Which functional groups are present in the chemical structure of every standard amino acid?

• A. A phosphate group and a carboxyl group
• B. An amino group and a carboxyl group (typically as $N_{H_3}+$ and $C_{O_2}-$ at physiological pH)
• C. An aldehyde group and a hydroxyl group
• D. A sulfhydryl group and an amino group

Answer: (B)

Verified step by step guidance

1. Recall the general structure of a standard amino acid, which includes a central (alpha) carbon atom bonded to four groups: an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group).
2. Identify the functional groups that are common to all standard amino acids regardless of their side chains. These are the amino group (–NH\_2) and the carboxyl group (–COOH).
3. Understand that at physiological pH (~7.4), the amino group typically exists in its protonated form as –NH\_3\^+, and the carboxyl group exists as its deprotonated form –COO\^−. This ionization state is important for amino acid behavior in biological systems.
4. Note that other groups listed in the options, such as phosphate, aldehyde, hydroxyl, or sulfhydryl, are not present in every standard amino acid. For example, only cysteine contains a sulfhydryl group, and phosphate groups are not part of amino acid structures.
5. Conclude that the correct functional groups present in every standard amino acid are the amino group and the carboxyl group, typically in their ionized forms at physiological pH.