California Science Test (CAST) Practice Exam

Molecules built from amino acids that become large and functional are proteins. Proteins are polymers formed when many amino acids link together through peptide bonds to create long chains that fold into specific three‑dimensional shapes. Those shapes enable a wide range of jobs in the cell, from speeding up chemical reactions (enzymes) to providing structure (collagen) and carrying substances (hemoglobin). The term here points to a particular kind of polymer that specifically consists of amino acids and has biological roles, which is why protein is the best answer. A general polymer is a large molecule made of repeating units, but it doesn’t name the specific type made from amino acids. A monomer is a single amino acid, not the entire molecule. An ester is a chemical group, not the large biomolecule described.