Membrane Structure And Function

Question 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.

Answer 1

a. phosphate head - hydrophilic
b. phospholipid bilayer
c. hydrocarbon tail - hydrophobic
d. hydrophobic region of protein
e. hydrophilic region of protein

Question 2

Cite some experimental evidence that shows that membrane proteins drift.

Answer 2

In hybrid human/mouse cells, membrane proteins rapidly intermingle (Frye and Edidin experiment).

Question 3

List the six major kinds of functions that membrane proteins may form

Answer 3

• Transport
• Enzymatic activity
• Signal transduction
• Intercellular joining
• Cell-cell recognition
• Attachment to cytoskeleton and ECM

Question 4

What types of molecules have difficulty crossing the plasma membrane? Why?

Answer 4

Ions and larger polar molecules, such as glucose. They are impeded by the hydrophobic center of the plasma membrane’s lipid bilayer. Passage through the center of a lipid bilayer is not fast even for small, polar water molecules.

Question 5

A solution of 1 M glucose (side A) is separated by a selectively permeable membrane from a solution of 0.2 M fructose and 0.7 M sucrose (side B). 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

Answer 5

Side A initially has fewer free water molecules. More water molecules are clustered around the glucose in the 1 M solution than around the .9 M combined concentration of the fructose and sucrose. Water will move by osmosis from side B to A.

Question 6

A) What osmotic problems do freshwater protists face?

B) What adaptations may help them osmoregulate?

Answer 6

A) The protists will gain water from their hypotonic environment.

B) They may have membranes that are less permeable to water and contractile vacuoles that expel excess water.

Question 7

A) The ideal osmotic environment for animal cells is ______.

B) The ideal osmotic environment for plant cells is ______.

Answer 7

A) isotonic

B) hypotonic

Question 8

Why is facilitated diffusion considered passive transport?

Answer 8

Although it may speed diffusion, facilitated diffusion is a till passive transport because the solute is moving down its concentration gradient; the process is driven by the concentration gradient and not the energy expended by the cell.

Question 9

The sodium-potassium 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?

Answer 9

Three sodium ions are pumped out of the cell for every two potassium ions pumped in, resulting in a net movement of positive charge from the cytoplasm to the extracellular fluid.

Question 10

A) How is cholesterol, which is used for the synthesis of other steroids and membranes, transported into human cells?

B) Explain why cholesterol accumulates in the blood of individuals with the disease familial hypercholesteremia.

Answer 10

A) Human cells use receptor-mediated endocytosis.

B) LDL receptor proteins in the plasma membrane are defective, and low-density lipoproteins cannot bind and and be transported from the blood into the cell.

Question 11

Create a concept map to illustrate your understanding of osmosis.

Answer 11

Question 12

The diagram illustrates passive and active transport across a plasma membrane.

a) Which section represents facilitated diffusion? How can you tell? Does the cell expend energy in this transport? Why or why not? What types of solute molecules may be moved by this type of transport?
b) Which section shows active transport? List two ways you can tell.
c) Which section shows diffusion? What types of solute molecules may be moved by this type of transport?
d) Which of these sections are considered passive transport?

Answer 12

a) II represents facilitated diffusion. The solute is moving through a transport protein and down its concentration gradient. The cell does not expend energy in this transport. Polar molecules and ions may move by facilitated diffusion.
b) 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) I illustrates diffusion through the lipid bilayer. The solute molecules must be nonpolar or very small polar molecules.
d) I and II. Both diffusion and facilitated diffusio are considered passive transport because the solute moves down its concentration gradient and the cell does not expend energy in the transport.

Question 13

Glycoproteins and glycolipids are important for

a) facilitated diffusion.
b) active transport.
c) cell-cell recognition.
d) cotransport.
e) signal-transduction pathways.

