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Through the Cell Membrane
Explain the dynamics of the
transport of substances
through the cell membrane,
including facilitated diffusion.
Design and carry out an
investigation to examine the
movement of substances
across a membrane.
Figure 1.30 Two different water
environments meet when the
Thompson River joins the
Fraser River, much like the
internal environment of a cell
and the extracellular fluid
meeting at the cell membrane.
The conditions inside every cell must remain
nearly constant for it to continue performing its
life functions. The steady state that results from
maintaining near-constant conditions in the
internal environment of a living thing is called
homeostasis. The structure chiefly responsible for
maintaining homeostasis inside a living cell is the
cell membrane.
You have seen that the cell membrane’s structure
is remarkably complex. The cell membrane uses
several methods to transport molecules of different
sizes and with different properties in and out of the
cell. The primary methods it uses rely on the fact
that the cell membrane is semi-permeable, allowing
some molecules to pass through it while preventing
others from doing so. This section will examine
those transport methods that involve substances
moving through the cell membrane.
On both sides of the cell membrane, water is
the solvent, the meeting place for all of the other
chemicals. As you learned in Section 1.1, water
has special properties that make it a functional
medium for living reactions. For example, the
external environment of a single-celled life form,
such as the amoeba shown in Figure 1.31, consists
primarily of water. This external environment also
contains other microscopic aquatic organisms,
decaying organic matter, and dissolved gases (such
as oxygen) and other inorganic substances.
Figure 1.31 This amoeba has little sensory equipment,
limited locomotion, and a seemingly fragile membranous
covering. Yet it copes with an external environment as
complex as yours.
In the case of a multicellular organism, every
cell is bathed in a thin layer of extracellular fluid.
The extracellular fluid consists of a variable
mixture of water and dissolved materials. Some are
substances that a particular cell type requires; some
are substances needed by all cells. Other materials
are wastes that the cell has already discarded —
and that the organism will eventually get rid of.
Diffusion and the Cell Membrane
One passive method by which small molecules
move through the cell membrane is diffusion.
Diffusion is the movement of molecules from a
region where they are more concentrated to one
Exploring the Micro-universe of the Cell • MHR
where they are less concentrated. Many molecules
— especially small, uncharged ones, such as
oxygen — can move easily through the cell
membrane as a result of this process.
How does diffusion work? You may remember
from earlier studies that molecules are in constant
motion — even in a solid. In a liquid, this means
that the molecules move about randomly all the
time. As they collide with each other and with the
walls of their container, they rebound, changing
speed and direction. This constant, random
movement of molecules in a liquid is called
Brownian motion. It drives the process of diffusion.
If molecules of another substance are added to
water, they will be bounced around by the motion
of the water molecules and each other until the
new substance is spread evenly throughout the
water. From earlier studies, you will recall that in
this case the water is acting as a solvent that
dissolves other substances, or solutes.
Diffusion always results in the net movement
of particles from a region of high concentration
toward a region of low concentration. The
difference in concentrations between these regions
is called the concentration gradient. For example,
in the river pictured in Figure 1.30 on the previous
page, the concentration of mud particles is very
high on one side and very low on the other. What
do you think happens to the concentration of the
mud particles farther downstream?
Over short distances, diffusion works well
to transport small molecules across the cell
membrane. For example, oxygen and carbon
dioxide cross the cell membrane by diffusion. As a
cell uses up dissolved oxygen, more oxygen enters
the cell; as a cell generates carbon dioxide, more
carbon dioxide leaves the cell.
A human – sized amoeba?
Substances diffuse
rapidly through the
cell membrane (in
less than a second).
0.6 m
1.65 m
0.5 m
1.65 m
Substances diffuse very slowly throughout
the cell’s internal fluid (taking more than a
week to reach the centre).
