Which of these is found in all amino acids?

• A. —COOH
• B. —COH
• C. —OH
• D. both —COOH and —NH₂

Answer: (D)

Verified step by step guidance

1. Understand the basic structure of an amino acid, which consists of a central carbon atom (known as the alpha carbon) bonded to four different groups: an amino group (—NH2), a carboxyl group (—COOH), a hydrogen atom, and a variable side chain (R group).
2. Identify the functional groups that are common to all amino acids. The amino group (—NH2) and the carboxyl group (—COOH) are present in every amino acid, regardless of the side chain.
3. Recognize that the —COH group is not a standard component of amino acids. Instead, the —COOH group is the carboxyl group, which is a defining feature of amino acids.
4. Note that the —OH group is not a universal feature of amino acids. While some amino acids may have an —OH group in their side chain (such as serine or threonine), it is not present in all amino acids.
5. Conclude that the correct answer is both —COOH and —NH2, as these are the functional groups found in all amino acids, forming the backbone of their structure.