7–3 Cell Boundaries
Section 7–3
1 FOCUS
W
hen you first study a country, you may begin by examining
a map of the country’s borders. Before you can learn
anything about a nation, it’s important to understand where it
begins and where it ends. The same principle applies to cells.
Among the most important parts of a cell are its borders, which
separate the cell from its surroundings. All cells are surrounded
by a thin, flexible barrier known as the cell membrane. Many
cells also produce a strong supporting layer around the membrane known as a cell wall.
Objectives
7.3.1 Identify the main functions of
the cell membrane and the
cell wall.
7.3.2 Describe what happens during diffusion.
7.3.3 Explain the processes of
osmosis, facilitated diffusion,
and active transport.
Key Concepts
• What are the main functions of
the cell membrane and the
cell wall?
• What happens during
diffusion?
• What is osmosis?
Vocabulary
cell membrane • cell wall
lipid bilayer • concentration
diffusion • equilibrium
osmosis • isotonic
hypertonic • hypotonic
facilitated diffusion
active transport • endocytosis
phagocytosis • pinocytosis
exocytosis
Vocabulary Preview
Suggest that students preview the
meaning of the Vocabulary terms in
the section by skimming the text to
find the highlighted boldface words
and their meanings.
The cell membrane regulates what enters and leaves
the cell and also provides protection and support. The
composition of nearly all cell membranes is a double-layered
sheet called a lipid bilayer. As you can see in Figure 7–12,
there are two layers of lipids, hence the name bilayer. The lipid
bilayer gives cell membranes a flexible structure that forms a
strong barrier between the cell and its surroundings.
In addition to lipids, most cell membranes contain protein
molecules that are embedded in the lipid bilayer. Carbohydrate
molecules are attached to many of these proteins. In fact, there are
so many kinds of molecules in cell membranes that scientists
describe the membrane as a “mosaic” of different molecules. A
mosaic is a work of art made of individual tiles or other pieces
assembled to form a picture or design. As you will see, some of the
proteins form channels and pumps that help to move material
across the cell membrane. Many of the carbohydrates act like
chemical identification cards, allowing individual cells to identify
one another.
Reading Strategy:
Summarizing As you read,
make a list of the ways in which
substances can move through the
cell membrane. Write one
sentence describing each process.
Reading Strategy
Before students read, have them
skim the section to identify and make
a list of the main ideas. Then, as they
read the section they should write
down supporting details for each
main idea.
Cell Membrane
왔
Figure 7–12
The cell
membrane regulates what enters
and leaves the cell. This illustration
of the cell membrane shows that it
is made up of a lipid bilayer in
which proteins are embedded.
2 INSTRUCT
Outside
of Cell
Cell Membrane
Use Visuals
Inside of Cell
(cytoplasm)
Protein
channel
Chapter 7
Lipid bilayer
SECTION RESOURCES
Print:
Technology:
• Laboratory Manual A, Chapter 7 Lab
• Teaching Resources, Section Review 7–3,
Save
Chapter 7 Real-World Lab
e
• Reading and Study Workbook A,
Section 7–3
• Adapted Reading and Study Workbook B,
Section 7–3
• Lesson Plans, Section 7–3
• iText, Section 7–3
• Animated Biological Concepts Videotape
Library, 5 Diffusion and Osmosis, 6 Passive
and Active Transport, 7 Endocytosis and
Exocytosis
• Transparencies Plus, Section 7–3
• Lab Simulations CD-ROM, Biomembranes 1:
Membrane Structure and Transport
• Virtual Labs, Lab 3, Lab 4, Lab 5
r
182
Carbohydrate
chains
Cell
C
C
Membrane
Tim
Figure 7–12 Ask students: What
does it mean that a cell membrane
has a “lipid bilayer”? (A cell membrane is composed of two layers of lipid
molecules.) What do the blue molecules represent in the illustration
of the cell membrane? (They represent carbohydrate chains attached to
the outside of the protein molecules
embedded in the lipid bilayer.) Explain
that these carbohydrate molecules
are particularly important in cell
recognition. Nearly all cells have special carbohydrate molecules on their
surfaces—cell markers—that are
unique.
