When does passive transport occur across a cell membrane?

• A. Only when transport proteins pump ions to maintain gradients regardless of concentration differences
• B. When molecules move against their concentration gradient using energy from ATP hydrolysis
• C. Only when vesicles fuse with the plasma membrane to release substances outside the cell
• D. When molecules move down their concentration or electrochemical gradient without requiring cellular energy input (ATP)

Answer: (D)

Verified step by step guidance

1. Understand that passive transport is the movement of molecules across the cell membrane without the use of cellular energy (ATP).
2. Recognize that passive transport occurs when molecules move down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration.
3. Note that passive transport can also involve movement down an electrochemical gradient, which combines both concentration and electrical charge differences across the membrane.
4. Distinguish passive transport from active transport, which requires energy to move molecules against their concentration gradient.
5. Conclude that passive transport happens naturally due to the tendency of molecules to move toward equilibrium, without the involvement of transport proteins pumping ions or vesicle fusion.