Lipids, amino acids and proteins

Proteins stand first among the organic substances of the cell in amount and importance; they contain about 50–54 % carbon, 21–23 % oxygen, 6.5–7.3 % hydrogen, 16 % nitrogen and a little sulfur. The monomer of a protein is the amino acid: general formula H₂N–CH(R)–COOH, that is an amino group (–NH₂), a carboxyl group (–COOH) and a radical (R); amino acids differ only in the radical. Proteins contain 20 kinds of amino acids. When two amino acids join, a molecule of water is released from the –COOH group of one and the –NH₂ group of the other, and they are linked by a peptide bond (–CO–NH–); two form a dipeptide, many a polypeptide, and a chain of n amino acids has n − 1 peptide bonds. Plants and many microorganisms synthesise all amino acids, while humans get about nine (the essential ones) only from food; children may need a few more. Today many of them are produced industrially with microorganisms and biotechnology.