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7– 2 Eukaryotic Cell Structure
Section 7–2
1 FOCUS
A
t first glance, a factory is a puzzling place. A bewildering
variety of machines buzz and clatter, people move quickly
in different directions, and the sheer diversity of so much
activity can be confusing. However, if you take your time and
watch carefully, before long you will begin to identify patterns.
What might at first have seemed like chaos begins to make
sense.
Objectives
7.2.1 Describe the function of the
cell nucleus.
7.2.2 Describe the functions of the
major cell organelles.
7.2.3 Identify the main roles of the
cytoskeleton.
Key Concept
• What are the functions of the
major cell structures?
Vocabulary
organelle
cytoplasm
nuclear envelope
chromatin
chromosome
nucleolus
ribosome
endoplasmic reticulum
Golgi apparatus
lysosome
vacuole
mitochondrion
chloroplast
cytoskeleton
centriole
Vocabulary Preview
Pronounce each Vocabulary word
and have students repeat the pronunciation as a class. Pay special
attention to words that are difficult
for English language learners.
Reading Strategy
Comparing the Cell to a Factory
In some respects, the eukaryotic cell is like a factory. The first
time you look at a microscope image of a cell, such as the one in
Figure 7–5, the cell seems impossibly complex. Look closely at a
eukaryotic cell, however, and patterns begin to emerge. To see
those patterns more clearly, we’ll look at some structures that
are common to eukaryotic cells, shown in Figure 7– 6. Because
many of these structures act as if they are specialized organs,
these structures are known as organelles, literally “little
organs.”
Cell biologists divide the eukaryotic cell into two major
parts: the nucleus and the cytoplasm. The cytoplasm is the
portion of the cell outside the nucleus. As you will see, the
nucleus and cytoplasm work together in the business of life.
Reading Strategy:
Building Vocabulary
To help students begin their understanding of the differences between
plant cells and animal cells, have
them preview Figure 7–6 and answer
the caption question.
Before you read, preview the
vocabulary by skimming the
section and making a list of the
highlighted boldface terms.
Leave space to make notes as
you read.
2 INSTRUCT
Comparing the Cell
to a Factory
Build Science Skills
Using Models Divide the class into
small groups, and have groups make
a labeled, two-dimensional drawing
of a typical cell. First, have groups
meet before reading the section to
discuss what the inside of a cell
might contain. Then, ask groups to
meet again after learning about the
structures of a cell to make the
labeled drawing.
왘
Figure 7– 5 This electron micrograph
of a plant cell shows many of the different
types of structures that are found in
eukaryotic cells. The cell has been artificially colored so that you can distinguish
one structure from another.
(magnification: 1500⫻)
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Technology:
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Section 7–2
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• Adapted Reading and Study Workbook B,
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• Lesson Plans, Section 7–2
174
Chapter 7
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PLANT
AND
ANIMAL CELLS
Figure 7–6 Both plant and animal cells contain a variety of organelles.
Some structures are specific to either plant cells or animal cells only.
Interpreting Graphics What structures do plant cells have that animal
cells do not?
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Plant Cell
Nucleus
Nucleolus
Nuclear envelope
Ribosome (free)
Smooth endoplasmic
reticulum
Rough endoplasmic
reticulum
Ribosome (attached)
Cell wall
Golgi apparatus
Cell membrane
Chloroplast
Mitochondrion
Vacuole
Smooth endoplasmic
reticulum
Nucleolus
Nuclear envelope
Build Science Skills
Predicting Ask students what specific functions a unicellular organism
would need to carry out in order to
live. Then, divide the class into small
groups, and ask each group to make
a table of predictions about what
structures would likely be found
inside a unicellular organism. The
table should have two columns:
Necessary Function and Structure
Needed to Carry Out Function.
Use Visuals
Animal Cell
Nucleus
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Students can interact with the
cell art online.
Ribosome (free)
Cell membrane
Figure 7– 6 Encourage students to
make copies of these labeled illustrations in their notebooks. As they
learn about the various structures
that make up a cell, they can add
definitions and descriptions of functions for each of the labels. Point out
that when they have completed this
task, they will have made the best
possible tool for review.
