Question

which is an example of coupled transport across a cell membrane? a) sodium ions diffuse down their concentration gradient while a sugar molecule is transported against its concentration gradient. b) sodium ions and a sugar molecule diffuse down their concentration gradients. c) sodium ions and potassium ions are pumped in different directions and against their concentration gradients. d) sodium ions diffuse down their concentration gradient.

Explanation:

Coupled transport involves the movement of one substance down its concentration gradient to drive the movement of another substance against its concentration gradient. In option A, sodium ions moving down their gradient while a sugar molecule is transported against its gradient is an example of coupled transport. Options B and D are simple diffusion scenarios. Option C is an example of active - transport of ions against their gradients but not coupled transport as described in the question.

Answer:

A. Sodium ions diffuse down their concentration gradient while a sugar molecule is transported against its concentration gradient.