Figure 1.32 This amoeba has two problems: (1) It would
take years for molecules critical to survival, such as oxygen,
to reach its centre via diffusion. (2) Relative to the volume
of the “body” that it has, it does not have much surface area
(cell membrane) across which substances can move in
and out.
Random Walking
In this lab, you will measure how long it takes for foodcolouring particles to diffuse different distances. Fill a 25 mL
graduated cylinder with warm tap water. Gently tap the side
of the cylinder to eliminate all air bubbles in the water. Use
a long pipette to take a 1 mL sample of undiluted blue or
red food colouring, and then rinse the outside of the pipette
with running water. Carefully insert the pipette into the
cylinder until the tip reaches the bottom of the water.
Release the food colouring into the water. Time how long it
takes for the colour to move 3 mL up the cylinder. Time
how long it takes for the colour to move up 10 mL.
Diffusion Limits Cell Size
Diffusion also explains how molecules move
around once inside the cell. But the concentration
gradient within a cell is not nearly as great as that
across the cell membrane. Once molecules have
diffused through the membrane, their rate of
diffusion slows down abruptly. How does this fact
limit the maximum size of cells? Figure 1.32 gives
some clues.
For the cell, having a large surface area relative
to its volume increases the area available for
materials to diffuse in and out.
MHR • Cellular Functions
1. Make a general statement about the speed of diffusion
and distance.
2. Explain why you think this MiniLab is called “Random
3. Make a prediction about what would happen to the rate
of diffusion if you (a) increased the temperature of the
water or (b) decreased the temperature of the water. In
each case, explain your reasons why.
4. State whether temperature is a dependent or an
independent variable in testing rates of diffusion.
As a cell increases in size, what happens to the amount of
surface area it has relative to the volume of its contents? Find
out by calculating the ratio of a cell’s surface area to its
volume. Start your calculations using a cube-shaped cell with
sides 1 unit long. Find the cell’s surface area and volume.
Divide the surface area by the volume. Increase the length of
each side by 0.5 units, and perform your calculations again.
Repeat this 10 times. Display your data in a spreadsheet.
Perform the same calculations for a spherical and a cylindrical
cell (both starting with a radius of 1 unit). Display your data in
a spreadsheet. How does changing the shape of the cell affect
the ratio of surface area to volume?
hypotonic environment. The figure also shows how
the concentration of solutes on either side of the
cell membrane affects the concentration of water.
One of the dangers a cell faces under hypotonic
conditions is that the cell membrane may burst.
The destruction of a cell through this process is
called lysis.
What would happen to this cell under hypertonic
conditions? When water diffuses out of the cell, the
process is called plasmolysis. A higher concentration
of solutes in the extracellular fluid than in the
intracellular fluid can cause plasmolysis.
Water concentrations
Osmosis: The Diffusion of Water
Water from the extracellular fluid and from inside
the cell also diffuses freely through the cell
membrane in such a way that the concentration
of water on either side of the membrane usually
remains equal. This diffusion of the solvent across
a semi-permeable membrane separating two
solutions is called osmosis. For cells, where the
solvent is water, the water molecules move from a
region of higher concentration to a region of lower
concentration — as in any other form of diffusion.
The direction of osmosis always depends on the
relative concentration of water molecules on either
side of the cell membrane:
When the water concentration inside the cell
equals the water concentration outside the cell,
equal amounts of water move in and out of the
cell (isotonic conditions).
When the water concentration outside the cell is
greater than that inside the cell, water moves into
the cell (hypotonic conditions).
When the water concentration inside the cell is
greater than that outside the cell, water moves
out of the cell (hypertonic conditions).
The cell membrane cannot prevent this
movement of water (because it is permeable to
water molecules), and the only energy involved
is the Brownian motion of the water molecules.
Hence, osmosis (like diffusion) is a passive process
that does not require energy from the cell. Clearly
though, the cell can remain healthy only if the
water concentrations inside it and in the extra
cellular fluid surrounding it stay in balance.
