Common Core Structure of Amino Acids

Steps:

• Recall that all amino acids share a central alpha carbon bonded to a hydrogen atom, a carboxyl group (-COOH), and an amino group (-NH2).
• Identify the variable part as the R group (side chain), which differs among amino acids.
• Examine each diagram's shaded region to find the one highlighting the alpha carbon with H, -COOH, and -NH2, excluding the R group.
• Select the diagram where shading covers only this universal backbone.

Why B is correct:

• Diagram B shades the alpha carbon and its attachments (H, -COOH, -NH2), matching the defining structure of all amino acids per standard biochemistry.

Why the others are wrong:

• A shades only the carboxyl group, which is common but incomplete without the full core.
• C shades the R group, which varies and is not universal.
• D shades the entire molecule, including the unique R group.

Final answer: B