Which element is found in the structure of proteins but not in carbohydrates or lipids?

• A. Carbon
• B. Nitrogen
• C. Oxygen
• D. Phosphorus

Answer: (B)

Proteins are unique among macromolecules because they are the only ones that contain nitrogen as a fundamental building block. This is due to the presence of amino acids, the monomers that make up proteins, each of which contains an amino group (–NH2) that includes nitrogen. Carbohydrates and lipids, on the other hand, consist primarily of carbon, hydrogen, and oxygen, and do not include nitrogen in their structures.

While phosphorus is sometimes found in certain lipids (such as phospholipids) and nucleic acids (like DNA and RNA), it is not a component of proteins. Therefore, nitrogen is the distinct element present in proteins but absent from both carbohydrates and lipids, making it the correct choice.