A protein molecule is a chain of up to several hundred amino acids and is folded onto a more or less compact structure. Because about 20 different amino acids are used by living matter in making proteins, the variety of protein types is enormous. In their biologically active states, proteins function as catalysts in metabolism and to some extent as structural elements of cells and tissues. Protein content in biomass (in mass percentage) can be estimated by multiplying the mass percentage nitrogen of the sample by 6.25.