Now, proteins are the molecules that basically act in signal transduction. So, for instance, some type of hormone can bind onto a protein and that's the protein that will create that signal inside the cell, that will lead to some type of process inside the cell. So proteins function as transporters. 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.