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Lect23.ChannelsPumps

Lect23.ChannelsPumps - LECTURE 23 Membrane Pumps...

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LECTURE 23: Membrane Pumps, Cotransporters and Channels Reading: Berg, Tymoczko & Stryer: Chapter 13 Lipid membranes are barriers to diffusion of aqueous-soluble molecules Pumps, transporters & channels enable SELECTIVE molecular transport across membranes PUMPS move molecules uphill AGAINST their concentration gradients Pumping is rendered energetically favorable by ACTIVE TRANSPORT: The coupling to an expenditure of energy, such as hydrolysis of ATP COTRANSPORTERS couple the passage of molecule A across a membrane to the passage Of molecule B across membrane in same or opposite direction Transport of molecule A uphill AGAINST its gradient is made energetically favorable By coupling to molecule B transport down its gradient CHANNELS allow passage of molecules downhill from high-to-low concentration Process is energetically favorable, and is called PASSIVE TRANSPORT or FACILITATED DIFFUSION PUMPS CHANNELS Na + - K + ATPase Gastric H + - K + ATPase Glucose Transporters Voltage-Gated Ion Channels Ligand-Gated Ion Channels COTRANSPORTERS Lactose Permease Na + - Ca + Exchanger
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Biological Functions of Pumps Increase concentration of an ion to enable a biochemical process (e.g., gastric proton ATPase lowers pH to favor food-digesting proteases) Decrease concentration of an ion to prevent a biochemical process (e.g. muscle sarcolemmal Ca+ - ATPase pumps calcium out of muscle cytoplasm to allow for muscle relaxation) Generate molecule concentration gradients that serve as energy reservoirs for subsequent physiological processes The free energy stored in a concentration gradient is quantifiable: G = 2.3RT log 10 (c out /c in ) When the pumped molecule is charged, both concentration gradient and voltage across the membrane determine the stored free energy G = 2.3RT log 10 (c out /c in ) + ZF V
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P-Type ATPase Pumps
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