AP Biology: Membrane Transport Practice Questions

Explanation. Oxygen is a small nonpolar molecule that can pass through the hydrophobic interior of a phospholipid bilayer. Its net movement down a concentration gradient is simple diffusion and does not require ATP or a transport protein.

Explanation. Facilitated diffusion uses a membrane protein to move a substance down its concentration gradient without an energy input. The use of a protein alone does not make transport active. Primary active transport requires an energy source, while phagocytosis and exocytosis involve vesicles.

Explanation. At equal pressure, the solution with the greater concentration of dissolved sucrose has lower water potential. Water initially moves by osmosis from X, the more dilute solution, toward Y, the more concentrated solution. Sucrose remains on its original side because the membrane blocks it.

Explanation. The external solution is hypertonic relative to the cell. Water can cross the plasma membrane and moves out by osmosis, causing the cell to shrink. Animal cells do not have a rigid cell wall that would prevent this volume change.

Explanation. Water entering the plant cell increases internal pressure. The cell wall resists expansion, producing turgor pressure that opposes additional water entry. Ribosomes, the nucleolus, and chromosomes do not provide the external mechanical resistance responsible for turgidity.

Explanation. The pump requires ATP directly, so its primary active transport is impaired when ATP is unavailable. The other listed movements are passive and do not directly consume ATP. Over time, loss of pumping can also change the gradients that drive passive movement.

Explanation. This is secondary active transport: sodium moving down its electrochemical gradient supplies energy to move glucose against its concentration gradient. The cotransporter does not use ATP directly, although an ATP-powered pump can maintain the sodium gradient.

Explanation. Aquaporins provide channels through which water can move rapidly. They increase water permeability and therefore the initial rate of osmosis under the same driving conditions. They do not use ATP or determine the direction of net water movement, which depends on the water-potential difference.

Explanation. In an isotonic environment, there is no sustained net movement of water into or out of the cell. Individual water molecules still cross the membrane in both directions. Equal average rates explain a stable cell volume without requiring water movement to stop.

Explanation. A fixed number of carriers can transport only a limited number of molecules per unit time. Once the carriers are occupied and cycling near their maximum rate, adding more nutrient produces little further increase. Saturation is compatible with facilitated diffusion and does not show that ATP is required.

Explanation. Receptor-mediated endocytosis selectively brings receptor-bound material into a cell by forming an inward vesicle. Exocytosis releases material when a vesicle fuses with the plasma membrane. Diffusion and osmosis do not package receptor-bound cargo into vesicles.

Explanation. Exocytosis occurs when an intracellular vesicle fuses with the plasma membrane and releases its contents outside the cell. Endocytosis brings material inward. Neither facilitated diffusion nor osmosis explains the fusion of a protein-containing vesicle with the plasma membrane.