Build Science Skills
Rough endoplasmic
reticulum
Ribosome (attached)
Centrioles
Golgi apparatus
Comparing and Contrasting
Set up microscope stations at several
locations around the room, and provide prepared slides of an animal cell
and a plant cell at each location.
Have students make labeled drawings
of each and write a paragraph comparing and contrasting the two types
of cells.
Mitochondrion
Less Proficient Readers
To reinforce students’ understanding of cell structures, draw an animal cell on the board. Include
and label the nucleus, the cell membrane, and the
cytoplasm. Have students make a copy of the
drawing on a sheet of paper. Then, as each
organelle is studied and discussed, add labeled
structures to the cell on the board, and have students add these structures to their own drawings.
English Language Learners
When students read about the cytoskeleton on
page 181, explain that cyto- means “cell,” and
thus cytoskeleton can be thought of as the “skeleton of the cell.” Explain that this is an analogy,
since the cytoskeleton is not like an animal skeleton. Also, explain that a filament is a threadlike
material and a tubule is a “very slender tube.”
Thus, the cytoskeleton can be thought of as composed of threads and slender tubes.
Answer to . . .
Figure 7– 6 Plant cells have a cell
wall and chloroplasts. Many plant cells
also have a large, central vacuole.
Cell Structure and Function
175
7–2 (continued)
FIGURE 7–7 THE NUCLEUS
Nucleus
The nucleus controls most cell processes and contains the
hereditary information of DNA. The DNA combines with protein
to form chromatin, which is found throughout the nucleus. The
small, dense region in the nucleus is the nucleolus.
Use Visuals
Figure 7–7 Ask students: What is
the nucleolus? (It is a small, dense
region of the nucleus where the assembly of ribosomes begins.) Where is the
DNA that a nucleus contains? (The
DNA is part of the chromatin, which is
spread throughout the nucleus most of
the time.) Why is DNA important?
(It holds coded instructions for making
proteins and other important molecules.) Point out that the genetic
information is the coded instructions
for making molecules.
Build Science Skills
Nucleolus
Nuclear
envelope
Chromatin
Inferring Remind students that
prokaryotes do not contain a nucleus.
Then, ask: If the nucleus controls
most cell processes in eukaryotes,
how can prokaryotes live without a
nucleus? (Some students might suggest that the lives of prokaryotes aren’t
as complex as those of eukaryotes.
Others might correctly infer that the
most important part of a nucleus is the
DNA it contains, and prokaryotes have
DNA without having a nucleus.)
Nuclear
pores
Nucleus
In the same way that the main office controls a large factory, the
nucleus is the control center of the cell.
The nucleus
contains nearly all the cell’s DNA and with it the coded
instructions for making proteins and other important
molecules. The structure of the nucleus is shown in Figure 7–7.
The nucleus is surrounded by a nuclear envelope
composed of two membranes. The nuclear envelope is dotted
with thousands of nuclear pores, which allow material to move
into and out of the nucleus. Like messages, instructions, and
blueprints moving in and out of a main office, a steady stream of
proteins, RNA, and other molecules move through the nuclear
pores to and from the rest of the cell.
The granular material you can see in the nucleus is called
chromatin. Chromatin consists of DNA bound to protein.
Most of the time, chromatin is spread throughout the nucleus.
When a cell divides, however, chromatin condenses to form
chromosomes (KROH-muh-sohms). These distinct, threadlike
structures contain the genetic information that is passed from
one generation of cells to the next. You will learn more about
chromosomes in later chapters.
Most nuclei also contain a small, dense region known as the
nucleolus (noo-KLEE-uh-lus). The nucleolus is where the
assembly of ribosomes begins.
What kind of information is contained in chromosomes?
HISTORY OF SCIENCE
The nucleus controls the cell
During the 1930s and 1940s, researchers performed a series of experiments that demonstrated the link between a cell’s nucleus and the
physical characteristics of the cell. Two species
of Acetabularia algae were used in the experiments. This marine alga, though 5 cm long,
consists of a single cell. Each cell includes a
holdfast at the bottom, a stalk, and a cuplike
cap at the top, and the cell’s nucleus is in the
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Chapter 7
holdfast. The two species that were used had
different-shaped caps. Researchers cut the cap
off one cell, removed the nucleus from its holdfast, and transplanted a nucleus from a cell of
the second species into the holdfast of the first
cell. The cell regenerated a new cap, and
researchers cut off that one. Eventually, the cap
that grew was the shape of the cap from the
second species, from which the transplanted
nucleus came, and not the shape of the first cap.
