Like other biological macromolecules such as polysaccharides and nucleic acids , proteins are essential parts of organisms and participate in virtually every process within cells . Many proteins are enzymes that catalyse biochemical reactions and are vital to metabolism . Proteins also have structural or mechanical functions , such as actin and myosin in muscle and the proteins in the cytoskeleton , which form a system of scaffolding that maintain cell shape . Other proteins are important in cell signaling , immune responses , cell adhesion , and the cell cycle . In animals , proteins are needed in the diet to provide the essential amino acids that cannot be synthesized . Digestion break the proteins down for use in the metabolism .