Up-regulation increases receptor numbers while down-regulation decreases them. Which statement correctly contrasts these processes?

• A. Both increase receptor numbers
• B. Both decrease receptor numbers
• C. Up-regulation is a reversible process; down-regulation is not necessarily
• D. Up-regulation increases receptor numbers; down-regulation decreases receptor numbers

Answer: (D)

Up-regulation and down-regulation describe opposite changes in how many receptors sit on a cell’s surface in response to signaling. Up-regulation increases receptor numbers, boosting the cell’s sensitivity to the ligand. Down-regulation decreases receptor numbers, dampening responsiveness. The statement that directly contrasts these two effects—up-regulation increases receptor numbers while down-regulation decreases them—captures the fundamental difference in how receptor density changes.

The other options don’t fit because they either claim the same directional change for both processes or bring in reversibility without addressing the direct change in receptor numbers. Contextual note: in many systems, both processes can be reversible when signaling conditions change, but the crucial contrast asked about here is the opposite effect on receptor density.