Active transport is mediated by integral membrane proteins or carriers that bind a substance on one side of the membrane, undergo a conformational change and then release the substance on the opposite side of the membrane. Carriers may be specific for a given substance (uniport), alternatively they may transport a combination of substances (symport) and finally they can exchange one substance for another (anti-port). Factors affecting rate of active transport include:
• Degree of carrier saturation
• Density of carriers on membrane
• The speed of carrier conformational change mernbrg ne mernbrg ne
Figure PG.14 Primary active transport
Active transport requires the expenditure of energy since it usually moves substances against a concentration or electrical potential gradient (i.e. against an electrochemical gradient). In primary active transport, energy is obtained directly from the hydrolysis of ATP and then catalysed by the carrier, which binds the released phosphate (Figure PG.14). Phosphorylation of the carrier produces covalent modulation of its structure. Na+K+ATPase is an antiport carrier responsible for maintaining transmembrane ion gradients of Na+ and K+. This carrier "pumps' three Na+ ions out of the cell in exchange for two potassium ions (Figure PG.15), both against their respective concentration gradients. Other ion pumps (uniport systems) move calcium (Ca2+ATPase) and hydrogen ions (H+ATPase) across cell and organelle membranes against electrochemical gradients.
Secondary Active Transport
In this process a symport carrier transports a substance and an ion (usually Na+) together. The carrier possesses two binding sites one for the substance and one for the ion. The substance binds to the first carrier site. The change in carrier conformation required to release the substance on the opposite side of the membrane is then powered by the ion binding to the second site, which produces allosteric modulation of the carrier structure. Since the ion always passes from high to low concentrations, there is no direct energy input required, and the energy for this secondary transport process is ultimately derived from the energy required to maintain the ion concentration gradient. The transported substance can travel in the same direction as the ion (co-transport) or in the opposite direction (counter transport). An example of such a system is the transport of glucose and Na+ in the gut (Figure PG.16).
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