Chapter 7: Membrane Structure and Function
Key Concepts
7.1 Cellular membranes are fluid mosaics of lipids and proteins
7.2 Membrane structure results in selective permeability
7.3 Passive transport is diffusion of a substance across a membrane with no energy investment
7.4 Active transport uses energy to move solutes against their gradients
7.5 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
Interactive Question 7.1
Label the components in this diagram of the fluid mosaic model of membrane structure. Indicate
the regions that are hydrophobic and those that are hydrophilic.
a.
b.
c.
d.
e.
phospholipids bilayer
hydrocarbon tail
phosphate
hydrophobic region
hydrophilic region
Interactive Question 7.2
a. Cite some experimental evidence that shows that membrane proteins drift.
Human/mouse cell hybrid shows quick intermingling of proteins.
b. How might the plasma membrane of a plant cell change in response to the cold
temperatures of winter?
The cell may increase the proportion of unsaturated phospholipids in its membrane.
Interactive Question 7.3
List the six major kinds of functions that membrane proteins may perform.
1.
2.
3.
4.
5.
6.
transport
enzymatic activity
signal tranduct
intercellular attachment
cell to cell recognition
attachment to ECM
Interactive Question 7.4
What types of molecules have difficulty crossing the plasma membrane? Why?
Ions, larger polar molecules such as glucose are impeded by hydrophobic region. Passage is not
easy even for small, polar molecules.
Interactive Question 7.5
A solution of 1 M glucose is separated by a selectively permeable membrane from a solution of
0.2 M fructose and 0.7 M sucrose. The membrane is not permeable to the sugar molecules.
Indicate which side initially has more free water molecules and which has fewer. Show the
direction of osmosis.
Side A has fewer free molecules. Side B initially has the higher amount of water. Water goes
from B to A.
Interactive Question 7.6
a. What osmotic problems do freshwater protists face?
The protists will gain water from their hydrophobic environment.
b. What adaptations may help them osmoregulate?
Membranes could be less permeable to water and contractible vacuoles expel water.
Interactive Question 7.7
a. The ideal osmotic environment for animal cells is isotonic
b. The ideal osmotic environment for plant cells is hypotonic.
Interactive Question 7.8
Why is facilitated diffusion considered passive transport?
Facilitated diffusion is still passive because solutes move down its concentrated gradient.
Interactive Question 7.9
The Na+-K+ pump, the major electrogenic pump in animal cells, exchanges sodium ions for
potassium ions, both of which are cations. How does this exchange generate a membrane
potential?
3 Na+ ions are pumped out and two K+ ions are pumped in, resulting in a net movement of +
charge from cytoplasm to extracellular flow.
Interactive Question 7.10
a. How is cholesterol, which is used for the synthesis of other steroids and membranes
transported into human cells?
Human cells are receptor-mediated endocytosis; to take in cholesterol.
b. Explain why cholesterol accumulates in the blood of individuals with the disease familial
hypercholestermia.
LDL receptor proteins in the plasma membrane are defective and low-density
lipoproteins cannot bind and be transported into the cell.
Structure Your Knowledge
1. Create a concept map to illustrate your understanding of osmosis. This exercise will help you
practice using the words hypotonic, isotonic, and hypertonic, and it will help you focus on the
effect of these osmotic environments on plant and animal cells. Explain your map to a friend.
Osmosis
is the
diffusion of water
through a
selectively permeable membrane
from a
no net movement if
hypotonic solution  hypertonic solution
in which
has
has
in which
isotonic solution
in which
animal cell will lyse
lower
higher
animal cell animal cell is normal
and
solute
solute
will shrivel
and
plant cell concentration concentration and
plant cell is flaccid
is turgid
and
and
plant cell
more water
less water will plasmolyse
available
available
to move
to move
2. The following diagram illustrates passive and active transport across a plasma membrane. Use
it to answer questions a-d.
a. Which section represents facilitated diffusion? How can you tell?
II represents facilitated diffusion. The solute is moving through a transport protein and
down a concentration gradient. The cell does not expend energy in this transport. Polar
molecules and ions may move by facilitated diffusion.
b. Which section shows active transport? List two ways how you can tell?
III represents active transport because the solute is clearly moving against its
concentration gradient and the cell is expending ATP to drive this transport against the
gradient.
c. Which section shows diffusion? What types of solute molecules may be moved by this
type of transport?
I illustrates diffusion through the lipid bilayer. The solute molecules must be nonpolar or
very small polar molecules.
d. Which of these sections are considered passive transport?
I and II, Both diffusion and facilitated diffusion are considered passive transport because
the solute moves down its concentration gradient and the cell does not expend energy in
the transport.
Multiple Choice
1. c. cell-cell recognition. pg. 129
2. b. the hydrocarbon tails. pg. 126
3. a. It is a passive process in cells without walls, but an active one in cells with walls.
pg. 132
4. e. all of the above. pg. 125
5. d. shriveling of the cell. pg. 132-133
6. a. electrochemical gradient. pg. 135
7. e. increasing the number of phospholipids with unsaturated hydrocarbon tails. pg. 126
8. c. become turgid. pg. 132
9. b. They can concentrate solute molecules on one side of the membrane. pg. 133-134
10. a. movement of cations into the cell. pg. 134-136
11. e. transport of one solute against its concentration gradient in tandem with another that is
diffusing down its gradient. pg. 136
12. a. ligands and coated pits. pg. 137
13. b. sodium-potassium pump. pg. 134
14. c. the pinching in of the plasma membrane around small droplets of external liquid. pg.
138
15. b. the soil solution becomes hypertonic, causing the cells to lose water. pg. 132
16. b. facing inside the ER lumen and inside the transport vesicle. pg. 127-128
17. e. a middle region composed of alpha-helical stretches of hydrophobic amino acids, with
hydrophilic regions at both ends of the protein. pg. 127-128
18. c. has an equal solute concentration. pg. 132-133
19. e. water and glucose. pg. 132-133
20. a. The water level is higher in side A than in side B. pg. 132-133
21. d. transport proteins…down. pg. 133-134
22. b. isotonic. pg. 132
23. d. receptor-mediated endocytosis. pg. 137-138
24. c. a phospholipids bilayer with proteins embedded in and attached to it. pg. 125
25. d. It is hypertonic to the plant cells, but its solute can cross the plant cell membranes.
pg. 132-133