Proteins
Cell Walls
Cell walls are present in many organisms, including plants,
algae, fungi, and many prokaryotes. Cell walls lie outside the
cell membrane. Most cell walls are porous enough to allow
water, oxygen, carbon dioxide, and certain other substances to
pass through easily.
The main function of the cell
wall is to provide support and protection for the cell.
Most cell walls are made from fibers of carbohydrate and
protein. These substances are produced within the cell and
then released at the surface of the cell membrane where they
are assembled to form the wall. Plant cell walls are composed
mostly of cellulose, a tough carbohydrate fiber. Cellulose is
the principal component of both wood and paper, so every
time you pick up a sheet of paper, you are holding the stuff of
cell walls in your hand.
N S TA
For: Links on cell
membranes
Visit: www.SciLinks.org
Web Code: cbn-3073
Cell Walls
Address Misconceptions
Diffusion Through Cell Boundaries
Every living cell exists in a liquid environment that it needs to
survive. It may not always seem that way; yet even in the dust
and heat of a desert like the one in Figure 7–13, the cells of
cactus plants, scorpions, and vultures are bathed in liquid.
One of the most important functions of the cell membrane is to
regulate the movement of dissolved molecules from the liquid
on one side of the membrane to the liquid on the other side.
Measuring Concentration The cytoplasm of a cell contains a solution of many different substances in water. Recall
that a solution is a mixture of two or more substances. The
substances dissolved in the solution are called solutes. The
concentration of a solution is the mass of solute in a given
volume of solution, or mass/volume. For example, if you dissolved 12 grams of salt in 3 liters of water, the concentration
of the solution would be 12 g/3 L, or 4 g/L (grams per liter). If
you had 12 grams of salt in 6 liters of water, the concentration would be 12 g/6 L, or 2 g/L. The first solution is twice as
concentrated as the second solution.
왔
Figure 7–13 The cells of living
things are bathed in liquid even in dry
environments. When it rains, these
cactus plants store the water in their
stems. Applying Concepts Which cell
structure could serve as a storage location
for water?
Some students may have the misconception that a cell wall takes the
place of a cell membrane. Emphasize
that all cells have a cell membrane.
Some cells have the added characteristic of having a cell wall outside the
cell membrane. Some students may
also have the misconception that a
cell wall is impenetrable, like the wall
of a building. Read aloud the sentence in their text that explains that
cell walls are porous. Point out that if
cell walls weren’t porous, nothing
could pass into or out of plant cells,
for instance.
Diffusion Through
Cell Boundaries
Make Connections
TEACHER TO TEACHER
To help students remember the structures and
functions of an animal cell, give each pair of students an acetate sheet and a marking pen.
Assign each student pair a different cell structure,
and ask them to draw the cell structure in a specific place on their acetate sheet. As you discuss
the parts of the cell, place all the sheets on an
overhead projector. Then, draw a cell membrane
surrounding all the cell structures on the top
N S TA
Download a worksheet
on cell membranes for students to
complete, and find additional
teacher support from NSTA
SciLinks.
acetate sheet. Have students identify each structure and tell what its function is, including the
cell membrane.
—Beverly Cea
Biology Teacher
Grimsley High School
Greensboro, NC
Mathematics Reinforce students’
understanding of concentration by
building on the example in the text.
Ask: If you dissolved 12 grams of
salt in 3 liters of water, what is the
concentration of salt in the solution? (4 g/L) Suppose you added 12
more grams of salt to the solution.
What would be the resulting concentration? (24 g/3 L = 8 g/L) What
if you then added another 3 liters
of water to that solution. What
would be the resulting concentration? (24 g/6 L = 4 g/L) Which
solution of the ones discussed
would be called the most concentrated? (The solution in which the
concentration is 8 g/L) Point out that
in each case the solute has a relatively small volume compared to the
volume of the solvent. Yet, small differences in solute can have a great
effect in living things.
Answer to . . .
