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Quiz: Cell Membranes and Transport

Test your understanding of membrane structure and the transport mechanisms that move substances across it with these review questions.

1. What model describes the plasma membrane as a dynamic phospholipid bilayer with proteins embedded and free to move laterally?

  1. Lipid raft model
  2. Unit membrane model
  3. Fluid mosaic model
  4. Endosymbiotic model
Show Answer

The correct answer is C. The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a fluid phospholipid bilayer in which a mosaic of different proteins is embedded and can drift laterally within the lipid sea. The other terms refer to specific membrane microdomains or unrelated organelle theories, not the overall structural model.

Concept Tested: Fluid mosaic model

2. Which molecule buffers membrane fluidity by inserting between phospholipid tails, restricting excessive motion at high temperature and preventing crystallization at low temperature?

  1. Cholesterol
  2. Collagen
  3. Clathrin
  4. Connexin
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The correct answer is A. Cholesterol wedges between the fatty acid tails of phospholipids in animal cell membranes, acting as a fluidity buffer across a range of temperatures. Collagen is an extracellular matrix protein, clathrin coats endocytic vesicles, and connexin forms gap junction channels — none of these regulate lipid bilayer fluidity.

Concept Tested: Cholesterol and membrane fluidity

3. Why can small nonpolar molecules such as oxygen cross the plasma membrane freely while ions such as sodium cannot?

  1. Ions are too large to fit through membrane pores
  2. Nonpolar molecules bind to carrier proteins that pull them across
  3. Ions dissolve readily in the hydrophobic tails of the bilayer
  4. Nonpolar molecules dissolve in the hydrophobic tail region of the bilayer, while charged ions are repelled by it and need protein assistance
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The correct answer is D. The interior of the phospholipid bilayer is a hydrophobic environment formed by fatty acid tails. Small nonpolar molecules like oxygen dissolve directly into this region and diffuse across, whereas charged ions are electrostatically repelled by the nonpolar core and must rely on channel or carrier proteins to cross the membrane.

Concept Tested: Selective permeability

4. Why is osmosis classified as passive transport even though water appears to move toward the side with more dissolved solute?

  1. Water is pumped by a membrane ATPase to sites of high solute concentration
  2. Water moves down its own concentration gradient toward the region with less free water, requiring no energy input
  3. Water is co-transported with solutes as a form of secondary active transport
  4. Water crosses the membrane only through voltage-gated channels
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The correct answer is B. Adding solute to a solution lowers the concentration of free water molecules there. Water simply diffuses from the side with more free water to the side with less, which is exactly the definition of passive movement down a concentration gradient. No ATP is spent moving water directly; only ion pumps consume energy, and water follows them osmotically.

Concept Tested: Osmosis and water potential

5. What distinguishes primary active transport from secondary active transport?

  1. Primary active transport is directly powered by ATP hydrolysis; secondary active transport uses an electrochemical gradient established by a primary pump
  2. Primary active transport moves solutes down their gradient; secondary active transport moves them against their gradient
  3. Primary active transport occurs only in plants; secondary active transport occurs only in animals
  4. Primary and secondary active transport both require no energy input
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The correct answer is A. Primary active transporters, such as the Na⁺/K⁺-ATPase, hydrolyze ATP directly to power conformational changes that move solutes against their gradient. Secondary active transporters do not use ATP directly; instead they harness the potential energy stored in an electrochemical gradient, such as the Na⁺ gradient the pump creates, to move a different solute against its own gradient.

Concept Tested: Active transport mechanisms

6. A student places red blood cells into a 5% NaCl solution, which is hypertonic relative to the cytoplasm. What will most likely happen to the cells?

  1. The cells will swell and may lyse
  2. The cells will remain unchanged in size
  3. The cells will shrink and crenate as water exits into the more concentrated external solution
  4. The cells will actively pump water back out using ATP
Show Answer

The correct answer is C. In a hypertonic solution, the external solute concentration exceeds that of the cytoplasm, so the external water potential is lower. Water moves out of the cells by osmosis, causing red blood cells to shrink and develop the crenated (spiky) appearance characteristic of hypertonic exposure. Cells never expend ATP to move water directly across the membrane.

Concept Tested: Tonicity and cell response to osmotic environments

7. A researcher blocks the Na⁺/K⁺-ATPase pump in intestinal epithelial cells with a specific inhibitor. What effect would this most likely have on glucose absorption via the SGLT1 cotransporter?

  1. Glucose uptake would increase because SGLT1 would compensate
  2. Glucose uptake would be unaffected because SGLT1 uses ATP directly
  3. Glucose uptake would increase because the Na⁺ gradient would strengthen
  4. Glucose uptake would decrease because SGLT1 depends on the Na⁺ gradient maintained by the Na⁺/K⁺-ATPase
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The correct answer is D. SGLT1 is a secondary active transporter that co-transports glucose into the cell using the steep inward Na⁺ gradient established by the Na⁺/K⁺-ATPase. Blocking the pump would collapse that gradient over time, removing the driving force for glucose cotransport and sharply reducing glucose absorption, since SGLT1 itself does not hydrolyze ATP.

Concept Tested: Secondary active transport (cotransport)

8. A macrophage extends pseudopods to surround and engulf a bacterium, forming a large internal vesicle that later fuses with a lysosome. Which process does this describe?

  1. Pinocytosis
  2. Phagocytosis
  3. Receptor-mediated endocytosis
  4. Facilitated diffusion
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The correct answer is B. Phagocytosis is the engulfment of large particles, such as bacteria or cellular debris, through pseudopod extension and membrane fusion, forming a phagosome that subsequently fuses with a lysosome for digestion. Pinocytosis takes up small fluid droplets non-specifically, receptor-mediated endocytosis targets specific bound macromolecules, and facilitated diffusion does not involve vesicle formation at all.

Concept Tested: Phagocytosis

9. How do tight junctions and desmosomes differ in their function between adjacent animal cells?

  1. Tight junctions seal the paracellular space to prevent leakage, while desmosomes anchor intermediate filaments between cells to resist mechanical stress
  2. Tight junctions transmit electrical signals between cells, while desmosomes seal the extracellular space
  3. Both junction types perform identical sealing functions
  4. Desmosomes allow direct cytoplasmic communication, while tight junctions provide mechanical anchoring
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The correct answer is A. Tight junctions, built from claudins and occludins, form a continuous seal near the apical surface of epithelial cells that blocks molecules from leaking between cells. Desmosomes instead use transmembrane cadherins to link the keratin intermediate filaments of neighboring cells, creating a mechanical rivet that resists shear and tensile forces in tissues like skin and heart muscle.

Concept Tested: Cell junction structure and function

10. A mutant plant produces cell walls but fails to form functional plasmodesmata between adjacent cells. What is the most likely direct consequence?

  1. Increased turgor pressure in all cells
  2. Enhanced active transport across the plasma membrane
  3. Loss of direct cytoplasmic (symplastic) communication and transport of small molecules between adjacent cells
  4. Formation of additional cell walls between cells
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The correct answer is C. Plasmodesmata are membrane-lined channels that connect the cytoplasms of adjacent plant cells, allowing ions, sugars, and signaling molecules to move symplastically without crossing a membrane. Without functional plasmodesmata, cells would lose this direct route for cell-to-cell communication and would have to rely entirely on transport across the plasma membrane and cell wall, disrupting coordinated development.

Concept Tested: Plasmodesmata and symplastic transport