Cell Membranes and Transport: Question 10

Worked solution

Why phagocytosis requires energy

Phagocytosis is a form of endocytosis: it moves large, solid material (here, a whole bacterium) into the cell by folding a region of the cell-surface membrane around it and pinching off a vesicle in the cytoplasm. Although this process does not rely on a single specific carrier protein moving one molecule at a time, it still requires ATP. Energy is needed to rearrange the cell’s cytoskeleton, extending the membrane folds around the bacterium and constricting the neck of the forming vesicle to pinch it off inside the cell. Without this energy, the membrane could not be actively remodelled in this way.

Why the other options are wrong

• A wrongly assumes that because no single carrier protein is involved, no energy is needed at all; energy is still required for the cytoskeletal movements that drive the process.
• C wrongly applies the logic of facilitated diffusion (movement of a small molecule down a concentration gradient through a protein) to phagocytosis, which is a bulk, energy-requiring process that engulfs a whole particle rather than moving individual molecules down a gradient.
• D incorrectly describes phagocytosis as the reverse of exocytosis in terms of membrane movement. In fact, phagocytosis (like all endocytosis) removes a piece of membrane from the cell surface to form the vesicle, while exocytosis adds a vesicle’s membrane to the cell-surface membrane, so phagocytosis is broadly the reverse of exocytosis, but not in the way option D describes (it does not remove membrane from the vesicle and add it to the cell surface).

Final answer

B. Phagocytosis requires ATP for the cytoskeletal rearrangements that extend the membrane around the bacterium and pinch off the vesicle, even though no single carrier protein transports the bacterium itself.