Blood plasma and the fluid that bathes our cells
are usually isotonic. Figure 1.33 illustrates what
happens to a red blood cell in an isotonic and a
Red blood cell
A Equal water concentrations
cell unchanged
B Greater water concentration
outside cell — pure water
cell bursts
water molecules
dissolved substances
Figure 1.33 If a cell is placed in a hypotonic solution, water
enters the cell by osmosis. Under these conditions, cells
without cell walls may burst.
Facilitated Diffusion
Although water, oxygen, and carbon dioxide can
pass through the cell membrane without assistance,
other substances cannot do so without help. This
makes the cell membrane a selectively permeable
membrane. For example, glucose cannot cross the
cell membrane by simple diffusion — even if the
glucose concentration outside a cell is greater than
that within. The glucose molecule is too big to
diffuse between the phospholipid molecules of the
Exploring the Micro-universe of the Cell • MHR
membrane and is insoluble in lipids, so it cannot
dissolve in the lipid bilayer.
How then do molecules such as glucose get in
and out of the cell? This is where many of the
proteins studded in the cell membrane play a role.
Specialized transport proteins in the cell membrane
help different kinds of substances move in and out
of the cell. The structure of these transport proteins
enables them to be highly selective. A particular
transport protein will recognize and help to move
only one type of dissolved molecule or ion based
on the particle’s shape, size, and electrical charge.
In the case of glucose, a type of membrane
protein called a carrier protein facilitates the
movement of glucose molecules from a region
where they are more concentrated to a region
where they are less concentrated. Because the
relative concentration of glucose still drives its
movement through the carrier protein, this
facilitated diffusion is also an example of passive
transport. Figure 1.34 shows how a carrier protein
1 • B
Osmosis in a Model Cell
Modelling concepts
The small size of living cells makes it difficult to observe osmosis
actually occurring across their membranes. However, you can make a
model of a cell to study the process of osmosis. You can use dialysis
tubing, which provides a synthetic membrane permeable to water
molecules, as the membrane of a model cell. In this investigation, you
will design and conduct an experiment to determine how the
composition of the extracellular fluid affects osmosis.
Initiating and planning
Analyzing and interpreting
How does the presence or absence of solutes in
the extracellular fluid affect the direction and
amount of osmosis through the model cell’s
Make a hypothesis about the effect of solutes in
the extracellular fluid on the flow of water
through the cell membrane by osmosis.
CAUTION: Follow your teacher’s directions for
conducting laboratory experiments safely. Do not
consume any food products in the laboratory.
dialysis tubing
string to tie tubing
support stand to hang
“cell” in beaker
large beakers (500 mL)
or jars
distilled water, egg white,
or Ringer’s solution
4 mol/L sucrose solution or
table syrup
sodium chloride (table salt)
tap water
any other materials
you require
Experimental Plan
1. Examine the materials provided by your
teacher. As a group, list the possible ways
MHR • Cellular Functions
Carrier proteins depend on their threedimensional shapes to do their jobs. A carrier
protein will accept only a non-charged molecule
with a specific shape, much as a shape-sorting toy
allows a child to insert a triangular piece only in a
triangular hole. However, carrier proteins allow
molecules to move both in and out of the cell.
To review cell membrane structure and enhance your
learning about membrane transport, go to your Electronic
Learning Partner.
Figure 1.34 Carrier proteins change shape to allow certain
molecules to cross the cell membrane.
you might test your hypothesis using these
2. Agree on one way that your group could
investigate your hypothesis.
3. Your experimental design should use a
control and test only one variable at a time.
Plan how you will collect quantitative data.
4. Write a numbered procedure for your
experiment that lists each step, and prepare
a list of materials that includes the
quantities you will need.
Checking the Plan
1. What will be your independent variable?
What will be your dependent variable(s)?
How will you set up your control?
2. What measurements will you make?
How will you determine if the solute
content in the extracellular fluid has
affected the direction or amount of osmosis?
