**Question:** True about charge in amino acid?

A. Charge depends on the amino acid's side chain
B. Charge remains constant for all amino acids
C. Charge depends on the amino acid's primary sequence
D. Charge depends on the amino acid's secondary structure

**Core Concept:** Charge in amino acids is determined by the side chain of the amino acid. Amino acids can have different side chains, which carry either a positive (+) or negative (-) charge at physiological pH (7). This varying side chain property contributes to the formation of proteins with diverse properties and functions.

**Why the Correct Answer is Right:** Charge in amino acids is determined by the side chain of the amino acid because the side chain carries the positive or negative charge. In the context of protein structure, this contributes to the formation of proteins with various properties and functions due to the diverse side chain properties.

**Why Each Wrong Option is Incorrect:**

A. Charge depends on the amino acid's side chain (correct)
B. Charge remains constant for all amino acids (incorrect) - This is because amino acids have different side chains, resulting in diverse charges.
C. Charge depends on the amino acid's primary sequence (incorrect) - The primary sequence refers to the order of amino acids in a polypeptide chain, not the charge.
D. Charge depends on the amino acid's secondary structure (incorrect) - Secondary structures, such as alpha-helices and beta-sheets, do not determine the charge of amino acids; it is the side chain that determines the charge.

**Clinical Pearl:** Understanding the charge of amino acids is essential for understanding protein structure, function, and interactions with other molecules. This knowledge aids in understanding phenomena like protein-protein interactions, binding affinities, and enzymatic reactions involving amino acids.