Figure 7–13 Vacuoles
Cell Structure and Function
183
DIFFUSION
7–3 (continued)
Solute
Cell
Membrane
For: Diffusion activity
Visit: PHSchool.com
Web Code: cbe-3073
Students explore the process of
diffusion online.
Build Science Skills
Using Models Have students act
out the process of diffusion. To
begin, group class members at the
classroom door. Then, tell them to
spread out through the classroom in
such a way that no two students are
closer to each other than to any
other students. Discuss how molecules randomly spread out through
a liquid or a gas.
Build Science Skills
Observing Students can observe
the action of diffusion through this
simple activity. Divide the class into
small groups, and give each group
two beakers, salt, a teaspoon, and
food coloring, as well as access to
water. Have groups follow this procedure. Fill one beaker about
one-third full of water, and then add
about a half teaspoon of salt and a
few drops of food coloring. Fill the
second beaker about half full of
water. Then, pour the contents of the
first beaker into the second beaker,
and observe what happens. (Students
should observe that the colored saltwater will sink to the bottom of the
beaker.) Let the second beaker stand
overnight, and observe any changes
that have occurred. (The next day,
students should observe that the liquid
in the beaker will be uniformly colored throughout.) Ask students:
What process occurred that
changed the mixture
overnight? (Diffusion)
B
A
There is a higher
concentration of
solute on one side of
the membrane as
compared to the
other side of the
membrane.
Chapter 7
When equilibrium is
reached, solute
particles continue to
diffuse across the
membrane in both
directions.
왖
Figure 7–14 Diffusion is the
process by which molecules of a
substance move from areas of higher
concentration to areas of lower
concentration.
Diffusion does
not require the cell to use energy.
For: Diffusion activity
Visit: PHSchool.com
Web Code: cbp-3073
Diffusion In a solution, particles move constantly. They
collide with one another and tend to spread out randomly. As a
result, the particles tend to move from an area where they are
more concentrated to an area where they are less concentrated,
a process known as diffusion (dih-FYOO-zhun). When the
concentration of the solute is the same throughout a system, the
system has reached equilibrium.
What do diffusion and equilibrium have to do with cell membranes? Suppose a substance is present in unequal concentrations on either side of a cell membrane, as shown in Figure 7–14.
If the substance can cross the cell membrane, its particles will
tend to move toward the area where it is less concentrated until
equilibrium is reached. At that point, the concentration of the
substance on both sides of the cell membrane will be the same.
Because diffusion depends upon random particle
movements, substances diffuse across membranes without requiring the cell to use energy. Even when equilibrium
is reached, particles of a solution will continue to move across
the membrane in both directions. However, because almost
equal numbers of particles move in each direction, there is no
further change in concentration.
What conditions are present when equilibrium is reached
in a solution?
Inclusion/Special Needs
To help students understand the difference
between facilitated diffusion and active transport,
use the analogy of going through a fence at an
open gate or going through a turnstile. Walking
through an open gate is like facilitated diffusion—
it takes no energy to pass through that “carrier
protein” in the fence. Moving through a turnstile
is like active transport—a person has to use energy
to move through that “molecular pump.”
184
C
Solute particles move
from the side of the
membrane with a
higher concentration
of solute to the side of
the membrane with a
lower concentration of
solute. The solute
particles will continue
to diffuse across the
membrane until
equilibrium is reached.
Advanced Learners
Point out that the human digestive system is
unable to digest cellulose, and thus cellulose
passes through the digestive tract without being
broken down. Explain that cellulose is nevertheless
an important part of a healthy diet. Cellulose in
the diet is called fiber, and consuming enough
fiber may help prevent some forms of cancer.
Encourage students who need a challenge to find
out how cellulose aids in digestion and helps prevent disease.
Osmosis
Osmosis
Although many substances can diffuse across biological membranes, some are too large or too strongly charged to cross the
lipid bilayer. If a substance is able to diffuse across a membrane,
the membrane is said to be permeable to it. A membrane is
impermeable to substances that cannot pass across it. Most
biological membranes are selectively permeable, meaning that
some substances can pass across them and others cannot.
