The selective transportation of molecules and ions across membrane barriers ensures that the necessary substances are moved into and out of cells and cell compartments at the appropriate time and at useful rates. Small, non-polar molecules such as 02 and ethanol cross the membrane by simple diffusion.
The transportation of all other solutes, including all ions and most molecules of biological relevance, is mediated by specific proteins that provide solute-specific mechanisms for passage through an otherwise impermeable membrane.
Each such protein has at least one, and frequently several or even many hydrophobic membrane-spanning sequences that embed the protein within the membrane and determine its molecular mechanism of action.
Transport can either be downhill or uphill in relation to a solute&’s concentration or electrochemical potential. Downhill transport, called facilitated diffusion, is mediated by carrier proteins and channel proteins. The former function by alternating between two conformational states; examples include the glucose transporter and the anion exchange protein found in the plasma membrane of the erythrocyte.
Transport of a single kind of molecule or ion is called uniport. The coupled transport of two or more molecules or ions at a time may involve the movement of both solutes in the same direction (symport) or in opposite directions (antiport).
Channel proteins facilitate diffusion by forming hydrophilic transmembrane channels. Three important categories of channel proteins are ion channels (which are used mainly for transport of Na+, K+, Ca2+ and H+) as well as porins and aquaporins (which facilitate the rapid movement of various solutes and water, respectively).
Uphill, or active, transport requires energy and may be powered by ATP hydrolysis, the electrochemical potential of an ion gradient, or light. Active transport powered by ATP hydrolysis utilises four major classes of transport proteins called P-type, V-type, F-type, and ABC-type ATPases.
One widely encountered example is the ATP-powered Na+/K+ pump (a P-type ATPase), which maintains electrochemical potentials for sodium and potassium across the plasma membrane of anin cells.
Transport driven by an electrochemical potential usually depends on a gradient of either sodium ions (animal cells) or protons (plant, fungal, and many prokaryotic cells).
The writer is associate professor, Head, Department of Botany, Ananda Mohan College, Kolkata, and also fellow, Botanical Society of Bengal, and can be contacted at [email protected]