Proteins are complex molecule composed of amino acids and necessary for the chemical processes that occur in living organisms.
Proteins are basic constituents in all living organisms.
The interior substance of plant cells is also composed partly of proteins. The importance of proteins is related more to their function than to their amount in an organism or tissue. Proteins are sometimes referred to as macromolecular polypeptides because they are very large molecules and because the amino acids of which they are composed are joined by peptide bonds. The vast majority of the proteins found in living organisms are composed of only 20 different kinds of amino acids, repeated many times and strung together in a particular order. Interactions among the amino acids cause the protein chain to assume a characteristic secondary structure and, under some circumstances, a tertiary structure.