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