ABSTRACT
This chapter deals with the rates of movement of substances across membranes during their transport or translocation. Two fundamentally different mechanisms of membrane transport exist: nonspecific permeation and specific, saturable transport. In the first type of transmembrane movement, no specific reactions take place between the permeant solute and the permeable membrane, although nonspecific interactions, such as stripping of hydration water, attraction by opposite electrical charges, and the like, are possible and, in fact, likely. Characteristics of specific transport type of transport are its specificity and its saturability, both indicating that a finite number of membrane proteins are involved in the process. Although the molecular mechanisms of specific transport type of membrane translocation are many, they all involve binding of the transported solute to a receptor site in the membrane, its movement across the membrane, and dissociation toward the other membrane face. From positive cooperativity in transport one can predict an interesting kinetic phenomenon that has been termed co-transport.