Ribosomes
FIGURE 7–8 ENDOPLASMIC RETICULUM
Build Science Skills
The endoplasmic reticulum synthesizes proteins for
export from the cell. The rough endoplasmic reticulum,
shown here, gets its name from the “rough” appearance of the
ribosomes on its surface.
Ribosomes
(magnification: 160,000⫻)
Ribosomes
Endoplasmic reticulum
(magnification: about 40,000⫻)
Using Analogies Read aloud the
sentence in the text that compares a
ribosome to a machine. Use this
comparison to discuss how a eukaryotic cell is like a factory. Then,
encourage students who need an
extra challenge to work together in
writing a short play based on the
analogy of the cell as a factory.
Explain that a good play needs some
conflict or danger. The “factory”
might be under economic threat or
some environmental threat. Advise
students to include the functions of
as many parts of the factory—cell
organelles—as possible. Once the
play has been written, encourage the
“playwrights” to recruit class members to act out the drama.
Endoplasmic
Reticulum
Ribosomes
One of the most important jobs carried out in the cellular
“factory” is making proteins.
Proteins are assembled on
ribosomes. Ribosomes are small particles of RNA and protein found throughout the cytoplasm. They produce proteins by
following coded instructions that come from the nucleus. Each
ribosome, in its own way, is like a small machine in a factory,
turning out proteins on orders that come from its “boss”—the
cell nucleus. Cells that are active in protein synthesis are often
packed with ribosomes.
Endoplasmic Reticulum
Eukaryotic cells also contain an internal membrane system
known as the endoplasmic reticulum (en-doh-PLAZ-mik
rih-TIK-yuh-lum), or ER.
The endoplasmic reticulum is
the site where lipid components of the cell membrane
are assembled, along with proteins and other materials
that are exported from the cell.
The portion of the ER involved in the synthesis of proteins is
called rough endoplasmic reticulum, or rough ER. It is given
this name because of the ribosomes found on its surface. Newly
made proteins leave these ribosomes and are inserted into the
rough ER, where they may be chemically modified.
Use Visuals
Figure 7–8 Ask students: What are
ribosomes composed of? (RNA and
protein) Where are ribosomes produced? (In the nucleolus) What do
ribosomes produce? (Proteins) What
happens to these proteins after
they’re produced by ribosomes?
(Membrane proteins are inserted
directly into the ER membrane. Many
of the proteins produced on the rough
ER are released or secreted from the
cell.) If this were an illustration of
smooth ER, how would it be different? (The ER would not have
ribosomes on its surface.) What is the
function of smooth ER? (The smooth
ER contains enzymes that help synthesize lipids, such as steroids. Smooth ER
also helps to detoxify and process
chemicals.)
HISTORY OF SCIENCE
Learning from sea urchin nuclei
The German cytologist Theodor Boveri
(1862–1915) performed an experiment before
the invention of microdissection that demonstrated the importance of the nucleus. By vigorous
shaking, Boveri removed the nuclei from the eggs
of sea urchins of the genus Sphaerechinus. He then
fertilized the eggs (which had no nuclei) with
sperm from sea urchins of the genus Echinus. In a
practical sense, fertilization resulted in the substitution of one nucleus for another. The larvae that
developed had only the traits of Echinus, even
though the sperm contributed little more than a
tiny bit of nucleus to the developing organism.
Answer to . . .
Chromosomes contain
the genetic information that is passed
from one generation to the next.
Cell Structure and Function
177
FIGURE 7–9 GOLGI APPARATUS
7–2 (continued)
Golgi Apparatus
The Golgi apparatus modifies, sorts, and packages proteins.
Notice the stacklike membranes that make up the Golgi apparatus
in this transmission electron micrograph.
Build Science Skills
Comparing and Contrasting
Students often confuse the Golgi
apparatus with the endoplasmic
reticulum, because both are usually
represented as folded membranes
within the cytoplasm. Have students
compare the illustrations in Figure
7–8 with those in Figure 7–9. Then,
call on students at random to explain
the differences in functions between
ER and the Golgi apparatus.
