Proteins are macromolecules made of smaller units called

Proteins are large biological macromolecules (polymers) built from smaller repeating units called amino acids. Each amino acid has a central carbon atom bonded to an amino group (\(-\text{NH}_2\)), a carboxyl group (\(-\text{COOH}\)), a hydrogen atom, and a variable side chain (R group) that gives each amino acid its unique chemical properties.

Amino acids are linked together by peptide bonds, formed through condensation reactions between the amino group of one amino acid and the carboxyl group of the next, releasing a molecule of water. A chain of two amino acids is a dipeptide; a long chain is a polypeptide. One or more polypeptide chains fold into a specific three-dimensional shape to form a functional protein.

There are about 20 different amino acids used in biological systems. The specific sequence, number, and types of amino acids in a polypeptide chain determine the protein's structure and function.

The other options are not the monomers of proteins:

• Glycerol is a three-carbon alcohol that forms part of lipids (fats and oils) along with fatty acids.
• Fatty acids are long hydrocarbon chains with a carboxyl group; they are components of lipids, not proteins.
• Carboxylic acid is a broad chemical class characterised by the \(-\text{COOH}\) group. While amino acids do contain a carboxyl group, the term "carboxylic acid" alone does not specifically describe the building blocks of proteins.

Exam tip: Match each macromolecule to its monomer: proteins to amino acids, carbohydrates to monosaccharides (simple sugars), lipids to glycerol and fatty acids, and nucleic acids to nucleotides.