16 - Mechanisms of Carrier-Mediated Transport: Facilitated Diffusion, Cotransport, and Countertransport

Abstract

This chapter discusses the mechanisms of carrier-mediated transport, including facilitated diffusion, cotransport, and countertransport. The selective passage of hydrophilic solutes across the hydrophobic barrier, a physiological property known as “membrane permeability,” is mediated by the presence of membrane transport proteins that span the phospholipid bilayer. The selective and regulated passage of ions and nonelectrolytes across the cell membrane is an essential component of cellular homeostasis. The maintenance of cell pH and volume and the accumulation of nutrients for protein synthesis and cell metabolism are physiological processes that depend on membrane transport for cells to thrive. Transport mechanisms can be thermodynamically distinguished as active and passive transport. The chapter discusses electrochemical potential and carrier-mediated transport mechanisms. Carrier-mediated transport mechanisms discuss facilitated diffusion, cotransport, and countertransport. The facilitated diffusion of a solute may be inhibited in the presence of other solutes that interact with, but are not necessarily transported by the same transporter. The nature of interaction of the inhibitor with the transporter may be assessed by observing the effect of the inhibitor on the kinetic parameters featuring the transport mechanism. Active transport is defined as movement of a solute from a region of low electrochemical potential on one side of the cell membrane to a region of higher electrochemical potential on the opposite side. Passive transport is defined as movement of a solute from a region of high electrochemical potential on one side of the cell membrane to a region of lower electrochemical potential on the opposite side. Facilitated diffusion or uniport is the simplest form of passive carrier-mediated transport and results in the transfer of large hydrophilic molecules across the cell membrane. Cotransport or symport is a form of secondary active transport. Countertransport or antiport is a form of secondary active transport that like cotransport may also mediate net transfer of a solute across the cell membrane in a direction against the electrochemical potential gradient of the solute. Countertransport mechanisms may mediate the exchange of the same solutes (homoexchange) or different solutes (heteroexchange) and will mediate net solute transport only when exchanging different solutes.