Broadly speaking, proteins have two kinds of function. The first is structural: they make up many of the key structures in cells and tissues. Muscle, hair, and skin are all mainly made of protein. The second is functional: as enzymes they greatly speed up the chemical reactions in a living cell. All cellular life consists of a thousand, or more, chemical reactions, called metabolism, which transform the molecules eaten into energy, or into other molecules the cell needs to survive. The function of proteins is to speed up these reactions, often by over a million times faster. In addition they cause chemical reactions to occur which would not without the action of the protein.