Have you designed a table for collecting data?
3. How many trials will you carry out?
How long will you allow each trial to run?
Data and Observations
Conduct your experiment, make your
measurements, and complete your data table.
Design and complete a graph or other visual
presentation of your results.
1. How did any changes you observed in
your model cell(s) relate to the solute
concentrations of the extracellular fluid
in which you placed the cell(s)?
2. What evidence do you have that the amount
of water inside the cell was or was not
changed by osmosis?
3. What evidence do you have that dialysis
tubing is permeable to water?
Conclude and Apply
4. How do solutes affect the concentration of
5. Based on your results, predict how the use
of road salt to melt ice or snow affects the
plants bordering the sidewalk, road, or
highway. Explain.
Exploring Further
6. Using what you learned in this investigation,
design an experiment that would help solve
a related problem: How will the presence
or absence of solutes in the model cell’s
intracellular fluid affect the direction and
extent of osmosis through the model cell’s
membrane? If time and materials are
available, perform the experiment and
anlayze your results.
Exploring the Micro-universe of the Cell • MHR
negatively charged
channel protein
No cellular energy is required regardless of
whether the substances move in or out of the cell.
Facilitated diffusion does require the participation
of specialized membrane proteins, but the proteins
do not require energy from the cell to do their job.
Active Transport
Figure 1.35 Channel proteins provide water-filled
passages through which small dissolved ions can diffuse.
Channel Proteins
Since carrier proteins cannot transport charged
particles across the cell membrane, a different type
of membrane protein called a channel protein does
this. Channel proteins have a tunnel-like shape that
allows charged particles (ions) to pass through the
lipid bilayer. Figure 1.35 illustrates how this
process works.
To pass through a channel protein, an ion in
solution must be small enough to fit through the
“tunnel.” It must also have the right charge. In
much the same way that like poles of two magnets
repel each other, a positively charged channel
protein repels positively charged ions and a
negatively charged channel protein repels
negatively charged ions.
A cell often needs to maintain an intracellular
environment vastly different from that outside the
cell. For example, it must concentrate nutrients for
growth and maintenance inside, and also carry out
any specialized functions it may have. In addition,
many of the cell’s waste products are highly
toxic and must be completely removed from the
intracellular environment. Passive transport would
allow some of these materials to remain in the cell.
How does the cell acquire control over the
substances it needs for life? To do this, the cell
must expend energy to transport substances
(solutes) from an area of lower concentration to one
of higher concentration, much like pushing an
object up a hill. This process of moving substances
against (or up) their concentration gradients is
called active transport.
How much energy does it take to move a
substance up a concentration gradient? That
depends on how steep the uphill gradient is.
Particles move down a concentration gradient with
the same ease that you might ride a bike down a
hill. A similar analogy can be made for particles
being moved up a concentration gradient. Like the
cyclist in Figure 1.36, the steeper the hill the
harder the you have to pedal to get up it.
In cystic fibrosis, a genetic disorder, faulty channel proteins
cause chloride ions to build up outside the cell and sodium
ions to build up inside the cell. This makes water move
into the cell (by osmosis). The cells that line the lungs,
intestines, and pancreas take water from the mucous layer
coating the passageways, leaving the mucous thick and
sticky. In the lungs, the thick mucous interferes with
breathing. It interferes with the absorption of nutrients in
the intestine. Intensive research has led to an increase in the
quality and length of life for people living with the disease.
In all of the forms of passive transport you have
looked at, any substances crossing the cell membrane
travel along (or down) their concentration gradient.
MHR • Cellular Functions
Figure 1.36 Like a cyclist pedalling up a steep hill, the cell
must expend a great deal of energy to pump molecules and
ions in or out of the cell against their concentration gradients.
Biology At Work
for drinking or to purify municipal and industrial wastewater
before discharge to the environment.
