In passive transport, what primarily determines the direction of diffusion of a solute across a membrane that is permeable to that solute?

• A. The direction of bulk flow generated by cytoskeletal motor proteins
• B. The solute’s molecular weight alone (heavier molecules always diffuse toward lighter molecules)
• C. The cell’s ATP availability, which powers solute movement toward higher concentration
• D. The solute’s concentration gradient (net movement from higher concentration to lower concentration)

Answer: (D)

Verified step by step guidance

1. Understand that passive transport is the movement of molecules across a membrane without the input of cellular energy (ATP).
2. Recognize that diffusion is a type of passive transport where solutes move from an area of higher concentration to an area of lower concentration.
3. Identify that the key factor determining the direction of diffusion is the concentration gradient of the solute across the membrane.
4. Note that molecular weight alone does not determine diffusion direction; heavier molecules do not necessarily move toward lighter molecules.
5. Acknowledge that ATP availability and cytoskeletal motor proteins are involved in active transport and bulk flow, not passive diffusion.