Proteins are also called polypeptides because the covalent bond between two amino acids is a peptide bond.

• A. The covalent bond between amino acids is a peptide bond.
• B. They are made of nucleotides.
• C. They are all identical.
• D. They are composed of lipids.

Answer: (A)

The idea being tested is that proteins are called polypeptides because amino acids are linked by peptide bonds, which are covalent bonds joining the carboxyl group of one amino acid to the amino group of the next. This covalent linkage forms a long chain of amino acids—the polypeptide backbone—that folds into functional proteins. Because proteins are essentially polymers built from many amino acid units connected by these bonds, the name polypeptide makes sense.

The other statements don’t fit: nucleotides build nucleic acids like DNA and RNA, not proteins; proteins are not all identical—there is vast variety in the sequence and composition of amino acids; and lipids are a different class of biomolecules altogether, not the building blocks of proteins.