
to moving materials in bulk. All of the needs of cells are accomplished by organelles
communicating through use of the different transport systems. Our story showed that
cell parts not only dictate only life processes, but influence human health and the social
structure of our society.
ChECk oUT
summary: key points
• Cell biology affects our lives in many ways, from diseases such as lysosome storage disorders to the
way society is structured based on human differences.
• Organelles have specific roles within cells, communicating with each other and the outside environ-
ment through membranes.
• The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotic cells
based on chemical and biological evidence.
• The development of microscopy led to the discovery of cells and cell parts.
• The plasma membrane has a unique structure that enables transport across a cell.
• Cells of the five kingdoms diverge based on just a few key differences.
• Passive and active transport mechanisms are used to move materials within a cell.
• Different chemicals use transport mechanisms differently to move across a cell.
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Chapter 3: The Cell As a City 111
active transport
aerobic
bulk transport
cell respiration
centriole
chloroplast
chromatin
chromosome
chromoplast
cilia
compartmentalization
compound light microscope
concentration, higher and lower
cristae
cytoplasm
desmosome
diffraction
diffusion
endocytosis
endoplasmic reticulum (ER)
endosymbionts
endosymbiotic theory
equilibrium
exocytosis
facilitated diffusion
flagella
fluid mosaic model
gap (communicating) junction
golgi apparatus
gradient
hypertonic
hypotonic
integral protein (transmembrane or
carrier protein)
intermediate fiber
intracellular digestion
intracellular transport
invagination
isotonic
leucoplast
lyse
lysosome
lysosome storage disease
magnification
microfilament
micrometer
microtubule
mitochondria
nanometer
nuclear envelop
nucleoli
nucleus
organelle (subcellular structure)
osmosis
oxygen revolution
passive transport
peripheral protein
phagocytosis
photosynthesis
pinocytosis
plasma (cell) membrane
plasmolysis
plastid
receptor
receptor-mediated endocytosis
recognition protein
resolution
ribosome
scanning electron microscope (SEM)
secondary active transport
selectively permeable
sodium–potassium pump
stoma
thylakoid membrane
tight junction
transmission electron microscope (TEM)
turgor pressure
vacuole
key terMS
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Chapter 3: The Cell As a City 113
Multiple Choice Questions
1. How might a chloroplast disease, preventing its key processes, affect human society?
a. There would be no more food.
b. There would be no more light.
c. There would be no more water.
d. There would be no more carbon dioxide.
2. If a plant cell absorbs water and becomes stiff, it becomes __________ through the
process of __________.
a. crenated; passive transport
b. turgid; osmosis
c. rigid; active transport
d. reactive; facilitated diffusion
3. Which sentence BEST describes a ribosome?
a. It is a large organelle that makes protein.
b. It is a small organelle that makes protein.
c. It is a large organelle that breaks down protein.
d. It is a small organelle that breaks down protein.
4. Which type of microscope can best view the cristae of a mitochondrion?
a. compound light microscope
b. phase contrast microscope
c. scanning electron microscope
d. transmission electron microscope
5. Which term best describes the plasma membrane?
a. static
b. dynamic
c. organized
d. rigid
6. Which term best differentiates between active transport and passive transport?
a. polarity
b. facilitation
c. concentration
d. energy
7. Which is NOT a passive transport mechanism across a plant cell?
a. turgor pressure
b. diffusion
c. osmosis
d. receptor-mediated endocytosis
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114 Unit 1: That’s Life
8. When a cell builds a new Golgi apparatus, new materials need to be brought into the
cell, sometimes against a concentration gradient. Which process best accomplishes
the task of building a new Golgi apparatus?
a. diffusion
b. osmosis
c. exocytosis
d. active transport
9. In question #8 above, which chemical drives the process for the correct answer?
a. sodium
b. water
c. ATP
d. adenine
10. An experiment places a skin cell within a petri dish containing pure, distilled water.
Predict what will happen to the skin cell within minutes:
a. The skin cell will shrink as water leaves the cell.
b. The skin cell will lyse as water leaves the cell.
c. The skin cell will shrink as water enters the cell.
d. The skin cell will lyse as water enters the cell.
short Answers
1. Describe how senescence is linked to lysosomes. Explain how lysosomes are both
linked to human diseases and to our proper functioning. Give an example of each.
2. Define the following terms: passive transport, active transport, and receptor-medi-
ated endocytosis. List one way each of the terms differs from the others.
3. Compare the two types of electron microscopes, explaining how their images are
similar and how they are different. Use a drawing to make the distinctions clear.
Show your art work. Which was more important in discovering subcellular structure?
4. There are several ways chemicals move within cells. At times they use passive trans-
port and at times they use active transport. List the types of passive transport and
explain how each works. Be sure to include the following terms in your explanation:
gradient, concentration, and ATP energy?
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Chapter 3: The Cell As a City 115
5. For question #4 above, list and draw an example of an active transport mechanism
within a cell. Include in the picture the electron arrangement around the atoms.
6. Name two differences and two similarities between cells found in the matched
kingdoms:
a. fungi–protist
b. animal–plant
c. plant–protist
d. bacteria–animal
7. ATP plays a major role in life’s processes. How does ATP energy function in sec-
ondary active transport processes? Explain how the sodium–potassium pump plays
a role in secondary active transport.
8. Draw the fluid mosaic model of the cell membrane. Describe how and why a charged
molecule such as sugar that needed by every cell enters through the fluid mosaic.
9. Explain the differences between passive and active transport. Explain the difference
using the following terms: ATP, spontaneous, gradient, concentration, ions.
10. A battle between an amoeba and a single-celled protist, chilomonas ensues. The
amoeba wins, but the large piece of chilomonas is too big to pass through the plasma
membrane of the amoeba to eat. Describe the process by which the amoeba absorbs
and digests the chilomonas.
Biology and society Corner: Discussion Questions
1. Many scientists argue that our most pressing resource crisis is not food shortage or
climate change, but water scarcity. How is water important for a cell? Predict how
the role of a limited water supply will affect society. Explain how the water crisis is
already impacting policy changes, the economy and overall human impacts on the
environment.
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116 Unit 1: That’s Life
2. How would van Leeuwenhoek be surprised by discoveries since his viewing of
cells? What role do you think microscopy will play in future scientific develop-
ments? How did microscopy affect society in the 20th century? Use either the scan-
ning electron microscope or transmission electron microscope to frame your thesis.
3. If a person is stranded on a desert island, without freshwater, what are the dangers
of drinking sea water. Explain the transport mechanism at work, which leads to
problems for a human cell in contact with seawater.
4. Mitochondrial diseases are attributed to human health problems, from sweating and
epilepsy to diabetes and heart disease. Do you support biotechnology research to
help people with these diseases? Do you support its use in potentially making “bet-
ter” mitochondria to help people live better lives, as seen in our story. What restric-
tions would you place, if any, on such developments?
5. Plasma membranes are delicate and can weaken in some cases causing human
health disorders. Research the effects of plasma membrane problems on the health
of humans and animals. Choose one of the organelles discussed in this chapter.
Describe what happens to the organelle and human or animal health as a result of
the delicate nature of plasma membranes.
Figure – Concept Map of Chapter 3 Big Ideas
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