Plasma Membrane Structure and Transport Questions and Answers

Answer: C

Lipids and many proteins drift sideways much as boats drift on water, while the sheet itself remains intact. The membrane is not a liquid, and calling it fluid describes the mobility of its parts rather than its state.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: an animal cell in a hypertonic solution loses water and crenates, and a plant cell in a hypotonic solution becomes turgid. Statement 3 has the direction wrong - a plant cell in a hypertonic solution loses water and the protoplast shrinks away from the wall, which is plasmolysis. Bursting requires water entering, and the wall prevents that in any case.

Answer: A

Water crosses in both directions at equal rates, so there is no net movement and the cell keeps its volume unchanged. Intravenous fluids are made isotonic to blood for exactly this reason, since anything else would swell or shrink the patient's own cells.

Answer: B

Water continues to cross the membrane in both directions; what ceases is any net movement. Confusing the absence of net change with the absence of any activity at all is a common misreading of what equilibrium means in a physical system.

Answer: C

Phospholipids form closed vesicles spontaneously in water, and these can be loaded with drugs to be delivered into cells by fusion. A lysosome, whose name sounds similar, is the organelle full of digestive enzymes and is quite a different thing.

Answer: A

The correct match is P-2, Q-3, R-4, S-1.

• Flippase moves aminophospholipids (phosphatidylserine, phosphatidylethanolamine) from the outer (extracellular) leaflet to the inner leaflet, using ATP (P-2).
• Floppase moves phospholipids the other way, from the cytosolic to the extracellular leaflet (Q-3).
• Lipase hydrolyses lipids, degrading phospholipids of either leaflet (R-4).
• Scramblase moves phospholipids in both directions down their concentration gradients, without ATP, randomising the two leaflets (S-1).

The other options swap the direction of flippase and floppase or give scramblase a directional role it does not have.

Answer: A

The Commission's answer is an enhancement in membrane fluidity.

Sterols act as a fluidity buffer in membranes. At low temperature, when lipid acyl chains tend to pack tightly into a rigid gel-like state, sterol molecules wedge between them and prevent that close packing, so the membrane stays more fluid than it otherwise would. Raising the sterol content is therefore one way cells keep their membranes functional in the cold.

A decrease in fluidity is the effect sterols have at higher temperatures, where they restrain chain motion, which is why it does not fit cells grown at 25 to 30 °C. Sterols reduce, not increase, permeability to water and small solutes, and stabilising membrane proteins is not their defining effect here.

Answer: A

The mechanism is group translocation.

In group translocation the solute is chemically modified as it crosses the membrane, so the product inside the cell differs from what entered. The bacterial phosphotransferase system (PTS) is the classic case: sugars such as glucose are phosphorylated during transport, using phosphoenolpyruvate as the energy and phosphate source, and arrive as sugar phosphates.

Simple diffusion, facilitated diffusion and osmosis are passive and leave the substance unchanged, so this multiple-select question has a single correct choice.

Answer: C

The correct option is P is false but Q is true.

After dilution the vesicles hold 50 mM KCl inside while the outside is almost pure water, so there is clearly a K⁺ (and Cl^-) concentration difference across the membrane: Q is true.

A membrane potential, however, needs a net movement of charge, which requires the membrane to be selectively permeable to one of the ions. A pure phospholipid bilayer is essentially impermeable to both K⁺ and Cl^-, so neither ion crosses and no charge separation develops: P is false. A potential would appear only if a K⁺-selective pathway such as the ionophore valinomycin were added, letting K⁺ leak out ahead of Cl^-.

So 'both true' and 'P true, Q false' fail on P, and 'both false' fails on Q.

Answer: B

The class that uses ATP is the pumps.

Pumps are ATP-driven transporters (ATPases) that move ions or molecules against their concentration gradient, using the energy of ATP hydrolysis directly. The Na⁺/K⁺-ATPase, which expels three Na⁺ and takes in two K⁺ per ATP, is the standard example; the Ca²⁺-ATPase and ABC transporters are others.

Ion channels let ions flow passively down their electrochemical gradient. Symporters and antiporters carry out secondary active transport: they move one solute against its gradient by coupling it to another solute (often Na⁺ or H⁺) moving down its gradient, so they use no ATP themselves.