Water passes quite easily across most membranes, even
though many solute molecules cannot. An important process
known as osmosis is the result.
Osmosis is the diffusion
of water through a selectively permeable membrane.
How Osmosis Works Look at the beaker on the left in
Figure 7–15. There are more sugar molecules on the left side of
the selectively permeable membrane than on the right side.
That means that the concentration of water is lower on the left
than it is on the right. The membrane is permeable to water but
not to sugar. This means that water can cross the membrane in
both directions, but sugar cannot. As a result, there is a net
movement of water from the area of high concentration to the
area of low concentration.
Water will tend to move across the membrane until equilibrium is reached. At that point, the concentrations of water and
sugar will be the same on both sides of the membrane. When this
happens, the two solutions will be isotonic, which means “same
strength.” When the experiment began, the more concentrated
sugar solution was hypertonic, which means “above strength,”
as compared to the dilute sugar solution. The dilute sugar
solution was hypotonic, or “below strength.”
Hypotonic comes from the
Greek word hupo, meaning
“under,” and the New Latin
word tonicus, meaning “tension”
or “strength.” So a hypotonic
solution is less strong, or less
concentrated, than another
solution of the same type. If
derma means “skin,” how
would you describe a hypodermic injection?
For: Osmosis activity
Visit: PHSchool.com
Web Code: cbp-3075
Figure 7–15
Osmosis is the diffusion of water through a selectively permeable
membrane. In the first beaker, water is more concentrated on the right side of the membrane.
As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.
Dilute sugar
solution
(Water more
concentrated)
Sugar
molecules
Selectively permeable
membrane
Demonstrate the concept of selective
permeability by using a kitchen
strainer. As students observe, pour
different sorts of materials through
the strainer, making sure to choose
some materials that will go through
the strainer and some that will not.
Materials might include water, sugar,
sand, small stones, marbles, and
paper clips. Students will observe
that the strainer is selectively
permeable.
Figure 7–15 Have students study
the figure and read the caption.
Then, ask: In the beaker on the left,
which solution is hypertonic and
which is hypotonic? (The solution on
the left side of the membrane is hypertonic, and the solution on the right side
is hypotonic.) In this model, to
which material is the membrane
permeable, water or sugar? (Water)
Emphasize that the membrane allows
one material—water—to pass
through it, while blocking the other
material—sugar. This makes the
membrane selectively permeable.
Finally, ask students to draw a third
beaker that would model a situation
in which the two solutions on either
side of the membrane are isotonic.
(Students should make a drawing in
which the same number of sugar molecules are evenly distributed on both
sides of the membrane.)
Movement
of water
SCIENCE UPDATE
Water finds its way
Osmosis is easy to observe in cells, yet it was long
a mystery as to how water can cross membranes
so quickly. Water is a polar molecule, and as such
it is not lipid soluble and should not be expected
to cross a lipid bilayer. Many puzzled physical
chemists suggested that biochemists should look
for some kind of channels in the membrane. In
Demonstration
Use Visuals
왔
Concentrated
sugar solution
(Water less
concentrated)
A hypodermic injection is one that is
administered under the skin.
the late twentieth century, such channels were in
fact discovered. These channels, which are
membrane-spanning proteins, are named aquaporins. They have been found in scores of cells,
and some scientists think that they might be present in nearly all cells. Their discovery is so recent
that a detailed analysis of how they work may be
years away.
For: Osmosis activity
Visit: PHSchool.com
Web Code: cbe-3075
Students interact with the art of
osmosis online.
Answer to . . .
Equilibrium is reached
when the concentration of the solute is
the same throughout the solution.
Cell Structure and Function
185
7–3 (continued)
Build Science Skills
Applying Concepts Ask students
to consider this real-life circumstance.
A homeowner contracts a lawn company to add fertilizer to the lawn in
order to make the grass grow better.
