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BIOL 205 :: Photosynthesis Lecture 1 Introduction and the light reactions

BIOL 205 :: Photosynthesis Lecture 1 Introduction and the light reactions. What is photosynthesis?. The process of converting solar energy into chemical energy. Can use only water and carbon dioxide to create sugars = chemical energy.

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BIOL 205 :: Photosynthesis Lecture 1 Introduction and the light reactions

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  1. BIOL 205 :: Photosynthesis Lecture 1 Introduction and the light reactions

  2. What is photosynthesis? • The process of converting solar energy into chemical energy. • Can use only water and carbon dioxide to create sugars = chemical energy. • Responsible for removal of ~ 200 billion tons of C from the atmosphere yearly.

  3. 6CO2 + 12 H2O + hv C6H12O6 +6O2 +6H2O What is photosynthesis? • hv designates light • you should memorize this equation!! you will see it again O2 = oxygenic photosynthesis

  4. What is photosynthesis? • 2 stages of photosynthesis • Light reactions and Dark reactions • Light reactions convert sunlight into chemical energy (ATP + NADPH) • Dark reactions use those products to form sugars (stored chemical energy)

  5. Overview of Photosynthesis Dark Reactions CO2 ATP NADPH Light Reactions Sugars H2O + CO2 O2

  6. Where does photosynthesis take place? • Prokaryotes • Prokaryotes have both anoxygenic and oxygenic • Cyanobacteria have oxygenic • -Photosynthesis on thylakoids (from plasma membrane) • -Cyanobacteria are source of eukaryotic photosynthesis • Eukaryotes • Oxygenic only • Takes place in chloroplasts

  7. Chloroplast Morphology

  8. Chloroplast Morphology-Terms • Inner Membrane • Outer Membrane • Thylakoid • Thylakoid lumen • Stroma • Granum

  9. How can light provide energy for plants? • Light is composed of particles: photons • Light behaves like a wave • Can e described w/ wavelength & frequency • Only a small portion of the electromagnetic spectrum.

  10. The electromagnetic spectrum PAR = photosynthetically available radiation

  11. Pigments • Pigment = a light absorbing molecule • Associated with the thylakoid membranes • Chlorophyll • Chl a and Chl b (Chl c in some algae) • Xanthophylls • Carotenoids • ß-carotene

  12. Porphyrin ring delocalized e- Phytol tail Chlorophyll • Chl a has a methyl group • Chl b has a carbonyl group

  13. Different pigments absorb light differently

  14. Different pigments absorb light differently

  15. 2-minute quiz 1. What is the relationship between the structure of chlorophyll and its location in the chloroplast? 2. Why are plants green?

  16. Light Reactions 1: Light capture and redox • 2 spatially & functionally distinct units = Photosystems • Photosystem II = 1st stage • Photosystem I = 2nd stage • Named after order of discovery

  17. 2. Sunlight is absorbed in the LHC and is passed from pigment to pigment Light capture 1 1. Most Chlorophyll is located in the Light Harvesting Complex Chl a Chl b Remember the porphyrin ring? ß-carotene LHC Reaction Center

  18. e-* 4. e- in P680 Chl a goes to excited state and is shed = Charge separation Light capture - 2 3. Energy finally ends up in a pair of special chlorophyll a molecules: P680 P680 chlorophylls * Optimal @ <680 nm LHC Reaction Center

  19. 5. High-energy e- accepted by quinone Q = primary e- acceptor 6. Q has been reduced; P680 Chl a has been oxidized Oxidation by light = photo-oxidation e- transport chain 7. The excited e- is shunted into the electron transport chain Light capture - 3 e-* Q + LHC Reaction Center

  20. O2 evolving complex e- H2O 1/2 O2 4H+ Light capture - 4 8. The O2 evolving complex + Chl a+ strip e- from H2O and reduce Chl a+ Chl a+ = most powerful biological oxidizing agent +

  21. Light capture - 5 9. The reaction center is reset and ready to go again

  22. Light Capture Movie

  23. H+ H+ 1. Q accepts 2 e- from P680 and removes 2 H+ from the stroma Q e- e- 2. Q passes the e- to cytochrome b/f complex & pumps the 2H+ into thylakoid lumen e- e- cytochrome b/f complex PSII electron transport chain 3. As the e- moves through b/f more H+ are pumped into lumen

  24. The PS II Electron transport chain H+ H+ Q e- e- cytochrome b/f complex e- 2H+ plastocyanin H+

  25. 4. e- end up on plastocyanin: a soluble electron carrier in the lumen 5. Plastocyanin serves as e- donor for PSI reaction center Chl a e- plastocyanin e- plastocyanin PS II to PS I

  26. e- plastocyanin Photosystem I e-* 1. Charge separation and photo-oxidation are similar to PSII Optimal wavelength = 700nm + P700 2. Plastocyanin acts as reducing agent on P700 Chl a

  27. e- Ferredoxin Fe/S 1. Ferredoxin recieves e- from P700* 2. e- moves throuh FAD (flavin adenine dinucleotide) FAD H+ e- FAD-NAPD Reductase 3. e- plus stromal H+ are used to reduce NADP+ to NADPH NADP+ NADPH = FINAL PRODUCT! PSI electron transport chain

  28. PSI electron transport chain e- Ferredoxin Fe/S e- e- FAD H+ e- FAD-NAPD Reductase NADP+ NADPH

  29. ATP synthesis 1. O2 evolving complex liberates H+ into lumen from water 2. Q and Cyt b/f pump H+ from stroma into lumen. 3. NADP+ scavenges protons from the stroma pH separation across membrane = Proton Motive Force! H+ H+ stroma lumen H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

  30. ATP synthesis ADP + Pi F0F1 complex H+ ATP stroma H+ The energy released as protons travel down their concentration gradient is used to fuel an ATP synthase lumen H+ H+ H+ H+ H+ H+ H+ H+ H+ H+

  31. Light Reactions Products In: CO2, H2O, sunlight Out: O2, ATP, NADPH

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