The basic structural unit (monomer) of a protein molecule is:

Concept

Polymers are large molecules built by joining smaller repeating units called monomers.

In proteins, amino-acid monomers join through peptide bonds to form polypeptide chains.

Application

The question asks for the monomer that builds a protein. Amino acids link in chains to form polypeptides, which fold into functional proteins.

Contrast

• Glucose is a monosaccharide and a building block of many carbohydrates, not proteins.

• Fatty acids combine with glycerol to form many lipids; they are not protein monomers.

• Vitamins are micronutrients that support metabolism; they are not repeating structural units of proteins.

Cross-check

Hydrolysing a polypeptide breaks its peptide bonds and yields amino acids, confirming the monomer.

Therefore, the basic structural unit of a protein molecule is an amino acid.