Chapter21

Chapter21 - BCH 4054 Fall 2000 Chapter 21 Lecture Notes...

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Chapter 21, page 1 BCH 4054 Fall 2000 Chapter 21 Lecture Notes Slide 1 Chapter 21 Electron Transport and Oxidative Phosphorylation Slide 2 Overview • Oxidation of NADH and CoQH 2 produced in TCA cycle by O 2 is very exergonic. • Some of the energy of oxidation is captured by synthesis of ATP from ADP and P i • The capture of energy requires a “coupling” of oxidation to phosphorylation • Coupling occurs through an intermediate proton electrochemical gradient across the inner mitochondrial membrane. Slide 3 Energetics of Redox Reactions • Oxidation-Reduction (Redox) reactions can be written as two “half-cell” reactions. A ox + B red A red + B can be written as the sum of: A ox + e A red and B red B ox + e • These “half-cell” reactions can be physically separated and the electron transferred by an electrical circuit. (See Fig 21.2)
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Chapter 21, page 2 Slide 4 Energetics of the Voltaic Cell • When the voltage difference between two half cell reactions is measured, the cells are at equilibrium. G overall = G chemical + G electrical = 0 G chemical = - G electrical = -nF E (under standard state conditions, G o ’ and E o ’) n = # of electrons, F = the Faraday, 96,485 coulombs/mol (Recall that 1 coulomb-volt = 1 Joule) The measurement of the voltage difference between two half cells is therefore a way to measure the free energy change of the chemical reaction. It tells you the amount of “useful work” (in this case electrical work) you can get out of the process. By conventions established, a spontaneous process (where G is negative) has a positive value for E. Slide 5 Standard Reduction Potentials • Voltage differences are additive If voltage between A and B is 0.15, and between B and C is 0.23, then the voltage between A and C would be 0.38 • Therefore we can create a scale of “relative” voltages by picking a “standard” half cell, and setting it to 0.0, measuring everything else relative to it. Slide 6 Standard Reduction Potentials, con’t. • The standard electrode is chosen as the “hydrogen” electrode, for the half cell reaction: H + + e & ½ H 2 (1 atm) (E o = 0.0 volt) • Voltages of half cells measured against this cell are “standard reduction potentials” the tendency for reduction to occur. The higher the number, the more easily reduced, or the stronger the substance is as an oxidizing agent.
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Chapter 21, page 3 Slide 7 Standard Reduction Potentials, con’t. • Reduction potentials vary with concentration, just as free energy change does. o x red re d ox oo o x red for AeA [ A ] [A] R T RT E E l n or E ln n F [ A ] n F [ A] +→ = -+ 37 ' 2 '7 o so correcting the hydrogen electrode to the "biological standard state" R T [H ] (8.3x1 0 kJ/mo l K)(298K ) [1 0] E E l n 0. 0 ln n F ] (1)(96.5 kJ/mol ) [1] EE (.0256)(ln1 0)E (.0256) ( 16.1 ) 0.41 volt + -- - - = + =+ = + = + - =- Note, the table in the book gives – 0.421 volt—difference due to rounding errors. Slide 8 Standard Reduction Potentials, con’t.
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Chapter21 - BCH 4054 Fall 2000 Chapter 21 Lecture Notes...

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