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Photosynthesis: Introduction & Light Reactions

Understand the process of converting solar energy into chemical energy through photosynthesis. Learn about the light reactions and dark reactions, where photosynthesis takes place, chloroplast morphology, pigments involved, and more. Dive into the structure of chlorophyll, why plants are green, and the role of light in providing energy for plants. Gain insights into light capture, redox reactions, Photosystems I and II, and the electron transport chain in this comprehensive lecture on photosynthesis.

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Photosynthesis: Introduction & 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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