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PhysBiochWeektwo2011

PhysBiochWeektwo2011 - Tue In summary the PS II reaction...

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1 1 In summary, the PS II reaction center contains a special pair of chlorophyll a molecules that upon illumination eject a single electron. One chlorophyll a molecule is then reduced by an electron from water via a tyrosine residue of D1 protein . Tue Light Reaction is a Redox Rx X + Y X + + Y - X reduces Y, X transfers electron to Y Y oxidizes X, Y removes electron from X. X + oxidizes Y - , X removes electron from Y Y - reduces X + , Y transfers electron to X.
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2 3 Measurement of Redox Potential. Apparatus for the measure- ment of the standard oxidation- reduction potential of a redox couple. Electrons , but not X or X - , can flow through the agar bridge. direction of electron flow Redox couple Half cell Salt 25 o C, 1 M solns. Results X - + H + -- ½ H 2 H + is reduced to hydrogen gas. Half reactions: X - X + electron transferred to H + H + + electron ½ H 2 (Written this way for biochem). Reduction potential of H + /H 2 couple is taken to be zero volts for this expt. Negative redox potential likes to lose electrons, hates to accept electrons. Is a good reducing agent 4
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3 5 Redox (Midpoint) Potential The electrodes are connected to a voltmeter, and an agar bridge establishes electrical continuity between the half-cells. Electrons then flow from one half-cell to the other. The reduction potential of the X : X - couple is the observed voltage at the start of the experiment (when X, X - , and H + and H 2 are 1 M). This is where molecule half oxidized and half reduced. 6 Oxidant (gains elect) Reductant (loses elec) n 0 (V) Succinate + CO 2 -Ketoglutarate 2 -0.67 Acetate Acetaldehyde 2 -0.6 Ferredoxin (oxidized) Ferredoxin (reduced) 1 -0.43 2 H + H 2 2 -0.42 NAD + NADH + H + 2 -0.32 NADP + NADPH + H + 2 -0.32 Lipoate (oxidized) Lipoate (reduced) 2 -0.29 Glutathione (oxidized) Glutathione (reduced) 2 -0.23 FAD FADH 2 2 -0.22 Acetaldehyde Ethanol 2 -0.2 Pyruvate Lactate 2 -0.19 Fumarate Succinate 2 0.03 Cytochrome b (+3) Cytochrome b (+2) 1 0.07 Dehydroascorbate Ascorbate 2 0.08 Ubiquinone (oxidized) Ubiquinone (reduced) 2 0.1 Cytochrome c (+3) Cytochrome c (+2) 1 0.22 Fe (+3) Fe (+2) 1 0.77 1 / 2 O 2 + 2 H + H 2 O 2 0.82 Table 18.1. Standard reduction potentials of some reactions Cytochrome a (+3) Cytochrome a (+2) 1 0.29
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4 7 Standard redox potential 1. Oxidation-reduction potentials are ranked to some standard couple. 2. The standard oxidation-reduction couple is H + / H 2 3. Rather than using a standard free energy change ( D G or D G o ’), is given a value of E o in volts. 4. This is the standard redox potential E’ o , or a measure of the ability of a compound to oxidize something (25 o C, 1M). 5. E o is the voltage produced by a half-cell in which each member of the couple is present at a standard concentration under standard conditions (1 M concentration, 25 o C, etc). 6. A half-cell is where only one member of the redox couple is present. One contains the oxidized form, the other cell contains the reduced form.
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