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Unformatted text preview: 1 1 Biochemistry Sixth Edition Chapter 13: Membrane Channels and Pumps Berg • Tymoczko • Stryer 2 single membrane channel ion fow? Patch-clamp technique channel status: time ----> N Y 3 Membrane permeability is provided by two types oF transporters : 1. Pumps: Use external energy source ( e.g . ATP hydrolysis; light absorption) to drive thermodynamically ‘uphill’ transport oF ions or molecules. ¡unction as energy transducers: hydrolysis-to- pumping. Carry out active transport . Move From low to high concentration=‘uphill’. 2. Channels: Ions fow thermodynamically downhill. e.g. oF passive transport , or Facilitated diFFusion . Move From high to low concentration=‘downhill’. •The expression oF transporters largely de¢nes the metabolic activities oF a given cell. e.g . glucose transporters have diFFerent aF¢nities For glucose and determine which cells have ‘¢rst call’ to this metabolite. 2 4 13.1) Many molecules require protein transporters to cross membranes •Lipophilic molecules freely diffuse across the membrane. •Spontaneously move from high to low concentration, as per 2nd Law of thermodynamics. •aka ‘simple diffusion’ •e.g. Steroid hormones (cholesterol) •Polar molecules cannot freely diffuse. • e.g. Na + is present at 143 mM outside cell, 14 mM inside cell; but Na + does not freely enter cell due to hydrophobic nature of membrane. •They pass through speciFc channels in the membrane. This process is facilitated diffusion (facilitated by the channel), aka passive transport (the energy is derived from the gradient itself). e.g. a nerve impulse allows Na + ions into cell. •Gradient is established in the Frst place by pumps against concentration gradient. This is called active transport. 5 ¡ree energy stored in concentration gradients can be quantiFed •¡or uncharged species: Δ G=RTln(c 2 /c 1 ), where the process is going from c1 to c2 across the membrane. •¡or charged species: Δ G=RTln(c 2 /c 1 ) + Z¡ Δ V, where the process is going from c 1 to c 2 across the membrane, which is -59 mV inside (i.e. membrane potential is -59 mV) •T is temp in K; R is the gas constant (0.001987 kcal/K mol); Z is the electrical charge of transported species; Δ V is the potential across the membrane; ¡ is the ¡araday constant (23.1 kcal/V mol). •59 mV is equivalent to a factor of 10 on LHS plot a.) ¡or neutral molecules b.) ¡or charged species 6 13.2) Two families of membrane proteins use ATP hydrolysis to pump ions and molecules across membranes: 1.) P-type and 2.) ABC transporters [Na + ] high (143 mM) [K + ] low (4 mM) [Na + ] low (14 mM) [K + ] high (157 mM) •Na +-K + ATPase •Mg 2+-dependent a mammalian cell •Energy to pump 3 Na + out and 2 K + in is +42.6 kJ/mol. (see HW--typo in text)....
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- Fall '07
- Chemistry, Ion channel, k+ channel, single membrane channel