What macromolecule is composed of long, folded chains of amino acids?

• A. Nucleic acids
• B. Carbohydrates
• C. Lipids
• D. Proteins

Answer: (D)

Proteins are macromolecules built from long chains of amino acids that fold into three-dimensional shapes. These chains, held together by peptide bonds, start as a linear sequence (primary structure) and then twist and fold into local motifs (secondary structure) like alpha helices and beta sheets, eventually forming the final three-dimensional form (tertiary, and sometimes quaternary structure). The specific sequence and resulting folding determine the protein’s function, from speeding up reactions as enzymes to providing structure or transporting molecules. Nucleic acids are made from nucleotides, not amino acids; carbohydrates are polymers of sugar units; lipids aren’t formed from long chains of amino acids. So the description matches proteins.