物化生物學 生物體系之電子與能量轉移

Applications of Electrochemistry ATP Synthase Potential for a resting nerve cell

Membrane Equilibrium In a closed electrochemical system, the phase equilibrium condition for two phases  and 

ATP Synthase

Free-energy change during proton movement across a concentration gradient The movement of protons from the cytoplasm into the matrix of the mitochondrion.

Proton Pumping Proton pumping maintains a pH gradient of 1.4 units, then  pH =  G = RTΔpH = (8.315 × kJ/mol)(298K)(1.4) = kJ/mol Proton concentration gradient

Free-energy change during solute movement across a voltage gradient In mitochondria, electron transport drives proton pumping from the matrix into the intermembrane space. There is no compensating movement of other charged ions, so pumping creates both a concentration gradient and a voltage gradient. This voltage component makes the proton gradient an even more powerful energy source.

Membrane Potential  m =  in –  out =0.14 V  G =-nF  m =-(1)(96485)(0.14 ) = kJ/mol

Proton-motive force Proton-motive force (  P) is a  that combines the concentration and voltage effects of a proton gradient.  G=-nF  P = RT  pH + nF  m =(-7.99 kJ/mol)+( kJ/mol) = kJ/mol

ATP synthesis Mitochondrial proton gradient as a source of energy for ATP synthesis Estimated consumption of the proton gradient by ATP synthesis is about 3 moles protons per mole ATP.  G = 50 kJ/mol for ATP synthesis  G = (- 21.5) = kJ/mol The synthesis of ATP is spontaneous under mitochondrial conditions.

Potential for a resting nerve cell Goldman-Hodgkin-Katz equation

Resting Nerve Cell of a Squid P(K + )/P(Cl - )=2 P(K + )/P(Na + )=25 △  (K + )=-95 mV △  (Na + )=+57 mV △  (Cl - )=-67 mV (mmol/dm 3 ) K+K+ Na + Cl - int ext The observed potential for a resting squid nerve cell is about -70 mV at 25 o C.

Resting Nerve Cell of a Squid The observed potential for a resting squid nerve cell is about -70 mV at 25 o C. Hence Cl - is in electrochemical equilibrium, but K + and Na + are not. Na + continuously flows spontaneously into the cell and K + flows spontaneously out. Na + -K + pump