ZeeWeed is composed of thin, hollow fibres. The
membrane of the fibres has pores small enough to block
the passage of viruses and of micro-organisms such as
bacteria. These fibres are mounted in an open frame that
can be immersed directly in the water to be treated. Like
living seaweed, ZeeWeed fibres float freely. ZeeWeed has
low energy requirements. A light stream of air bubbles
keeps the Zeeweed fibres moving, thereby exposing the
fibre membranes to incoming water currents. A slight
suction on the clean water side draws water through the
pores of the membranes into the hollow interior of the
fibres, leaving the micro-organisms and viruses behind.
Dr. Pierre Côté with a sample of ZeeWeed membrane and
a glass of water that has passed through the membrane.
Civil Engineer
Traditionally, “membrane” is used to refer to a thin film
with microscopic pores that admit only particles small
enough to pass through. Membrane filtration technology
beyond teabags and coffee filters is nothing new. For
example, hospital dialysis machines use membranes to
filter the blood of patients whose kidneys no longer
function. However, the use of this technology is costly.
Until recently, membrane technology for water filtration
has been costly, too.
Enter Dr. Pierre Côté, civil engineer, with a new kind of
membrane. A book about the potential environmental
impact of Earth’s rapidly growing human population
motivated him to focus on environmental engineering,
especially water treatment.
A typical water treatment plant passes water through
clean sand as a primary filter and then gel-like coagulants
to trap fine sediments, such as clay. After this, chlorine
is added to kill remaining bacteria. (A conventional
“activated sludge” sewage treatment plant uses bacteria
to break down wastes.) Now, Dr. Côté’s prize-winning
membrane technology offers a new approach.
Designing an Award-Winning Membrane
Dr. Côté received a Manning Innovation Award of
$100,000 for his development of ZeeWeed, a unique
filtration membrane that represents a revolution in water
treatment. It can be used to treat ground or surface water
Dr. James M. Dickson of McMaster University’s
Membrane Research Group describes such membranes
as “not very smart” when compared with the cell
membrane. “They’re designed to do a very specific job,
and that’s all they do.” The membrane around a living cell
must sense and respond to every aspect of the internal
and external environment. It must perform dozens of
distinctly different sensing, separation, and transportation
tasks — some of them thousands of times per minute.
Teamwork Pays Off
As the chief technical officer at Oakville’s Zenon
Environmental Inc. — a global leader in advanced
membrane technologies for water purification,
wastewater treatment, and water recycling — Dr. Côté
loves his job. “Just coming to work is fun. It’s not
working,” he says. “I’m doing research and development,
so it’s always something new.” When asked what he’d do
with the prize money from the Manning Award, he replied
“I will share it with my [research] team and the workers at
Zenon.” Like other researchers in both the pure sciences
and the technologies, Dr. Côté recognizes that new
developments almost always result from a team effort.
Career Tips
1. Research further to discover how civil engineers are
solving other real-world environmental problems.
2. What knowledge of cell biology should a manager
at a water-treatment facility have to do the job
effectively? How does this person work with the
Ministry of the Environment, water-testing facilities,
and local landowners to ensure the delivery of safe
drinking water?
Exploring the Micro-universe of the Cell • MHR
When a person is resting, his or her cells use up
to 40% of their energy on active transport. Many
types of specialized cells use much more. For
example, the cells in your kidneys that filter your
blood use up to 90% of their energy on active
What kind of substances do cells need to pump
in or out by active transport? A few examples
Kidney cells pump glucose and amino acids out
of the urine and back into the blood.
Intestinal cells pump in nutrients from the gut.
Root cells pump in nutrients from the soil.
Gill cells in fish pump out sodium ions (their
extracellular fluid is less salty than sea water).
Figure 1.37 How does the work done by a refrigerator
resemble the work done by active transport in cells?
