Which or the following substances is a polypeptide?

1. Analyze the question The question asks which substance is a polypeptide. This is a biochemistry (organic chemistry) question about natural polymers.

2. Explain the concept

• Amino acids are the building blocks of proteins. Each contains an amino group (-NH₂) and a carboxyl group (-COOH).
• When amino acids join, the -COOH of one reacts with the -NH₂ of another, losing water. This forms a peptide bond (-CO-NH-).
• Many amino acids joined by peptide bonds form a polypeptide. Proteins are made of one or more polypeptide chains.

3. Solution: check each option

• A. Starch: a polymer of glucose units joined by glycosidic bonds. It is a polysaccharide, not a polypeptide.
• B. Glycogen: also a polysaccharide of glucose (the animal energy store). It is not a polypeptide.
• C. Protein: a polymer of amino acids joined by peptide bonds. This is a polypeptide.
• D. Fats: esters of glycerol and fatty acids (triglycerides). They are not polymers and contain no peptide bonds.

4. Answer The correct option is C. Protein.

5. Exam tip Match the polymer to its monomer:

• Polysaccharides (starch, glycogen, cellulose) come from monosaccharides, with glycosidic links.
• Polypeptides (proteins) come from amino acids, with peptide links.
• Fats and oils come from glycerol + fatty acids, with ester links.

The "pep" in polypeptide points to peptide bonds, and so to proteins.