Lysosomes
(magnification: about 45,700⫻)
Build Science Skills
Observing Divide the class into
small groups, and give each group
access to a paramecium culture and
a yeast suspension, as well as to a
microscope slide, coverslip, toothpick, dropper pipette, and
microscope. (Prepare the yeast suspension by adding a pinch of Congo
red indicator to a thick mixture of
yeast and water. Then, bring it to a
gentle boil for 5 minutes. Cool before
using. Transfer some paramecium
culture from the stock culture at least
a day ahead of time, and then limit
the food supply to the transferred
culture.) Have each group prepare a
slide of live paramecia using the
dropper pipette. Students should
focus the slide under the low-power
objective of the microscope. They
should then obtain a small sample
of the yeast solution. The indicator
in the solution is red above pH 5 and
blue below pH 3. The next step is
to use a toothpick to transfer a small
drop of yeast suspension to the
edge of the slide and observe the
paramecia under the microscope for
5 minutes. (Students should observe
that the paramecia sweep the yeast
through their oral grooves and form
vacuoles to enclose it. The vacuoles
become blue at first and eventually red,
as lysosomes fuse with the vacuole and
release acids that digest the yeast.)
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Chapter 7
Proteins that are released, or exported, from the cell are
synthesized on the rough ER, as are many membrane proteins.
Rough ER is abundant in cells that produce large amounts of
protein for export. Other cellular proteins are made on “free”
ribosomes, which are not attached to membranes.
The other portion of the ER is known as smooth endoplasmic reticulum (smooth ER) because ribosomes are not found on
its surface. In many cells, the smooth ER contains collections of
enzymes that perform specialized tasks, including the synthesis
of membrane lipids and the detoxification of drugs. Liver cells,
which play a key role in detoxifying drugs, often contain large
amounts of smooth ER.
Golgi Apparatus
Proteins produced in the rough ER move next into an organelle
called the Golgi apparatus, discovered by the Italian scientist
Camillo Golgi. As you can see in Figure 7–9, Golgi appears as a
stack of closely apposed membranes.
The function of the
Golgi apparatus is to modify, sort, and package proteins
and other materials from the endoplasmic reticulum for
storage in the cell or secretion outside the cell. The Golgi
apparatus is somewhat like a customization shop, where the
finishing touches are put on proteins before they are ready to
leave the “factory.” From the Golgi apparatus, proteins are then
“shipped” to their final destinations throughout the cell or
outside of the cell.
FACTS AND FIGURES
Important products of the Golgi apparatus
One of the most important cell components
packaged and distributed by the Golgi apparatus
is material for the membranes of the cell and its
organelles. Lysosomes, which are essentially
membranous bags filled with enzymes, are
products of the Golgi apparatuses. These
enzymes would destroy the cell if they were
not surrounded by membrane. An example of
how lysosomes function in cells can be seen in
the way paramecia digest their food. Upon
contact with a food organism or some other
particle, the paramecium envelops the food in a
vacuole. Lysosomes then fuse with the vacuole
and release acids. The acids quickly digest the
contents of the vacuole.
Lysosomes
Even the neatest, cleanest factory needs a cleanup crew, and
that’s what lysosomes (LY-suh-sohmz) are. Lysosomes are small
organelles filled with enzymes. One function of lysosomes is the
digestion, or breakdown, of lipids, carbohydrates, and proteins
into small molecules that can be used by the rest of the cell.
Lysosomes are also involved in breaking down organelles
that have outlived their usefulness. Lysosomes perform the vital
function of removing “junk” that might otherwise accumulate
and clutter up the cell. A number of serious human diseases,
including Tay-Sachs disease, can be traced to lysosomes that fail
to function properly.
Figure 7–10 Vacuoles have a variety
of functions. In the Coleus plant cell
(top), the large blue structure is the
central vacuole that stores salts,
proteins, and carbohydrates. The
paramecium (bottom) contains
contractile vacuoles that fill with water
and then pump the water out of the
cell. Applying Concepts How do
vacuoles help support plant structures?
Vacuole
What is the role of lysosomes?
