Saturday, December 30, 2006

active transport

Active transport pumps require expenditure of energy, most often in the form of ATP, to transport hydrophilic macromolecules and ions across membranes against chemical gradients. The number of integral protein transporters in the membrane limits active transport.

Primary, or direct transport involves an energy expending conformational change in the membrane protein to transport a specific molecule or ion across the membrane. Secondary, or indirect transport utilizes energy directly to generate a transmembrane gradient down which ions move and then up or down which the coupled molecule or ion of interest is transported indirectly.

ATPases couple the hydrolysis of ATP (to ADP and Pi) with the transmembrane transport of ions against a concentration gradient. An example is Na+K+ ATPase, which pumps 3 Na+ ions out of the cell for 2 K+ ions it pumps into the cell. Because the pump moves ejects three Na+ for every two K+ moved inward, it generates a net electrical differential necessary for polarization. This electrical potential energy is essential for neuronal activity, and it supplies the energy needed for other types of transport such as symport and antiport. animation - Na K ATPase

Active transport of some substances against a concentration gradient employs the ATPase-derived energy stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to drive transporter membrane proteins. In a symport, the ATPase transported molecule and the coupled, co-transported ion move in the same direction. Conversely, the ATPase transported molecule and the coupled, co-transported ion move in the opposite direction in an antiport. animation - Na glucose symport

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