This process is normally done by
spraying a mixture of fertilizer and
water onto the lawn. Ask: What
would happen if too much fertilizer and too little water were
sprayed onto the lawn? (Students
may know that the grass would appear
to be burned.) Can you suggest
what happened to the cells of the
grass? (They lost water because of the
concentrated solution of fertilizer
around them.) In that case, was the
fertilizer-water mixture hypotonic
or hypertonic compared to the
grass cells? (The mixture was hypertonic compared to the grass cells.)
Demonstration
Place a small number of paramecia in
a petri dish on a microprojector.
Have students observe the paramecia
as you discuss contractile vacuoles,
which some unicellular organisms
have to pump water out of the cell.
Flood the environment of the paramecia with distilled water. As
students continue to observe, point
out the action of the contractile vacuoles. Ask: What was added to the
dish? (Pure water) How do you
know? (The action of the contractile
vacuoles increased.) What will eventually happen to the paramecia?
(They will explode.) Why? (The vacuoles cannot keep up with the inward
movement of water because of osmosis.) What would happen if a small
amount of salt water were added?
(Vacuole action would probably return
to normal.)
The Effects of Osmosis on Cells
Solution
Animal Cell
Plant Cell
Isotonic:
The concentration of
solutes is the same inside
and outside the cell.
Vacuole
Water in
Water in
Water out
Cell wall
Water out
Cell
membrane
Hypertonic:
Solution has a higher
solute concentration
than the cell.
Water out
Water out
Hypotonic:
Solution has a lower
solute concentration
than the cell.
Water in
왖 Figure 7–16 Cells placed in an
isotonic solution neither gain nor lose
water. In a hypertonic solution,
animal cells shrink, and plant cell
vacuoles collapse. In a hypotonic
solution, animal cells swell and burst.
The vacuoles of plant cells swell,
pushing the cell contents out against
the cell wall. Predicting What
would happen to the animal cell in the
isotonic solution if it were placed in pure
water?
Water in
Osmotic Pressure For organisms to survive, they must have
a way to balance the intake and loss of water. Osmosis exerts a
pressure known as osmotic pressure on the hypertonic side of a
selectively permeable membrane. Osmotic pressure can cause
serious problems for a cell. Because the cell is filled with salts,
sugars, proteins, and other molecules, it will almost always be
hypertonic to fresh water. This means that osmotic pressure
should produce a net movement of water into a typical cell that
is surrounded by fresh water. If that happens, the volume of a
cell will increase until the cell becomes swollen. Eventually, the
cell may burst like an overinflated balloon.
Fortunately, cells in large organisms are not in danger of
bursting. Most cells in such organisms do not come in contact
with fresh water. Instead, the cells are bathed in fluids, such
as blood, that are isotonic. These isotonic fluids have concentrations of dissolved materials roughly equal to those in the cells
themselves.
Other cells, such as plant cells and bacteria, which do come
into contact with fresh water, are surrounded by tough cell
walls. The cell walls prevent the cells from expanding, even
under tremendous osmotic pressure. However, the increased
osmotic pressure makes the cells extremely vulnerable to
injuries to their cell walls.
What structures protect plant and bacterial cells from
potential damage resulting from osmotic pressure?
FACTS AND FIGURES
Penicillin works by osmosis
Penicillin, one of the most important antibiotic
drugs in the history of medicine, depends on
osmosis for its killing action. Penicillin inhibits an
enzyme with which many bacteria produce
chemical cross-links in their cell walls. This leads to
186
Chapter 7
the formation of a weakened cell wall that cannot
stand the stress of osmotic pressure. Gradually,
the cell wall becomes weaker and weaker until it
breaks, and the bacterium bursts under the inrush
of water caused by osmosis.
Glucose
molecules
Facilitated Diffusion
A few molecules, such as the sugar glucose, seem to pass
through the cell membrane much more quickly than they
should. One might think that these molecules are too large or
too strongly charged to cross the membrane, and yet they
diffuse across quite easily.
