Transport Systems in Pseudomonas

Abstract

Nutrient acquisition and waste excretion are important functions of all living cells including bacteria. Cellular membranes composed of lipid bilayers are inherently impermeable to most nutrients and wastes because of their hydrophilicity. Therefore, living organisms have developed a variety of transport systems that accumulate or excrete a particular solute across the cytoplasmic membrane. These transport systems either equilibrate solutes across the membrane (facilitated diffusion) or use energy to concentrate solutes (active transport). In bacteria, most solutes are translocated across the cytoplasmic membrane by active transport systems, which allow accumulation of solute in chemically unmodified form against a concentration gradient. Such transport systems can be classified into two classes according to the mode of energy coupling (Harold, 1972; Konings et al., 1989): primary transport systems that are directly coupled to biochemical reaction to translocate solutes across the cytoplasmic membrane; and secondary transport systems that utilize electrochemical energy of some compound (mostly electrochemical potential of protons or Na+ ions) generated by metabolic reaction. Another important mechanisms for accumulation of solute employed by bacteria is group translocation, which couples the translocation of a solute across the membrane to the chemical modification of the solute.