Plasma Membrane Structure and Transport Questions and Answers

Answer: B

The protein provides a route, not a push, so the movement is still driven by the concentration gradient alone. Requiring a protein and requiring metabolic energy are quite separate matters, and only active transport needs both of them together.

Answer: C

Water moves down its own potential gradient, which means towards the solution with more dissolved particles dissolved in it. Aquaporins greatly speed the process, though a little water also crosses the bilayer directly between the phospholipids.

Answer: D

They form narrow channels selective for water and exclude ions, which is why they speed osmosis without dissipating the cell's electrical gradients. Kidney tubule cells are especially rich in them, which is how urine is concentrated.

Answer: A

Water enters because the surroundings are more dilute than the cytoplasm, and with no cell wall to resist the rising pressure the membrane may rupture. In the case of a red blood cell this is called haemolysis, and it releases haemoglobin into the surrounding fluid.

Answer: B

Water leaves the vacuole and the protoplast shrinks away from the rigid wall. The wall itself does not prevent water movement at all, since it is freely permeable to water and small solutes alike. Only the plasma membrane controls what crosses.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Water enters by osmosis and the protoplast presses outward against the wall, which resists and develops turgor pressure. That pressure opposes further entry, so a point is reached where the net flow stops and the cell is turgid but intact. An animal cell has no wall to develop such a pressure, which is why it lyses instead.

Answer: D

It swells and bursts, because distilled water is hypotonic to the cytoplasm and water moves inward. Shrinkage happens in the opposite case, when the surroundings are hypertonic, and the puckered appearance it produces is called crenation.

Answer: A

Working against a gradient costs energy, supplied directly by ATP or indirectly by another gradient. This is how a cell maintains internal concentrations quite different from those outside it, and why transport of this kind stops when the energy supply fails.

Answer: B

Three sodium ions per ATP over forty molecules gives 120. The same forty would import eighty potassium ions in return, and it is exactly this unequal exchange of three for two that makes the pump electrogenic and contributes to the membrane potential.

Answer: C

Correct answer: moves more positive charge out than in

The pump exports three sodium ions for every two potassium ions imported, so each cycle removes one net positive charge from the cell. That imbalance contributes directly to the resting membrane potential - a small part of it directly, and a much larger part indirectly by maintaining the gradients on which the potential depends.