Answer 13

c) cell-cell recognition

Question 14

A single layer of phospholipid molecules coats the water in a beaker. Which part of the molecules will face the air?

a) the phosphate groups
b) the hydrocarbon tails
c) both head and tail because the molecules are amphipathic and will lie sideways
d) the glycolipid regions
e) the phospholipids would dissolve in the water and not form a membrane coat

Answer 14

b) the hydrocarbon tails

Question 15

Which of the following is not true about osmosis?

a) it is a passive process in cells without walls, but an active one in cells with walls.
b) water moves from a hypotonic to a hypertonic solution.
c) solute molecules bind to water and decrease the water available to move.
d) it can occur more rapidly through channel proteins known as aquaporins.
e) there is no set osmosis between isotonic solutions

Answer 15

a) it is a passive process in cells without walls, but an active one in cells with walls

Question 16

Support for the fluid mosaic model of membrane structure comes from

a) the freeze-fracture technique of electron microscopy.
b) the movement of the proteins in hybrid cells.
c) the amphipathic nature of many membrane proteins.
d) both a and c.
e) all of the above.

Answer 16

e) all of the above

Question 17

Facilitated diffusion of ions across a cellular membrane requires _____ and the ions move _____.

a) energy and transport proteins…against their electrochemical gradient
b) energy and transport proteins…against their concentration gradient
c) cotransport proteins…against their electrochemical gradient
d) transport proteins…down their electrochemical gradient
e) transport proteins…down their concentration gradient

Answer 17

d) transport proteins…down their electrochemical gradient

Question 18

Which of the following is the most probable description of an integral, transmembrane protein?

a) amphipathic with a hydrophiic head and a hydrophobic tail region
b) a globular protein with hydrophobic amino acids in the interior and hydrophilic amino acids arranged around the outside
c) a fibrous protein coated with hydrophobic fatty acids
d) a glycolipid attached to the portion of the protein facing the exterior of the cell and cytoskeletal elements attached to the portion facing inside the cell
e) a middle region composed of (symbol) helical stretches of hydrophobic amino acids, with hydrophilic regions at both ends of the protein

Answer 18

e) a middle region composed of (symbol) helical stretches of hydrophobic amino acids, with hydrophilic regions at both ends of the protein

Question 19

The fluidity of membranes in a plant in cold weather may be maintained by increasing the

a) number of phospholipids with saturated hydrocarbon tails.
b) action of an H⁺ pump.
c) concentration of cholesterol in the membrane.
d) proportion of peripheral proteins.
e) number of phospholipids with unsaturated hydrocarbon tails.

Answer 19

e) number of phospholipids with unsaturated hydrocarbon tails.

Question 20

An animal cell placed in a hypotonic environment will

a) plasmolyze.
b) shrivel.
c) become turgid.
d) become flaccid.
e) burst (lyse).

Answer 20

e) burst (lyse).

Question 21

Which of the following is not true of carrier molecules involved in facilitated diffusion?

a) They increase the speed of transport across a membrane.
b) They can concentrate solute molecules on one side of the membrane.
c) They may have specific binding sites for the molecules they transport.
d) They may undergo a change in shape upon binding of solute.
e) They do not require an energy investment from the cell to operate.

Answer 21

b) They can concentrate solute molecules on one side of the membrane.

Question 22

The membrane potential of a cell favors

a) the movement of cations into the cell.
b) the movement of anions into the cell.
c) the action of an electrogenic pump.
d) the movement of sodium out of the cell.
e) both b and d.

Answer 22

a) the movement of cations into the cell.

Question 23

Cotransport may involve

a) active transport of two solutes through a transport protein.
b) passive transport of two solutes through a transport protein.
c) ion diffusion against the electrochemical gradient created by an electrogenic pump.
d) a pump such as the sodium-potassium pump that moves ions in two different directions.
e) transport of one solute against its concentration gradient in tandem with another that is diffusing down its concentration gradient.

Answer 23

e) transport of one solute against its concentration gradient in tandem with another that is diffusing down its concentration gradient.

Question 24

Exocytosis may involve all of the following except

a) ligands and coated pits.
b) the fusion of a vesicle with the plasma membrane.
c) a mechanism to export some carbohydrates during the formation of plant cell walls.
d) a mechanism to rejuvenate the plasma membrane.
e) a means of exporting large molecules.