Specialized cells in your stomach lining secrete acid so that you
can digest foods. They maintain the concentration of acid at
about 0.000 15 g/mL of fluid. That number may seem low, but
the concentration of acid inside the cells is much lower — less
than 0.000 000 000 05 g/mL. Compare the acidity of the fluid
inside your stomach to the acidity inside these cells: divide the
stomach’s acid concentration by the cell’s acid concentration.
Write your answer using scientific notation. What does this
ratio tell you about the concentration gradient faced by the
cells? Why must the cells consume large amounts of energy to
pump acid out across their cell membranes?
Relative Concentration Challenge
In pond water
In cytoplasm
Nitella, a green alga, is a common pond organism that has
very long cells and a plantlike structure. If left undisturbed,
It may help to think of the cell as a refrigerator.
Although food comes into the kitchen, some food
must be actively concentrated in the refrigerator
and some items in the refrigerator need to be taken
out. The refrigerator has to maintain a special
environment inside in order to keep the food fresh.
To do this, the refrigerator uses a mechanism to
cool the inside air and pump out excess heat. It
also removes the water from the air inside and
sends it outside (the water comes from the food
that enters the refrigerator). Otherwise, the water
would condense on surfaces in the cold
MHR • Cellular Functions
Nitella can spread across the bottom of a pond, providing
refuge for many pond dwellers. Pond water contains
several of the dissolved ions needed by pond organisms,
such as sodium (Na+ ), potassium (K+ ), calcium (Ca2+),
magnesium (Mg2+ ), and chloride (Cl− ). In this lab, you will
investigate the concentration of these ions inside and
outside of Nitella. The vertical blue bars on the graph
represent ion concentrations inside the Nitella cells. The
green bars represent the concentration of ions in the pond
water outside Nitella’s cell membrane.
You Try It
1. If ions simply diffuse through Nitella’s cell membrane,
how would the blue and green bars for each ion
compare? Suggest why simple diffusion cannot
account for the data presented in the graph.
2. What evidence is there that Nitella must somehow be
forcing ions inward against a concentration gradient?
environment. Every time you open the refrigerator,
warm air flows in and cold air flows out, both
along their concentration gradients. Even with the
door closed, the refrigerator pump must still come
on frequently and use energy to maintain the
special internal environment.
binding site
Active Transport Pump
The cell uses an elegant system to actively
transport substances in and out against their
concentration gradients. The engine that drives this
system is a pump, which runs on energy from cell
metabolism. The pump is a cell membrane protein.
This transporter protein actively pumps ions across
the membrane against their concentration gradients.
Cells have several different transporter pumps.
The best-understood example of an active
transport pump is the sodium-potassium pump in
animal cells. The cell membrane of every cell in
your body uses these pumps. As Figure 1.38 shows,
this transporter pumps sodium and potassium ions.
When three (positive) sodium ions inside the
cell and two (positive) potassium ions from the
extracellular fluid bind to the transporter’s protein
complex, the transporter taps a form of cellular
energy (ATP). This allows the protein to change
shape. In its new shape, the three sodium ions
move to the outside of the cell and the two
potassium ions move inside — the transporter has
flipped the ions. Then it releases all of the ions and
returns to its original shape.
To learn more about cellular respiration and ATP, turn to
Chapter 3, Section 3.3.
To view an animation of and explore active transport, go to
your Electronic Learning Partner.
Tapping the Energy Stored by Active Transport
The cell uses the artificial concentration gradient it
has created for sodium ions to push molecules it
needs, such as glucose and amino acids, into the
cell. The cell cannot function if it only gets as many
of these molecules as diffusion will allow into the
cell, so the cell must move extra glucose and amino
acids in against a concentration gradient.
Figure 1.38 The primary components of the active-
transport system driven by the sodium-potassium pump
How can a sodium ion exert a pushing force on a
molecule? Think of the sodium ions as skiers at the
top of a ski hill with nowhere to go but down. The
cell pushed them up the chair lift. Now it makes
them take a molecule down the hill with them. A
type of carrier membrane protein helps the sodium
ion and a molecule (such as glucose) enter the cell.
