Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

A free amino acid dissolved in water at neutral pH carries both a negative charge and a positive charge. Which feature of its functional groups produces this state?

A.Its sulfhydryl group donates protons to nearby bases.
B.Its methyl group attracts electrons strongly enough to separate charges permanently.
C.Its carboxyl group donated a proton while its amino group accepted one.
D.Its hydroxyl group splits into free ions when placed in solution.

Explanation

Core Concept

The carboxyl group behaves as an organic acid and releases a proton at neutral pH, leaving behind a negatively charged COO- group. The amino group acts as a base and picks up an extra proton, becoming a positively charged NH3+ group. Both ionizations occur readily in water, so the molecule exists as a zwitterion bearing one full positive and one full negative charge and a net charge near zero. These opposite charges later permit salt bridges that help stabilize folded protein structures.

Correct Answer

CIts carboxyl group donated a proton while its amino group accepted one.

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