A Level Biology Practice Exam 2026 – 400 Free Practice Questions to Pass the Exam

Polypeptides and proteins are composed of amino acids, which serve as their fundamental building blocks. Each amino acid contains a central carbon atom, an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain (R group) that determines the specific characteristics and chemical properties of that amino acid.

When amino acids link together through peptide bonds, they form polypeptides, which can fold and coalesce into the three-dimensional structures that define proteins. The order and arrangement of these amino acids determine the protein’s unique structure and function, making amino acids essential for the synthesis of proteins essential for various biological processes, including catalyzing metabolic reactions, DNA replication, responding to stimuli, and transporting molecules from one location to another within cells.

Other choices present distinct classes of biomolecules with different roles; carbohydrates are primarily involved in energy storage and structural functions, nucleotides are the building blocks of nucleic acids like DNA and RNA, and fatty acids are components of lipids, which serve as energy sources and structural components of cell membranes. Thus, amino acids are the exclusive building blocks for polypeptides and proteins.