Notes to Instructors
Chapter 7 Membrane Structure and Function
What is the focus of these activities?
To be alive and to continue to survive in changing environments, organisms must be able
What are the particular activities designed to do?
Activity 7.1 What controls the movement of materials into and out of the cell?
Activity 7.2 How is the structure of a cell membrane related to its function?
These activities are designed to help students begin to understand:
What misconceptions or difficulties can these activities reveal?
Activity 7.1
Question 1: Many students visualize the phospholipid bilayer as a static structure rather
than as a fluid structure. As a result, they have a difficult time understanding how
anything could move across a membrane made of phospholipids alone.
Notes to Instructors 27
Question 2: Many students don’t understand how molecular size affects osmotic potential.
Once they understand this, it will be easier for them to understand why systems don’t store
ATP or sugar molecules, but instead store energy in the form of starches and fats.
Activity 7.2
Modeling the different types of transport as they occur in a single membrane allows
Answers
Activity 7.1 What controls the movement of materials into
and out of the cell?
1. To be alive, most cells must maintain a relatively constant internal environment. To do
this, they must be able to control the movement of materials into and out of the cell.
What characteristics of the cell membrane determine what gets into the cell and
what doesn’t? That is, what determines the permeability of a cell or organelle
membrane? To answer these questions, first consider the answers to the following
questions:
28 Activity 7.1
Using your understanding of the answers in a-c, now answer these questions: What
characteristics of the cell membrane determine what gets into the cell and what
doesn’t? That is, what determines the permeability of a cell or organelle membrane?
Only very small uncharged or nonpolar molecules are capable of diffusing across the
phospholipid bilayer. Specific membrane transport proteins are required for the
Activity 7.1 29
a. If a cell membrane were
composed of only a
phospholipid bilayer,
what properties would it
have?
b. What different roles or
functions do membrane
proteins serve?
c. Why are some cell
types more permeable
to a substance (for
example, sodium ions)
than others?
Phosopholipids contain a
hydrophilic phosphate
Membrane proteins allow
charged and larger
The difference in
permeability is the result
2. You design an experiment to test the effect(s) various compounds have on the
osmotic potential of a model cell. You know that substances dissolved in aqueous or
gaseous solutions tend to diffuse from regions of higher concentration to regions of
lower concentration.
You fill each of three (20 mL) dialysis bags half full with one of these substances:
• 5% by weight of glucose in distilled water
• 5% by weight of glass bead (one glass bead) in distilled water
The dialysis bag is permeable to water but impermeable to glucose, albumin, and
glass bead.
a. If the final weight of each bag is 10 g, how many grams of glucose, albumin, and
glass bead were added to each bag?
Five percent of the weight in each bag is glucose, albumin, or glass bead.
b. The molecular weight of the protein is about 45 kilodaltons, and the molecular
weight of glucose is about 180 daltons. How can you estimate the number of
molecules of glucose in the 5% solution compared to the number of albumin
molecules in its 5% solution?
Each molecule of protein is 45,000/180 (or 250) times heavier than each
c. You put the dialysis bags into three separate flasks of distilled water. After
2 hours, you remove the bags and record these weights:
30 Activity 7.1
How do you explain these results? (Hint: Consider the surface area-to-volume
ratio of each of the three substances and review pages 50 and 51 of Campbell
Biology, 9th edition.)
Dialysis bag Weight
Glucose 13.2 g
Albumin 10.1 g
Glass bead 10.0 g
Five grams of glucose equals 5g/180g/M = approximately 0.027 M of glucose.
Five grams of albumin equals 5g/45,000g/M = approximately 0.00011M of
albumin. In other words, 5 g of glucose contains many more molecules of solute
d. What results would you predict if you set up a similar experiment but used 5%
glucose and 5% sucrose?
Glucose has a molecular weight of 180 daltons. Sucrose has a molecular weight
Activity 7.2 How is the structure of a cell membrane related to
its function?
Membranes compartmentalize the different functions of living cells. The cell membrane
is a barrier between the cell or organism and its environment. Similarly, within the cell,
membranes of organelles separate the different reactions of metabolism from each other.
Activity 7.2 31
Use the understanding you gain from your model to answer the following questions.
1. Substances can move across the membrane via simple diffusion, facilitated
diffusion, or active transport.
32 Activity 7.2
d. What functions might each of the three types of diffusion serve in an independent
cell such as a Paramecium or an amoeba?
Many possible examples can be used to answer this. For example, water and
oxygen could enter these cells via simple diffusion. Fatty acids, produced as a
e. What functions might each of the three types of diffusion serve in a multicellular
organism—for example, a human or a tree?
Again, many possible examples can be used here. For example, in humans and
a. Where does it
occur in
membrane?
b. Does it require
transport
protein?
c. Does it require
input of energy?
Simple diffusion Across the
phospholipid
bilayer
No No
Building the Model
• Include in the membrane the phospholipid bilayer (phosphate heads and fatty
acid tails) as well as the integral proteins.
2. What would you need to observe or measure to determine whether a substance was
moved across a membrane via each type of diffusion?
Activity 7.2 33
Simple diffusion Facilitated diffusion Active transport
No energy required—that
No energy required. Does
Energy required. Occurs
3. The ratios of saturated to unsaturated phospholipids in an organism’s membranes
can change in response to changes in environmental conditions.
a. How do the properties of a membrane that contains a low percentage of
unsaturated phospholipids compare with those of a membrane that contains a
high percentage of unsaturated phospholipids?
Unsaturated phopholipids remain fluid at lower temperatures. Therefore, a
b. Considering what you know about the properties of saturated and unsaturated
fatty acids, would you expect an amoeba that lives in a pond in a cold northern
climate to have a higher or lower percentage of saturated fatty acids in its
membranes during the summer as compared to the winter? Explain your answer.
For the membranes to remain fluid in cold temperatures, they must contain a
higher concentration of unsaturated phospholipids in the winter than in the
34 Activity 7.2
i) Toxin level in fish
1,600
i) Toxin level in water
1,600
4. A fish is removed from a contaminated lake. You determine that a
particular toxin is present in its cells at concentration X = 1500 g/L. You place the
fish in a tank full of clean water (X = 0 g/L), and measure the toxin concentration
in the fish cells over the next 10 days.
a. On the graphs below, predict how the toxin concentrations in the fish and in the
water will change over time if:
i. the toxin is water soluble
ii. the toxin is fat soluble
b. After making your hypothesis, you test it by measuring the toxin levels in the fish
at various times during its 10 days in the tank. You observe that the level
of toxin in the fish drops from 1,500 g/L to 750 g/L and then stabilizes at
750 g/L. You test the water in the tank and find that after it stabilizes, toxin
is present in the water at 750 g/L also.
• Which of your predictions fit these data?
• Which of the following processes is most likely eliminating the toxin from the
fish?
i. Passive transport
ii. First active, then passive transport
c. Given the situation in part b, what should you do, in the short term, to continue to
reduce the toxin level in the fish below 750 g/L?
The easiest method would be to continue to change the water on a daily basis.
5. A particular amino acid is transported from the extracellular medium against its
concentration gradient. The integral membrane protein that transports the amino acid
also binds and transports Na+. Using your model of the cell membrane, develop a
transport mechanism that will permit the amino acid uptake to be coupled to the Na+
transport so that the amino acid’s entry is linked only indirectly to ATP hydrolysis.
Activity 7.2 35