How does this happen? The answer is that cell membranes
have protein channels that make it easy for certain molecules to
cross the membrane. Red blood cells, for example, have a cell
membrane protein with an internal channel that allows glucose
to pass through it. Only glucose can pass through this channel,
and it can move through in either direction. This cell membrane
protein is said to facilitate, or help, the diffusion of glucose
across the membrane. The process, shown in Figure 7–17, is
known as facilitated (fuh-SIL-uh-tayt-ud) diffusion.
Hundreds of different protein channels have been found that
allow particular substances to cross different membranes.
Although facilitated diffusion is fast and specific, it is still
diffusion. Therefore, a net movement of molecules across a cell
membrane will occur only if there is a higher concentration of
the particular molecules on one side than on the other side. This
movement does not require the use of the cell’s energy.
Protein
channel
왖
Figure 7–17 During facilitated
diffusion, molecules, such as glucose,
that cannot diffuse across the cell
membrane’s lipid bilayer on their
own move through protein channels
instead. Applying Concepts Does
facilitated diffusion require the cell to
use energy?
Starch solution
Iodine solution
How can you model permeability
in cells?
Materials graduated cylinder, plastic sandwich
bag, starch, twist tie, 500-mL beaker, iodine
solution
Procedure
4. Place the sandwich bag of water and starch into
the beaker of water and iodine.
5. After 20 minutes, look at the sandwich bag in the
beaker. Observe and record any changes that
occurred.
1. Pour about 50 mL of water into a plastic sandwich
bag. Add 10 mL of starch. Secure the bag with a
twist tie, and shake it gently to mix in the starch.
2. Put on your goggles, plastic gloves, and apron.
3. Pour 250 mL of water into a 500-mL beaker.
CAUTION: Handle the beaker carefully. Add 15
drops of iodine. CAUTION: Iodine is corrosive and
irritating to the skin and can stain skin and clothing.
Be careful not to spill it on yourself.
1. Using Models What cell structure does the
sandwich bag represent?
2. Observing What did you see inside the sandwich
bag? Outside the sandwich bag?
3. Inferring Iodine turns blue-black in the presence
of starch. What process do you think occurred that
caused the results you observed? Explain.
Analyze and Conclude
Facilitated Diffusion
Objective Students will be able to
make and investigate a model of
permeability in cells.
Skills Focus Using Models,
Inferring
Time 20 minutes
Advance Prep To save time and
reduce possible spills, you may want
to prepare the starch mixture and
iodine solution yourself in advance.
Safety Read the MSDS on iodine.
If you prepare the iodine solution
yourself, be sure to wear goggles,
plastic gloves, and an apron.
Strategy It will take up to 20 minutes for the iodine to pass through
the bag and react with the starch,
so you may want to set up this lab
at the beginning of the class and
return to it at the end.
Expected Outcome Students
should observe that the water inside
the sandwich bag turned blue-black.
The water outside the sandwich bag
did not change.
Analyze and Conclude
1. The sandwich bag represents the
cell membrane.
2. The water inside the sandwich
bag turned blue-black. The water
outside the sandwich bag did not
change.
3. Diffusion occurred. The iodine
molecules diffused through the
sandwich bag and reacted with the
starch inside. The starch molecules
were too big to pass through the
bag, and so there was no reaction
outside the bag.
FACTS AND FIGURES
Rate of facilitated diffusion
Facilitated diffusion depends on a difference in
concentration. In simple diffusion, that is the
only factor that affects rate. In facilitated diffusion, the rate also depends on the number of
specific carrier protein molecules in the membrane, because the diffusing molecules can
move across the membrane only through those
proteins. An example is the diffusion of glucose
into cells, as described on this page. Such diffusion occurs most of the time as facilitated
diffusion. No matter how much the cell “needs”
the glucose—no matter how great the difference is in concentration inside and outside the
cell—the rate at which the glucose can diffuse
into the cell has a limit because of the limited
number of glucose carrier protein molecules in
the lipid bilayer.
Answers to . . .
Cell walls prevent the
cells from expanding, even under
tremendous osmotic pressure.
Figure 7–16 The volume of the cell
would increase until it became swollen.
Eventually, it could burst like an overinflated balloon.