Vacuoles
Every factory needs a place to store things, and cells contain
places for storage as well. Some kinds of cells contain saclike
structures called vacuoles (VAK-yoo-ohlz) that store materials
such as water, salts, proteins, and carbohydrates. In many plant
cells there is a single, large central vacuole filled with liquid. The
pressure of the central vacuole in these cells makes it possible for
plants to support heavy structures such as leaves and flowers.
Vacuoles are also found in some single-celled organisms and
in some animals. The paramecium in Figure 7–10 contains a
vacuole called a contractile vacuole. By contracting rhythmically,
this specialized vacuole pumps excess water out of the cell. The
control of water content within the cell is just one example of an
important process known as homeostasis. Homeostasis is the
maintenance of a controlled internal environment.
Mitochondria and Chloroplasts
All living things require a source of energy. Factories are hooked
up to the local power company, but what about cells? Most cells get
energy in one of two ways—from food molecules or from the sun.
Vacuoles
Use Visuals
Figure 7–10 After students have
studied the figure and read the caption, explain that the Coleus cell is
from a multicellular, leafy plant,
whereas the paramecium is a microscopic unicellular organism that is
part of the kingdom Protista, which
students will learn about in Chapter
20. Then, ask: How is the function
of a vacuole in a plant cell different
from that in a unicellular organism? (A vacuole in a plant cell stores
materials such as water, salts, proteins,
and carbohydrates. It also helps support plant structures. A vacuole in a
unicellular organism is specialized to
pump water out of the cell.)
Mitochondria and
Chloroplasts
(magnification: about 3000⫻)
Contractile
vacuole
Mitochondria Nearly all eukaryotic cells, including plants,
contain mitochondria (myt-oh-KAHN-dree-uh; singular: mitochondrion).
Mitochondria are organelles that convert
the chemical energy stored in food into compounds that
are more convenient for the cell to use. Mitochondria are
enclosed by two membranes—an outer membrane and an inner
membrane. The inner membrane is folded up inside the organelle.
One of the most interesting aspects of mitochondria is the
way in which they are inherited. In humans, all or nearly all of
our mitochondria come from the cytoplasm of the ovum, or egg
cell. This means that when your relatives are discussing which
side of the family should take credit for your best characteristics, you can tell them that you got your mitchondria from Mom!
Build Science Skills
Using Analogies Explain to students that mitochondria have long
been called the “powerhouses” of
cells. Ask: What is a “powerhouse”?
(A powerhouse is another name for a
power plant, which produces electricity
for cities and regions.) How is a mitochondrion like a powerhouse?
Powerhouses convert one source of
energy to another more useful form.
For example, energy from coal, oil, or
gas is often converted to electricity, a
more useful form for homes and industry. A mitochondrion also converts
energy to a more useful form. It uses
energy from food to make high-energy
compounds that the cell can use in
growth, development, and movement.
TEACHER TO TEACHER
When I introduce the structure of the cell, I try
to analogize the cell with the students’ city.
Taking this analogy a step further, I organize a
cooperative learning activity in which I ask
teams of students to “create” an imaginary city
that correlates cell structures with city components. Teams should include most of the
organelles in their city. For example, they
might use a mitochondrion as the local power
plant or microtubules as major thoroughfares.
For this activity, each team will need a large
sheet of paper or poster board for drawing the
city, as well as colored pencils or similar materials. If possible, have teams display their work
and explain their “creations” to the class.
—Jorge E. Sanchez
Biology Teacher
Green Valley High School
Henderson, NV
Answers to . . .
Lysosomes break down
lipids, carbohydrates, and proteins.
They also break down organelles that
have outlived their usefulness in the cell.
Figure 7–10 The pressure exerted
by the liquid in the vacuole makes it
possible for plants to support heavy
structures.
Cell Structure and Function
179
7–2 (continued)
Objective Students will make
models of cell organelles and a large
class model of a cell.
Skill Focus Using models
Materials craft supplies, index
cards
Time 20 minutes
Advance Prep Collect a variety of
craft supplies, including scissors,
construction paper, cardboard
tubes, plastic bags, yarn, glue, and
beads.
Safety Caution students about the
use of pins and about standing on
chairs as they hang up their models.
Supervise them as they do so.
Strategies
• You may want students to build a
model of a different kind of cell. A
model of a plant cell is suggested
because plant cells have a great
variety of structures and organelles.
