Cells are surrounded by a membrane which separates intracellular from extracellular processes and allows controlled contact and exchange of the intracellular space with the environment. Communication is made possible by integral membrane proteins at the interface between intra-and extracellular space, thus establishing the contact to neighbouring cells and the surrounding medium. The membrane proteins can function either as signal transducers or as transporters. Transport proteins (permeases) transfer molecules across cell membranes, which can have nutritional or informational value for the cell. By using these transduction pathways, external signals can exert control on biochemical processes inside the cell. As a matter of fact, one molecule can be a nutrient and a regulatory signal at the same time.