Amino acids are the alphabet of protein structure and determine many of the important properties of proteins. Beside the 20 amino acids found in building blocks of proteins, many additional biologically occurring amino acids serve other functions in the cells. Different organisms vary considerably in their ability to synthesise amino acids and therefore require these amino acids preformed in the diet, produced by plant or bacteria. These are called essential amino acids. Man and the Albino rat can make only 10 of the 20 common amino acids. The non-essential amino acids can be synthesised directly by the organism but they are added also to cell culture media to save energy for the cells.