Proteins are composed of amino acids and have a defined three-dimensional structure. This form dictates the function. Proteins are responsible for all the reactions and activities of the cell. The structure of the individual proteins is encoded in DNA in the cell nucleus. Cytoskeletal proteins control the shape and movement of the cell.

Proteins are synthesized on ribosomes. Mitochondrial proteins are responsible for cell respiration and the synthesis of ATP that provides cellular energy. Enzymes in the cell catalyze chemical reactions. Storage vesicles contin, and release, hormones and neurotransmittors. They act on receptors and control ion channels. In this way cells can communicate with each other and order proteins in the cell to work in concert with the entire organism.