Proteins are chains of amino acids. There are 20 standard amino acids that cells use to build proteins. Each has a central carbon, called the alpha carbon, bonded to an amino group, a carboxyl group, a hydrogen atom, and a side chain called the R group. The R group is what makes each amino acid different. It can be as simple as a hydrogen atom, as in glycine, or as complex as an indole ring, as in tryptophan.

Amino acids link together through peptide bonds. The carboxyl group of one amino acid reacts with the amino group of another, releasing water and forming a bond. The resulting chain is a polypeptide. The sequence of amino acids determines how the protein folds, and the folded shape determines its function. A single wrong amino acid can cause disease. Sickle cell anemia results from a single substitution of valine for glutamic acid in hemoglobin.

Nine of the 20 standard amino acids are essential, meaning humans cannot make them and must obtain them from food. The others are non-essential, meaning the body can synthesize them. Complete proteins, found in meat, eggs, and dairy, contain all nine essential amino acids. Plant proteins often lack one or more, which is why combining foods like rice and beans provides a complete protein profile.

Amino acids are also used as supplements, flavor enhancers, and precursors to neurotransmitters. Glutamate is an excitatory neurotransmitter and the source of umami taste. Tryptophan is a precursor to serotonin. Glycine is a calming neurotransmitter.