Equilibrative and Concentrative Transport Mechanisms

Publisher Summary This chapter elaborates equilibrative and concentrative transport mechanisms. The processes of drug absorption, distribution, metabolism, and elimination include membrane transport steps that traditionally have been thought of as being mediated by passive diffusion. Research in vitro and in vivo has defined four basic mechanisms of transport across biological membranes: passive diffusion, facilitated diffusion, carrier-mediated transport, and carrier-mediated active transport. Active transport may be subdivided into primary transport, which is directly coupled with substrate oxidation or high-energy phosphate hydrolysis, or secondary transport, which is coupled with cotransport of another molecule or ion down its thermodynamic gradient. The basic principles of transport across a semipermeable membrane and the relevant thermodynamic and flux equations governing transport are well established. In the case of active transport, the movement of the substrate is coupled with some other energetic process, such as cotransport of another substrate or ion according to its electrochemical potential gradient or the hydrolysis of adenosine triphosphatase (ATP). The uptake mechanisms dependent on membrane trafficking are elaborated in the chapter.