Answer 24

a) ligands and coated pits.

Question 25

The proton pump in plant cells is the functional equivalent of an animal cell’s

a) cotransport mechanism.
b) sodium-potassium pump.
c) contractile vacuole for osmoregulation.
d) receptor-mediated endocytosis of cholesterol.
e) ATP pump.

Answer 25

b) sodium-potassium pump.

Question 26

Pinocytosis involves

a) the fusion of a newly formed food vacuole with a lysosome.
b) receptor-mediated endocytosis that involves binding of a ligand.
c) the pinching in of the plasma membrane around small droplets of external fluid.
d) the secretion of cell fluid.
e) the accumulation of specific molecules in a cell.

Answer 26

c) the pinching in of the plasma membrane around small droplets of external fluid.

Question 27

Watering a houseplant with too concentrated a solution of fertilizer can result in wilting because

a) the uptake of ions into plant cells make the cells hypertonic.
b) the soil solution becomes hypertonic, causing the cells to lose water.
c) the plant will grow faster than it can transport water and maintain proper water balance.
d) diffusion down the electrochemical gradient will cause a disruption of membrane potential and accompanying loss of water.
e) the plant will suffer fertilizer burn due to a caustic soil solution.

Answer 27

b) the soil solution becomes hypertonic, causing the cells to lose water.

Question 28

A cell is manufacturing receptor proteins for cholesterol. How would those proteins be oriented in the following membranes before they reach the plasma membrane?

a) facing inside the ER lumen but outside the transport vesicle membrane.
b) facing inside the ER lumen and inside the transport vesicle.
c) attached outside the ER and outside the transport vesicle.
d) attached outside the ER but facing inside the transport vesicle.
e) embedded in the hydrophobic center of both the ER and transport vesicle membranes.

Answer 28

b) facing inside the ER lumen and inside the transport vesicle.

Question 29

Initially the solution in side A, with respect to that in side B,

a) has a lower solute concentration.
b) has a higher solute concentration.
c) has an equal solute concentration.
d) is lower in the tube.
e) is higher in the tube.

Answer 29

c) has an equal solute concentration.

Question 30

During the period before equilibrium is reached, which molecule(s) will show net movement through the membrane?

a) water
b) glucose
c) sucrose
d) water and sucrose
e) water and glucose

Answer 30

e) water and glucose

Question 31

• After* the system reaches equilibrium, what changes are observed?
  a) The water level is higher in side A than in side B.
  b) The water level is higher in side B than in side A.
  c) The molarity of glucose is higher in side A than in side B.
  d) The molarity of sucrose has increased in side A.
  e) Both a and c have occurred.

Answer 31

a) The water level is higher in side A than in side B.
* Explanation:* This problem involves both osmosis and diffusion. Although the solutions are initially equal in molarity, glucose will diffuse down its concentration gradient until it reaches dynamic equilibrium with a 1.5 M concentration on both sides. The increasing colute concentration on side A will cause water to move into this side, and the water level will rise.

Question 32

You observe plant cells under a microscope as they are placed in an unknown solution. First the cells plasmolyze; after a minute, the plasmolysis reverses and the cells appear normal. What would you conclude about the unknown solution?

a) It is hypertonic to the plant cells, and its solute cannot cross the plant cell membranes.
b) It is hypotonic to the plant cells, and its solute cannot cross the plant cell membranes.
c) It is isotonic to the plant cells, but its solute can cross the plant cell membranes.
d) It is hypertonic to the plant cells, but its solute can cross the plant cell membranes.
e) It is hypotonic to the plant cells, but its solute can cross the plant cell membranes.

Answer 32

d) It is hypertonic to the plant cells, but its solute can cross the plant cell membranes.
* Explanation:* As the solute in the solution crosses the cell membrane, it increases the concentration of solutes within the cell, reducing the hypertonicity of the solution to the cell. As the solute reaches an equal concentration inside and outside the cell, it no longer causes osmotic changes in the cell.