Assume that the import of a particular amino acid across the plasma membrane is observed (1) to occur onlydown its concentration gradient and (2) to slow when a different but 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 fact that the import of the amino acid occurs only down its concentration gradient suggests that it is not actively transported, ruling out option D. The fact that the addition of a similar amino acid slows the import suggests that there is a specific and selective mechanism involved in its import, ruling out options A and B. Facilitated diffusion, option C, involves the use of a specific carrier protein to transport molecules down their concentration gradient, so this is the best choice. Option E, pinocytosis, involves the engulfing of extracellular fluid and is not a mechanism for the import of specific molecules like amino acids.

Assume That the Import of a Particular Amino Acid Across