Nutition
Whether found in humans or in single-celled bacteria, proteins are composed of units of about 20 different amino acids , which, in turn, are composed of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. In a protein molecule these acids form peptide bonds—bonds between amino and carboxyl (COOH) groups—in long strands (polypeptide chains). The almost numberless combinations in which the acids line up, and the helical and globular shapes into which the strands coil, help to explain the great diversity of tasks that proteins perform in living matter. To synthesize its life-essential proteins, each species needs given proportions of the 20 main amino acids. Although plants can manufacture all their amino acids from nitrogen, carbon dioxide, and other chemicals through photosynthesis , most other organisms can manufacture only some of them. The remaining ones, called essential amino acids, must be derived from food. Eight essential amino acids are needed to maintain health in humans