[Secondary active transport]

Abstract

Secondary active transport is defined as the transport of a solute in the direction of its increasing electrochemical potential coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential. The coupling agents are membrane proteins (carriers), each of which catalyzes simultaneously the facilitated diffusion of the driving ion and the active transport of a given solute. The review starts with some considerations on the energetics followed by a presentation of the kinetics of secondary active transport. Examples of information which may be gained by such studies are discussed. In the second part, some examples of secondary transport are given; we also describe the characteristics of the corresponding carriers. The various transport systems presented are: the D-glucose/Na+ symport in brush-border membranes, the lactose/H+ symport in E. coli, the Na+/H+ antiport, the different transport systems in the inner mitochondrial membrane.