Chapter22

Chapter22 - BCH 4054 Fall 2000 Chapter 22 Notes Slide 1...

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Chapter 22, page 1 BCH 4054 Fall 2000 Chapter 22 Notes Slide 1 Photosynthesis 2 Magnitude of the Process • 1.5 x 10 22 kJ energy per day from the sun • 1% transduced by photosynthesis into chemical energy • 10 11 tons of CO 2 fixed globally per year • In green plants, the process occurs in the chloroplast (See Fig. 22.2) Notice membrane organization of the chloroplast. An internal thylakoid membrane is folded into structures called lamellae that stack to form grana. The interior of the thylakoid vesicles is the “lumen”, while the space outside the thylakoid vesicles but inside the chloroplast membrane is called the stroma. Chloroplasts, like mitochondria, contain some DNA that codes for some of its proteins. 3 Overall Reaction In photosynthetic bacteria, H 2 A can be H 2 S, isopropanol, or other oxidizable substrate. light 2 2 6 1 2 62 Opposite of respiration: 6 C O 6 H O CHO + 6 O +  → o k J kJ G ' 286 0 , or +477 mol of glucos e mol of C reduced =+ light 2 222 Hydrogen Hydrogen Reduced Oxidized acceptor van Niel's generalized equation: CO + 2 HA (CHO) 2 A HO donor acceptor donor
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Chapter 22, page 2 Slide 4 Overall Reaction, con’t. • Water is split, not CO 2 . • Shown by experiments with H 2 18 O • Can be separated into two reactions: • O 2 generation (the light reaction) • CO 2 reduction (the dark reaction) • See Figure 22.4. light 2 2 2 22 Better representation: C O 2H O (CHO )O HO +   ++ 5 The Light Reaction 2 H 2 O + 2 NADP + + x ADP + x P i nh ν O 2 + 2 NADPH + 2 H + + x ATP + x H 2 O + o' o Reducing NADP to NADPH requires k J kJ G = -nFE ' (2)(96. 5 )(1.136 volts) = + 219 mo l vol t mol NADPH or 438 mol = -- - 2 of O produced 2 Making ATP requires 30.5 mol of ATP or an additional 30.5x mol of O produced We will see that at least 3 ATP’s will be required in the Dark Reaction, so that more than 530 kJ of light energy are needed per mole of oxygen produced. 6 The Dark Reaction 6 CO 2 + 12 NADPH + 12 H + + 18 ATP + 12 H 2 O C 6 H 12 O 6 + 12 NADP + + 18 ADP + 18 P i o k J kJ 12 x -21 9 for NADPH and 18 x -30.5 for ATP mo l mol or a G ' of -2628 kJ - 549 kJ = -3177 kJ energy input 2 More than enough for the reduction of 6 CO which required + 2860 mol Remember, these numbers are standard free energies . The actual energy need is greater in order to push the reaction towards completion.
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Chapter 22, page 3 Slide 7 Harvesting Light Energy • Chlorophyll is the main pigment for trapping the energy of light quanta. • See structure, Fig. 22.5 • See Absorption spectrum, 22.6 • Two major energy levels corresponding to major absorption peaks • ~ 450 nm and ~ 700 nm • (slightly different for chlorophyll a and b) 8 Harvesting Light Energy, con’t. • Photochemistry occurs generally from the lowest singlet state. (Higher energy states decay rapidly to lowest state). dec ay 700 nm 450 nm E E 1 E 2 3 4 81 9 -1 9 23 h c (6.625x1 0 Js)(3.00x10m s) E 700x1 0m J quanta = 2.84x1 0 x6.02x10 quantu m mol = 171 kJ per mol of quanta l -- - == 9 Harvesting Light Energy, con’t.
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Chapter22 - BCH 4054 Fall 2000 Chapter 22 Notes Slide 1...

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