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Chapter 22

Chapter 22. Photosynthesis to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham.

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Chapter 22

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  1. Chapter 22 Photosynthesis to accompany Biochemistry, 2/e by Reginald Garrett and Charles Grisham All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777

  2. Outline • 22.2 The Photoreactivity of Chlorophyll • 22.4 The Z Scheme of Photosynthesis • 22.7 Light-Driven ATP Synthesis - Photophosphorylation • 22.8 Carbon Dioxide Fixation • 22.9 The Calvin-Benson Cycle • 22.10 Regulation of Carbon Dioxide Fixation • 22.12 The C-4 Pathway of CO2 Fixation

  3. The Sun - Ultimate Energy 1.5 x 1022 kJ falls on the earth each day • 1% is absorbed by photosynthetic organisms and transformed into chemical energy • 6CO2 + 6H2O  C6H12O6 + 6O2 • 1011 tons (!) of CO2 are fixed globally per year • Formation of sugar from CO2 and water requires energy • Sunlight is the energy source!

  4. Photosynthesis General Aspects • Photosynthesis occurs in thylakoid membranes of chloroplasts - structures involving paired folds (lamellae) that stack to form "grana" • The soluble portion of the chloroplast is the "stroma" • The interior of the thylakoid vesicles is the "thylakoid space" or "thylakoid lumen" • Chloroplasts possess DNA, RNA and ribosomes

  5. Photosynthesis Consists of Both Light Reactions and Dark Reactions • The light reactions capture light energy and convert it to chemical energy in the form of reducing potential (NADPH) and ATP with evolution of oxygen • The dark reactions use NADPH and ATP to drive the endergonic process of hexose sugar formation from CO2 in a series of reactions in the stroma

  6. Water is the electron donor for Photosynthetic NADP+ Reduction • Equations 22.2 and 22.3 describe the light and dark reactions in green plants, respectively! • Equation 22.4 provides a more general version • Photosynthetic bacteria use H2S, isopropanol or other oxidizable substrates • The O2 we depend upon depends in turn on large amounts of photosynthesis on the earth!

  7. Chlorophyll Photoreactive, isoprene-based pigment • A planar, conjugated ring system - similar to porphyrins • Mg in place of iron in the center • Long chain phytol group confers membrane solubility • Aromaticity makes chlorophyll an efficient absorber of light

  8. The Photosynthetic Unit Many chlorophylls but only a single reaction center • The "unit" consists of several hundred light-capturing chlorophylls plus a pair of special chlorophylls in the "reaction center" • Light is captured by one of the "antenna chlorophylls" and routed from one to the other until it reaches the reaction center • See Figure 22.9

  9. Eukaryotic Photosystems PSI (P700) and PSII (P680) • All chlorophyll is part of either LHC, PSI or PSII • PSI absorbs at 700 nm • PSII absorbs at 680 nm • Chloroplasts given light at 680 and 700 nm simultaneously yield more O2 than the sum of amounts when each is used alone.

  10. What does each photosystem do? See Figure 22.11 • PSII oxidizes water (termed “photolysis") • PSI reduces NADP+ • ATP is generated by establishment of a proton gradient as electrons flow from PSII to PSI

  11. The Z Scheme An arrangement of the electron carriers as a chain according to their standard reduction potentials • PQ = plastoquinone • PC = plastocyanin • "F"s = ferredoxins • Ao = a special chlorophyll a • A1 = a special PSI quinone • Cytochrome b6/cytochrome f complex is a proton pump

  12. Oxygen evolution by PSII requires accumulation of four oxidizing equivalents • PSII (P680) cycles through five oxidation states • 1 e- is removed in each of four steps • Fifth step involves H2O oxidized to O2 + 4H+

  13. Structures of Reaction Centers R. viridis is a model! • Membrane proteins (as always) are resistant to crystallization (and X-ray diffraction studies) • Deisenhofer, Michel and Huber solved R.viridis structure in 1984 (Nobel Prize same year!) • Four peptides: L, M, H and cytochrome • No electron transfer appears to occur through M • See Figures 22.16, 22.18

  14. The Quantum Yield Amount of O2 evolved per photon • Four photons per reaction center - 8 total - drive the evolution of 1 O2, reduction of 2 NADP+, and the phosphorylation of 2 and 2/3 ATP

  15. Photophosphorylation Light-Driven ATP Synthesis • Electron transfer through the proteins of the Z scheme drives the generation of a proton gradient across the thylakoid membrane • Protons pumped into the lumen of the thylakoids flow back out, driving the synthesis of ATP • CF1-CFo ATP synthase is similar to the mitochondrial ATP synthase

  16. Cyclic Photophosphorylation ATP without NADPH! • The photo-excited electron removed from P700 returns to P700 in a pathway indicated by the dashed line in Figure 22.12 • Cyclic photophosphorylation depends only on PSI, not on PSII

  17. Carbon Dioxide Fixation A unique ability of plants, algae, etc. • Melvin Calvin at Berkeley in 1945 showed that Chlorella could take up 14CO2 and produce 3-phosphoglycerate • What was actually happening was that CO2 was combining with a 5-C sugar to form a 6-C intermediate • This breaks down to two 3-P glycerates

  18. Ribulose-1,5-Bisphosphate The CO2 Acceptor • Fixation is accomplished by ribulose bisphosphate carboxylase (oxygenase), aka rubisco • Probably the world's most abundant protein • Study the mechanism in Figure 22.24 • Rubisco is activated when carbamylated (CO2 added to Lys-201) and with Mg bound • RuBP (substrate!) is inhibitor and must be released from inactive rubisco by rubisco activase. Carbamylation and Mg then activate.

  19. The Calvin-Benson Cycle aka The Calvin Cycle • The set of reactions that transform 3-P- glycerate into hexose sugar • The only net CO2 fixation pathway in nature • A disguised gluconeogenesis pathway! • With some pentose phosphate pathway reactions thrown in.... • See Figure 22.25

  20. Regulation of CO2 Fixation Activities of Calvin cycle enzymes (in the stroma!) are coordinated with photosynthesis • Three effects: • Light-induced pH changes • Light-induced generation of reducing power (reduced ferredoxin and NADPH) • Light-induced Mg2+ Efflux from Thylakoids

  21. The C-4 Pathway for CO2 Fixation aka the Hatch-Slack Pathway • Not an alternative to Calvin cycle, nor even a net CO2 fixation pathway • Rather, it is a CO2 delivery system, which carries CO2 from the O2-rich leaf surface to interior cells where O2 won't compete in the rubisco reaction • Oxaloacetate and malate are the CO2 transporters • Read about Crassulacean acid metabolism

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