Question
A molecule moves down its concentration gradient using a transport protein in the plasma membrane. This is an example of:- A. diffusion- B. exocytosis- C. endocytosis- D. active transport- E. facilitated diffusion
Answer
The correct answer is:
E. facilitated diffusion
Two clues determine the answer:
- The molecule moves down its concentration gradient. Its net movement is from higher concentration toward lower concentration, so the process is passive rather than active transport.
- The molecule uses a membrane transport protein. Protein assistance distinguishes facilitated diffusion from simple diffusion directly through the lipid bilayer.
The other choices do not fit:
- Simple diffusion does not require a transport protein.
- Active transport moves a substance against its relevant gradient and requires an energy source, directly or indirectly.
- Endocytosis and exocytosis move material through membrane-bound vesicles rather than through a transport protein.
The prompt does not provide enough information to decide whether the protein is a channel or a carrier; either type can mediate facilitated diffusion.
Evidence boundary
The prompt establishes down-gradient, protein-mediated transport, which is enough to identify facilitated diffusion. It does not identify the solute, net direction across the named membrane sides, rate, or whether the protein is a channel or carrier.
Sources
These references support the concepts and methods used in the explanation above.