Active transport and vesicles

ATP hydrolysis splits off the last phosphate group, making ADP and releasing energy that proteins can use to do work.

• Correct: It splits off a phosphate group and releases energy:
• It adds a phosphate group and stores energy:
• It releases oxygen for the cell to use:

Passive transport moves a substance down its gradient, and the gradient itself supplies the energy. The cell spends no ATP.

• ATP split by the membrane protein:
• Correct: The substance's own gradient:
• Vesicles fusing with the membrane:

A vesicle is a small sphere of membrane that buds off one membrane, carries cargo, and fuses with another.

• A protein tunnel through the membrane:
• A fiber of the cytoskeleton:
• Correct: A small sphere of membrane that carries cargo:

Potassium leaks out of cells all the time, and the sodium–potassium pump brings it back in. With many pumps blocked, potassium keeps leaking out but is not returned, so it builds up in the plasma.

• Digoxin carries potassium into the blood: Digoxin contains no potassium. It raises plasma potassium by blocking the pump.
• Correct: Potassium is no longer pumped back into cells: Correct. Blocked pumps stop returning leaked potassium to the cells.
• His cells are now pumping extra potassium out into the plasma: The pump moves potassium into cells, not out. Blocking it cannot make cells pump more potassium out.
• Sodium is being converted into potassium: Ions are not converted into other ions in the body. The change comes from where potassium sits, not from making new potassium.

Stopping the pump lets sodium and potassium run down their gradients. The cell swells as sodium and water enter, and secondary active transport slows because the sodium gradient it spends is shrinking.

• Sodium concentration inside the cell: up. Sodium keeps leaking in down its gradient, and the blocked pumps no longer send it back out.
• Potassium concentration inside the cell: down. Potassium keeps leaking out, and the pumps no longer bring it back in.
• Cell volume: up. Sodium builds up inside and is no longer effectively held outside. Water follows the extra solute into the cell by osmosis, and the cell swells.
• Glucose uptake by the symporter: down. The symporter spends the sodium gradient. As sodium builds up inside, the gradient shrinks, so there is less energy to drag glucose in.

Sodium rushes into the cell down its steep electrochemical gradient, and the symporter couples that downhill movement to glucose's uphill movement. The sodium gradient itself was built by the sodium–potassium pump using ATP.

• Correct: Sodium moving into the cell down its electrochemical gradient: Correct. The sodium gradient powers the symporter; the pump built that gradient with ATP.
• Glucose moving down its own concentration gradient: Glucose is moving against its gradient here, so its own gradient cannot supply the energy.
• Heat released by the kidney tubule: Heat is not a usable energy source for moving a solute across a membrane.
• The pressure of fluid in the tubule: Fluid pressure drives filtration, not carrier transport of glucose into a cell.

The shape change opens the pump toward the outside, so the sodium ions are released into the extracellular fluid, not back into the cytoplasm.

• Three sodium ions bind to the pump on the cytoplasm side: This step is right. The cycle starts with sodium binding on the inside.
• ATP is split and a phosphate attaches to the pump: This step is right. Phosphorylation by ATP powers the shape change.
• Correct: The pump changes shape and releases sodium into the cytoplasm: This is the error. Sodium is released outside the cell; releasing it back inside would move nothing.
• Two potassium ions bind from the extracellular fluid: This step is right. The outward-facing pump picks up potassium from outside.
• The phosphate comes off, and potassium is released into the cytoplasm: This step is right. Losing the phosphate returns the pump to its inward-facing shape, releasing potassium inside.

Cells take up cholesterol-carrying particles by receptor-mediated endocytosis: surface proteins bind the particles and gather in coated pits that pinch off as vesicles. With those proteins defective, the particles stay in the blood. This is familial hypercholesterolemia.

• Phagocytosis: Phagocytosis engulfs large particles such as bacteria and does not depend on these specific surface proteins.
• Correct: Receptor-mediated endocytosis: Correct. Without working surface proteins, the specific uptake of cholesterol-carrying particles fails.
• Exocytosis: Exocytosis releases material from cells. The problem here is taking material in.
• Simple diffusion of cholesterol: The particles are far too large to diffuse across the bilayer. They enter by endocytosis.

Intestinal lining cells take up glucose on a symporter that carries sodium in with it. Each glucose molecule pulls sodium into the cells, and water follows the absorbed solutes by osmosis.

• Glucose makes the gut contents hypertonic to the cells: Making the gut contents hypertonic would draw water into the gut, which is the opposite of what helps.
• Glucose blocks the sodium–potassium pump: Blocking the pump would shrink the sodium gradient and slow the symporter, reducing absorption.
• Glucose opens aquaporins directly: Glucose does not open aquaporins. Water follows the absorbed solutes by osmosis.
• Correct: Glucose and sodium enter on a shared symporter: Correct. The sodium–glucose symporter brings sodium in with glucose, and water follows.