• Make sure at least one group is
working on these major structures:
cell wall, cell membrane, nucleus,
microtubules, microfilaments, ribosomes, smooth ER, rough ER, Golgi
apparatus, lysosomes, vacuoles,
mitochondria, and chloroplasts.
Expected Outcomes Students
will make models of plant-cell structures and arrange them to form a
complete cell.
Analyze and Conclude
1. Students’ answers should reflect
an understanding of the functions of
plant cell organelles.
2. Scales will vary depending on the
size of the cell model. A typical scale,
assuming that the classroom is 5 m
across, would be 5/0.00005
(50 micrometers 0.00005 meters),
or 100,000 : 1.
3. The model should be similar in
shape and structure to a real cell
part. The model is different in that it
is much larger, is made of different
materials, and does not function.
4. Students should explain how
their model would be an improvement on their previous model.
180
Chapter 7
How can you make a model
of a cell?
Materials variety of craft supplies, index cards
Procedure
1. Your class is going to make a model of a plant cell
using the whole classroom. Work with a partner or
in a small group to decide what cell part or
organelle you would like to model. (Use Figure
7–6 as a starting point. It will give you an idea of
the relative sizes of various cell parts and their
possible positions. Figures 7–7 through 7–10
can provide additional information.)
2. Using materials of your choice, make a threedimensional model of the cell part or organelle you
chose. Make the model as complete and as accurate as you can.
3. Label an index card with the name of your cell part
or organelle and list its main features and functions. Attach the card to your model.
4. Attach your model to an appropriate place in the
room. If possible, attach your model to another
related cell part or organelle.
Analyze and Conclude
1. Inferring What are the functions of the different
organelles in plant cells?
2. Calculating Assume that a typical plant cell is 50
micrometers wide. Calculate the scale of your
classroom cell model. (Hint: Divide the width of the
classroom by the width of a cell, making sure to
use the same units.)
3. Comparing and Contrasting How is your
model cell part or organelle similar to the real cell
part or organelle? How is it different?
4. Evaluating Based on your work with this model,
describe how you could make a better model.
Specify what new information the improved model
would demonstrate.
Chloroplasts Plants and some other organisms contain
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chloroplasts.
Chloroplasts are organelles that
capture the energy from sunlight and convert it into
chemical energy in a process called photosynthesis.
Chloroplasts are the biological equivalents of solar power
plants. Like mitochondria, chloroplasts are surrounded by two
membranes. Inside the organelle are large stacks of other
membranes, which contain the green pigment chlorophyll.
Organelle DNA Unlike other organelles that contain no
DNA, chloroplasts and mitochondria contain their own genetic
information in the form of small DNA molecules. Lynn
Margulis, an American biologist, has suggested that mitochondria and chloroplasts are actually the descendants of ancient
prokaryotes. Margulis suggests that the prokaryotic ancestors of
these organelles evolved a symbiotic relationship with early
eukaryotes, taking up residence within the eukaryotic cell. One
group of prokaryotes had the ability to use oxygen to generate
ATP. These prokaryotes evolved into mitochondria. Other
prokaryotes that carried out photosynthesis evolved into chloroplasts. This idea is called the endosymbiotic theory.
FACTS AND FIGURES
The origin of eukaryotes?
The idea that chloroplasts and mitochondria
originated in symbiotic relationships with
prokaryotic cells is called the endosymbiont
hypothesis. According to this hypothesis, the
ancestors of eukaryotic cells were smaller
species of prokaryotes living within larger
species of prokaryotes. Chloroplasts possibly
originated when cyanobacteria became
established in larger prokaryotes either as
parasites or as prey that were not digested.
Mitochondria were once possibly anaerobic
heterotrophs that found “safe harbor” inside
larger prokaryotes as the world became
increasingly aerobic. As the host and symbionts
over time became more and more interdependent, the organisms merged to become a single
organism.
Cytoskeleton
A supporting structure and a transportation system
complete our picture of the cell as a factory. As you
know, a factory building is supported by steel or
cement beams and by columns that support its walls
and roof. Eukaryotic cells have a structure—the
cytoskeleton —that helps support the cell.
The
cytoskeleton is a network of protein filaments
that helps the cell to maintain its shape. The
cytoskeleton is also involved in movement.