When one sodium ion and one glucose molecule
bind to this carrier protein, it changes shape. It
now allows the sodium ion to ride down its
concentration gradient into the cell — providing
the energy to move the glucose molecule as well.
Plant and bacterial cells use hydrogen instead of
sodium ions to do this.
Another protein in the cell membrane also taps
the energy stored in the sodium-ion concentration
gradient — this time to push another positive ion
out of the cell. A common use for this exchange
of one ion for another is pumping unwanted
hydrogen ions (H+ ) out of the cell against their
concentration gradient. This keeps the cell interior
from becoming too acidic.
The artificial concentration gradients that active
transport creates for sodium and potassium ions
result in a constant tendency for potassium ions
to diffuse out of the cell and for sodium ions to
diffuse back into it. So the sodium-potassium
pump must work constantly. In fact, even when
you are resting it consumes nearly one third of the
energy generated by your cells. This high-energy
requirement is thought to be caused by the need for
Exploring the Micro-universe of the Cell • MHR
rapid, repeated changes of shape in the transporter
protein complex.
Through its active transport system, the cell
stockpiles nutrients it needs for maintenance and
growth and pumps out unwanted particles. In
addition, it creates an electrical potential across
the cell membrane that allows nerves and muscles
to work. The higher concentration of positive ions
outside the cell creates an electrical charge across
the cell membrane.
Define diffusion using one specific example.
Explain the concept of a concentration gradient,
and use diagrams to clarify your explanation.
K/U Identify three different molecules that diffuse
into cells.
K/U What is homeostasis? Why is homeostasis
important to cells?
Diffusion allows for the effective movement of
substances over short distances. How is this
important for the cell?
Some potato cells are immersed in tap water.
Make diagrams showing the relative concentration
of water in both the cells and in the water.
Describe the movement of water inside a cell in
a hypertonic environment.
How is facilitated diffusion different from
Distinguish between osmosis and diffusion.
For a nerve impulse to travel the length of a nerve cell, it
must keep changing the electrical charge across the cell
membrane as it goes. The change in charge in one part of
the nerve cell membrane causes special ion channels to
open in the next part. During the millisecond that each
of these sodium ion channels remains open, some
7000 sodium ions pass into the cell through each one. To
allow for a new nerve impulse, the cell must pump out all of
these sodium ions.
Cytoplasmic ion concentration (mmol/dL)
Identify two situations where cells need to use
active transport. Describe an additional possible
situation where active transport might make sense.
I Cystic fibrosis is the result of genetic mutations
within a gene identified as cystic fibrosis
transmembrane conductance regulator (CFTR),
which codes for a channel protein. An experiment
was conducted to identify the specific ion being
transported by CFTR. The candidate ions were
chloride (Cl− ), magnesium (Mg2+ ), and sodium (Na+ ).
The experiment involved comparing cytoplasmic
levels of these ions between normal cells and cystic
fibrosis cells. Interpret the results below, and give a
possible explanation for them.
Normal CFTR
Abnormal CFTR
C Plan a dramatic presentation involving students in
the class to show the difference between passive
transport and active transport in the cell.
Make a diagram of the sodium-potassium
pump, and briefly explain how it works.
Cells need to bring in (absorb) nutrients.
• What method(s) of transport do cells in the intestine
use to absorb nutrients? Explain why.
What would happen to a cell if its cell membrane
were permeable rather than semi-permeable?
• Draw a picture showing how glucose and galactose
enter a cell. Use a different shape for each simple sugar.
K/U Why does oxygen continue to diffuse into cells
on an ongoing basis?
Explain why water pollution has such a profound
effect on living organisms.
• If a cell has adequate glucose in its extracellular
environment, explain why a cell might not have enough
glucose inside. Give several possible reasons.
MHR • Cellular Functions