They also function as enzyme receptors, and they mediate energy storage. For instance, one will discuss the electron transport chain. The electron transport chain is basically a series of proteins that are found on the inner membrane of the mitochondria. And what the electron transport chain does is it ultimately establishes a proton gradient. And what that means is the electron transport chain are these proteins that create an electric potential difference between the two sides of the membrane of the mitochondria, and that stores energy across the cell membrane. So proteins are important because they mediate energy storage, as well as act as enzymes, receptors and transporters.