Microfilaments and microtubules are two of the principal protein filaments that make up the cytoskeleton.
Microfilaments are threadlike structures made of
a protein called actin. They form extensive networks
in some cells and produce a tough, flexible framework
that supports the cell. Microfilaments also help cells
move. Microfilament assembly and disassembly is
responsible for the cytoplasmic movements that allow
cells, such as amoebas, to crawl along surfaces.
Microtubules, as shown in Figure 7–11, are hollow
structures made up of proteins known as tubulins. In
many cells, they play critical roles in maintaining cell
shape. Microtubules are also important in cell division, where they form a structure known as the
mitotic spindle, which helps to separate chromosomes.
In animal cells, tubulin is also used to form a pair of
structures known as centrioles. Centrioles are
located near the nucleus and help to organize cell
division. Centrioles are not found in plant cells.
Microtubules also help to build projections from
the cell surface, which are known as cilia (singular:
cilium) and flagella (singular: flagellum), that enable
cells to swim rapidly through liquids. Cilia and flagella can produce considerable force; and in some cells
they move almost like the oars of a boat, pulling or
pushing cells through the water. You will learn more
about cilia and flagella in later chapters.
Your students can extend their
knowledge of cell structure
through this online experience.
Cytoskeleton
Build Science Skills
(magnification: 1000⫻)
Cell membrane
Endoplasmic
reticulum
Microtubule
Microfilament
Ribosomes
3 ASSESS
Mitochondrion
왖
Figure 7–11
The cytoskeleton is a
network of protein filaments that helps the
cell to maintain its shape and is involved in
many forms of cell movement. The micrograph shows the microtubules of kidney cells.
Microtubules are part of the cytoskeleton that
help maintain cell shape.
Key Concept Describe
the functions of the endoplasmic
reticulum, Golgi apparatus,
chloroplast, and mitochondrion.
2. Describe the role of the nucleus
in the cell.
3. What are two functions of the
cytoskeleton?
Have students make a Venn diagram
to show organelles that are found
only in prokaryotic cells, those that
are found only in eukaryotic cells,
and those that are found in both
types of cells.
Ask students to make a compare/
contrast table that lists all the parts of
a typical cell. Column heads might
include Name, Structure, and
Function.
4. How is a cell like a factory?
5. Critical Thinking Inferring
You examine an unknown cell
under the microscope and
discover that the cell contains
chloroplasts. What type of
organism could you infer that
the cell came from?
Persuasive Writing
Image that you are Lynn
Margulis. Write a persuasive
letter to the editor of a
magazine, explaining your
idea. Your explanation should
be clear to people who do not
have a biology background.
Hint: Review the concept of
symbiosis in Section 4–2.
7–2 Section Assessment
1. Rough ER makes membranes and secretory
proteins. Smooth ER makes lipids and helps in
detoxification. The Golgi apparatus modifies,
sorts, and packages proteins and other
materials from the ER for storage or secretion.
Chloroplasts capture the energy of sunlight
and convert it into chemical energy.
Mitochondria convert stored chemical energy
into compounds that the cell can use.
2. It is the control center of the cell.
Evaluate Understanding
Reteach
7–2 Section Assessment
1.
Using Analogies Show students a
photo of a house that’s being built,
with only the foundation laid and the
basic frame constructed. Builders call
this initial stage “framing” the house.
Ask: How is this house frame like a
cell’s cytoskeleton? (Just as a
cytoskeleton is a network of protein filaments that helps a cell maintain its
shape, the frame of a house is a network of boards and timbers that forms
the shape of the house.)
3. It helps the cell maintain its shape and also is
involved in movement.
4. Answers may vary. A typical response will
compare ribosomes to factory machines and
the cytoskeleton to a supporting structure.
Students should also compare other
organelles to various parts of a factory.
5. Students should infer that the organism
would either be a plant or some other organism that carries out photosynthesis.
Students’ letters may vary. The
focus of all letters, though, should
be to explain the endosymbiotic
theory. Because this explanation
must be clear to people without a
biology background, students
should explain in simple but accurate terms what mitochondria,
chloroplasts, and prokaryotes are,
as well as how such symbiotic relationships could evolve.
If your class subscribes to the
iText, use it to review the Key
Concepts in Section 7–2.
Cell Structure and Function
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