Assume that the transport of a particular amino acid across the plasma membrane is observed (1)to occur only down its concentration gradient and (2)to slow when a similar amino acid is added to the extracellular fluid.The movement of the amino acid through the membrane is most likely by
A)osmosis.
B)diffusion.
C)facilitated diffusion.
D)active transport.
E)pinocytosis.

Question

Quizplus · Accepted Answer

The observations suggest that the transport of the amino acid across the plasma membrane is passive, as it only occurs down its concentration gradient. Therefore, options D (active transport) and E (pinocytosis) can be eliminated. Moreover, the fact that the presence of a similar amino acid slows down the transport suggests that the amino acid is transported by facilitated diffusion, which involves the use of protein channels or carriers to facilitate the movement of solutes down their concentration gradient. Therefore, the best choice is C (facilitated diffusion). Osmosis (option A) is the movement of water molecules across a selectively permeable membrane, whereas diffusion (option B) is the movement of any solute across a membrane down its concentration gradient, regardless of whether it occurs through a protein channel/carrier or not.

Assume That the Transport of a Particular Amino Acid Across