Microbes import the materials needed for growth and survival from their environment and export metabolites. As described in the pre- vious chapter, the cytoplasm is separated from the enviromnent by the hydrophobic cytoplasmic membrane, which is impermeable to hydrophilic solutes. Because of this permeability barrier exerted by the phospholipid component, almost all hydrophilic compounds can only pass through the membrane by means of integral membrane proteins. These are called carrier proteins, transporters or permeases (a Website devoted entirely to transport can be found at www-biology. ucsd-edu/~msajer/transport/). _ _ _ Solute transport can be classified as diffusion, active transport or group translocation according to the mechamsms involved. Diffusion does not require energy; energy is invested for active trans- port; and solutes transported by group tmnslocanon are Chenflcany modified during this process, Some solutes are accumulated in the cell against a concentration gradient of several orders of magnitude, and energy needs to be invested for such accumulation.