Membrane Structure and Transport Practice Quiz (11 questions)

1. Which arrangement is described by the fluid mosaic model of a cell membrane?

A. A protein bilayer with phospholipids attached to its outer surfaces

B. A rigid phospholipid layer covered on both sides by continuous sheets of protein

C. A fluid phospholipid bilayer with proteins embedded in it or attached to its surface

D. A fluid carbohydrate bilayer with lipids and proteins attached to its surface

Answer: (C)

Explanation

The model describes a fluid phospholipid bilayer containing a mosaic of proteins embedded within it or attached to its surface. The continuous protein layers describe the older sandwich model, not the fluid mosaic arrangement.

2. What feature makes a phospholipid amphipathic?

A. It alternates between hydrophobic and hydrophilic states over time.

B. It has two hydrophobic regions and no hydrophilic region.

C. It has both hydrophobic and hydrophilic regions.

D. It has two hydrophilic regions and no hydrophobic region.

Answer: (C)

Explanation

A phospholipid is amphipathic because the same molecule contains hydrophobic and hydrophilic regions. A molecule lacking either type of region would not have this defining combination.

3. Two membranes are examined at the same temperature: one is rich in unsaturated fatty acids and the other in saturated fatty acids. Which comparison is expected?

A. The unsaturated-fatty-acid-rich membrane is more fluid.

B. The saturated-fatty-acid-rich membrane is more fluid.

C. The unsaturated-fatty-acid-rich membrane is less fluid.

D. The two membranes have equal fluidity because temperature is matched.

Answer: (A)

Explanation

Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids. Matching temperature does not erase the fluidity difference associated with fatty-acid saturation.

4. How does cholesterol affect phospholipid movement in a membrane at warm and cool temperatures?

A. It promotes tight packing at both temperatures, reducing fluidity in each case.

B. It restrains movement when warm and helps maintain fluidity when cool.

C. It increases movement when warm and promotes tight packing when cool.

D. It prevents tight packing when warm and restrains movement when cool.

Answer: (B)

Explanation

At warm temperatures, cholesterol restrains phospholipid movement, while at cool temperatures it helps maintain fluidity by preventing tight packing. The idea that cholesterol has the same rigidifying effect at both temperatures misses this temperature-dependent role.

5. What is the role of short carbohydrate chains on the external surface of a plasma membrane?

A. They act as markers that help cells recognize one another.

B. They catalyze reactions at the membrane surface.

C. They form channels that allow ions to cross the lipid bilayer.

D. They anchor peripheral proteins to the cytoskeleton.

Answer: (A)

Explanation

Surface carbohydrates vary among species, individuals, and cell types, enabling cell recognition. Channels that facilitate ion passage are formed by transport proteins, not carbohydrate chains.

6. At what rate can each aquaporin channel facilitate the passage of water molecules?

A. About $3 \times 10^{12}$ molecules per second

B. About $3 \times 10^6$ molecules per second

C. Up to $3 \times 10^3$ molecules per second

D. Up to $3 \times 10^9$ molecules per second

Answer: (D)

Explanation

Aquaporins are channel proteins that can facilitate the passage of up to three billion water molecules per second. The smaller rates underestimate the stated capacity, while the rate of $3 \times 10^{12}$ overestimates it.

7. What is the role of osmosis in a plant cell placed in a hypotonic solution?

A. Water moves equally in both directions, leaving the cell flaccid.

B. Water leaves the cell, causing the membrane to pull away from the cell wall.

C. Water enters the cell, making it swell until the cell wall resists further uptake.

D. Solutes enter the cell, increasing its concentration without changing its water content.

Answer: (C)

Explanation

In a hypotonic solution, the surrounding fluid has a lower solute concentration than the cell, so water enters by osmosis and the plant cell becomes turgid as its wall opposes further swelling. Water loss and membrane separation characterize a plant cell in a hypertonic environment.

8. During one cycle of the sodium-potassium pump, which sequence of ion movements occurs?

A. Two potassium ions move out and three sodium ions move in.

B. Two sodium ions move out and three potassium ions move in.

C. Three sodium ions move out and two potassium ions move in.

D. Three potassium ions move out and two sodium ions move in.

Answer: (C)

Explanation

The sodium-potassium pump exports three Na+ ions and imports two K+ ions per cycle, resulting in a net outward movement of one positive charge. The reversed ion directions describe the pump incorrectly.

9. How does receptor-mediated endocytosis differ from pinocytosis in the substances it takes up?

A. It selectively concentrates substances bound to membrane receptors, whereas pinocytosis takes up extracellular fluid and dissolved molecules nonspecifically.

B. It releases vesicle contents outside the cell, whereas pinocytosis brings secreted materials into the cell.

C. It moves substances against concentration gradients through carrier proteins, whereas pinocytosis moves them down gradients.

D. It encloses large particles with pseudopodia, whereas pinocytosis packages those particles in food vacuoles.

Answer: (A)

Explanation

Receptor-mediated endocytosis uses receptors to selectively concentrate specific bound substances in coated pits, while pinocytosis nonspecifically internalizes extracellular fluid and dissolved molecules. Pseudopodia and food vacuoles characterize phagocytosis, not receptor-mediated endocytosis.

10. Who proposed the 1935 sandwich model in which a phospholipid bilayer lies between two layers of globular proteins?

A. Theodor Schwann and Matthias Schleiden

B. J. Singer and G. Nicolson

C. Rosalind Franklin and Maurice Wilkins

D. Hugh Davson and James Danielli

Answer: (D)

Explanation

Hugh Davson and James Danielli proposed the sandwich model in 1935, describing a phospholipid bilayer between layers of globular proteins. Singer and Nicolson proposed the later fluid mosaic model, rather than the sandwich model.

11. Why does winter wheat increase the proportion of unsaturated lipids in its membranes during fall?

A. To reduce lateral movement by causing phospholipids to switch between membrane layers

B. To increase protein movement by anchoring proteins to the cytoskeleton

C. To make the membranes less permeable by promoting tighter lipid packing

D. To help prevent the membranes from solidifying as temperatures drop

Answer: (D)

Explanation

Unsaturated fatty acids help maintain membrane fluidity in cold conditions, reducing the risk of solidification. Tighter lipid packing would instead make the membrane less fluid.