Figure 7–17 No
Cell Structure and Function
187
7–3 (continued)
Size of Molecules
Answers
1. Students may predict that water
will diffuse most quickly because it is
the smallest and glucose will diffuse
most slowly because it is the largest.
2. Students’ hypotheses may include:
The smaller a molecule is, the faster
it diffuses.
3. Students’ experiments should be
designed to test their hypotheses
and control variables.
Crossing the Cell Membrane
The cell membrane regulates what enters and leaves
the cell and also provides protection and support. The
core of nearly all cell membranes is a double-layered
sheet called a lipid bilayer. Most materials entering the
cell pass across this membrane by diffusion. The
graph shows the sizes of several molecules that can
diffuse across a lipid bilayer.
1. Predicting Which substances do you think will
diffuse across the lipid bilayer most quickly? Most
slowly? Explain your answers.
2. Formulating Hypotheses Formulate a hypothesis about the relationship between molecule size
and rate of diffusion.
3. Designing Experiments Design an experiment
to test your hypothesis.
Use Visuals
Figure 7–18 After students have
studied the image and read the caption, ask: Why is phagocytosis an
example of active transport and
not facilitated diffusion?
(Phagocytosis requires the input of
energy, because the organism uses
energy to surround and engulf a large
particle.)
Figure 7–18 Phagocytosis is one
form of active transport. During
phagocytosis, extensions of cytoplasm
surround and engulf large particles.
Amoebas are one type of organism that
uses this process to take in food and
other materials.
188
Chapter 7
Glucose
Glycerol
Oxygen
Urea
Water
0
50
150
100
Size (daltons)
200
As powerful as diffusion is, cells sometimes must move
materials in the opposite direction—against a concentration difference. This is accomplished by a process known as
active transport. As its name implies, active transport
requires energy. The active transport of small molecules or
ions across a cell membrane is generally carried out by
transport proteins or “pumps” that are found in the membrane itself. Larger molecules and clumps of material can
also be actively transported across the cell membrane by
processes known as endocytosis and exocytosis. The transport of these larger materials sometimes involves changes
in the shape of the cell membrane.
Molecular Transport Small molecules and ions are
carried across membranes by proteins in the membrane
that act like energy-requiring pumps. Many cells use such
proteins to move calcium, potassium, and sodium ions
across cell membranes. Changes in protein shape, as shown
in Figure 7–19, seem to play an important role in the
pumping process. A considerable portion of the energy used
by cells in their daily activities is devoted to providing the
energy to keep this form of active transport working. The
use of energy in these systems enables cells to concentrate
substances in a particular location, even when the forces of
diffusion might tend to move these substances in the
opposite direction.
Build Science Skills
Using Analogies Set up a ramp
using a board propped up on one
end by a stack of books. Then, as students observe, roll a ball down the
ramp and push it back up again. Ask:
Which is like facilitated diffusion
and which is like active transport,
rolling the ball down the ramp or
pushing it up? Explain. (Rolling the
ball down is like facilitated diffusion,
because neither requires addition of
energy. Pushing the ball back up is like
active transport, because it requires
addition of energy.)
Ethanol
Active Transport
왔
Active Transport
Carbon
dioxide
Molecule
As students examine the bar graph of
the different molecules, point out
that some molecules move through
the lipid bilayer by simple diffusion
and others move through by facilitated diffusion. In most cases, for
example, water moves through selectively permeable membranes by
diffusion, whereas glucose moves
through by facilitated diffusion.
FACTS AND FIGURES
Protein molecules and active transport
One of the most important examples of active
transport is known as the sodium potassium
pump, in which sodium ions are maintained at a
lower concentration inside the cell and potassium
ions are maintained at a higher concentration
inside the cell. The active transport by protein
molecules of these ions is central to the production of electrical impulses by nerve cells. At one
time, scientists thought that the protein molecules actually rotated as they transported
substances through the cell membrane, picking
up their parcels on the outside and dumping
them on the inside. Now, scientists think that the
transported molecules are somehow squeezed
through the transport proteins, as the proteins
change their configuration to accommodate their
riders.
