Tripeptide molecule. Computer-generated molecular model of a tripeptide molecule (made up of three amino acids). The amino acids are from left: glycine (C2.H5.NO2), cysteine (C3.H7.NO2.S), and alanine (C3.H7.NO2). Colored spheres represent different atoms: carbon (large white), hydrogen (small white), sulphur (yellow), nitrogen (blue), oxygen (red). Amino acids are the building blocks of proteins, which control the biochemical activity inside all living cells. There are 20 types of amino acids but millions of different proteins. Genes specify exactly how the types of amino acids are put together to make a protein, in this case a short tripeptide molecule.