Endocytosis and Exocytosis Larger molecules
and even solid clumps of material may be transported by
movements of the cell membrane. One of these movements is called endocytosis (en-doh-sy-TOH-sis).
Endocytosis is the process of taking material into the
cell by means of infoldings, or pockets, of the cell membrane. The pocket that results breaks loose from the
outer portion of the cell membrane and forms a vacuole
within the cytoplasm. Large molecules, clumps of food,
and even whole cells can be taken up in this way. Two
examples of endocytosis are phagocytosis (fag-oh-syTOH-sis) and pinocytosis (py-nuh-sy-TOH-sis).
Phagocytosis means “cell eating.” In phagocytosis,
extensions of cytoplasm surround a particle and package
it within a food vacuole. The cell then engulfs it.
Amoebas use this method of taking in food. Engulfing
material in this way requires a considerable amount of
energy and, therefore, is correctly considered a form of
active transport.
In a process similar to endocytosis, many cells take
up liquid from the surrounding environment. Tiny
pockets form along the cell membrane, fill with liquid,
and pinch off to form vacuoles within the cell. This
process is known as pinocytosis.
Many cells also release large amounts of material
from the cell, a process known as exocytosis (ek-soh-syTOH-sis). During exocytosis, the membrane of the
vacuole surrounding the material fuses with the cell
membrane, forcing the contents out of the cell. The
removal of water by means of a contractile vacuole is
one example of this kind of active transport.
For: Active Transport activity
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Molecule to
be carried
For: Active Transport activity
Visit: PHSchool.com
Web Code: cbe-3076
Students interact with the art of
active transport online.
3 ASSESS
Evaluate Understanding
Make up a list of fictitious substances.
Have students describe the method
of transport a cell would use to move
each substance through the cell
membrane and why. Students need
not be accurate; what you are looking for is correct reasoning.
Energy
Reteach
Molecule
being carried
Have students write definitions of diffusion, osmosis, facilitated diffusion,
and active transport. Then, discuss
ways in which they are similar and
ways in which they are different.
왘 Figure 7–19 Active transport of particles against a
concentration difference requires transport proteins and
energy. Interpreting Graphics What is happening in
the illustration?
7–3 Section Assessment
1.
Key Concept Describe the
functions of the cell membrane
and cell wall.
2.
Key Concept What
happens during diffusion?
3.
Key Concept Describe
how water moves during osmosis.
4. What is the basic structure of a
cell membrane?
5. What is the difference between
phagocytosis and pinocytosis?
6. Critical Thinking Comparing
and Contrasting What is the
main way that active transport
differs from diffusion?
Homeostasis
What is the relationship
between active transport and
homeostasis? Give one
example of active transport in
an organism, and explain how
the organism uses energy to
maintain homeostasis. You
may want to refer to
Section 1–3.
If your class subscribes to the
iText, use it to review the Key
Concepts in Section 7–3.
7–3 Section Assessment
1. The cell membrane regulates what enters
and leaves the cell and also provides protection and support. The cell wall provides
support and protection for the cell.
2. Particles tend to move from an area where
they are more concentrated to an area
where they are less concentrated.
3. Osmosis is the diffusion of water through a
selectively permeable membrane.
4. The basic structure is a double-layered sheet
Students should state the definition
of both active transport and homeostasis, which they learned about in
Section 1–3. Examples may vary.
Students might suggest that any
material taken into a cell by active
transport that is necessary for normal cell activities helps maintain
homeostasis, and active transport is
a process that uses energy.
called a lipid bilayer, in which proteins are
embedded.
5. In phagocytosis, extensions of cytoplasm surround a particle and package it within a food
vacuole. In pinocytosis, tiny pockets form
along the cell membrane, fill with liquid, and
pinch off to form vacuoles within the cell.
6. Active transport requires the input of energy, but diffusion does not require additional
energy.
Answer to . . .
Figure 7–19 A molecule is moving
across the cell membrane from an area
of low concentration to an area of high
concentration with the help of a transport protein and the use of energy.
Cell Structure and Function
189