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PHOTOSYNTHESIS Chapter 7

PHOTOSYNTHESIS Chapter 7. Where It Starts - Photosynthesis. Impacts, Issues: Sunlight and Survival. Plants are autotrophs , or self-nourishing organisms The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O 3 ) layer

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PHOTOSYNTHESIS Chapter 7

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  1. PHOTOSYNTHESISChapter 7 Where It Starts - Photosynthesis

  2. Impacts, Issues: Sunlight and Survival • Plants are autotrophs, or self-nourishing organisms • The first autotrophs filled Earth’s atmosphere with oxygen, creating an ozone (O3) layer • The ozone layer became a shield against deadly UV rays from the sun, allowing life to move out of the ocean

  3. Electromagnetic Spectrum Shortest Gamma rays wavelength X-rays UV radiation Visible light Infrared radiation Microwaves Longest Radio waves wavelength

  4. Photons • Packets of light energy • Each type of photon has fixed amount of energy • Photons having most energy travel as shortest wavelength (blue-violet light)

  5. Visible Light • Wavelengths humans perceive as different colors • Violet (380 nm) to red (750 nm) • Longer wavelengths, lower energy Figure 7-2Page 108

  6. Visible Light shortest wavelengths (most energetic) longest wavelengths (lowest energy) range of most radiation reaching Earth’s surface range of heat escaping from Earth’s surface gamma rays x rays ultraviolet radiation near-infrared radiation infrared radiation radio waves microwaves VISIBLE LIGHT 400 450 500 550 600 650 700 Wavelengths of light (nanometers) Fig. 7-2, p.108

  7. Pigments • Color you see is the wavelengths not absorbed • Light-catching part of molecule often has alternating single and double bonds • These bonds contain electrons that are capable of being moved to higher energy levels by absorbing light

  8. Variety of Pigments Chlorophylls a and b Carotenoids Anthocyanins Phycobilins

  9. Chlorophylls Main pigments in most photoautotrophs chlorophyll a Wavelength absorption (%) chlorophyll b Wavelength (nanometers)

  10. Accessory Pigments Carotenoids, Phycobilins, Anthocyanins beta-carotene phycoerythrin (a phycobilin) percent of wavelengths absorbed wavelengths (nanometers)

  11. Pigments in Photosynthesis • Bacteria • Pigments in plasma membranes • Plants • Pigments and proteins organized into photosystems that are embedded in thylakoid membrane system

  12. T.E. Englemann’s Experiment Background • Certain bacterial cells will move toward places where oxygen concentration is high • Photosynthesis produces oxygen

  13. T.E. Englemann’s Experiment

  14. T.E. Englemann’s Experiment Fig. 7-4c, p.110

  15. Linked Processes Photosynthesis • Energy-storing pathway • Releases oxygen • Requires carbon dioxide Aerobic Respiration • Energy-releasing pathway • Requires oxygen • Releases carbon dioxide

  16. Chloroplast Structure two outer membranes stroma inner membrane system (thylakoids connected by channels) Fig. 7-6, p.111

  17. Photosynthesis Equation LIGHT ENERGY 12H2O + 6CO2 6O2 + C2H12O6 + 6H2O Water Carbon Dioxide Oxygen Glucose Water In-text figurePage 111

  18. Photosynthesis Fig. 7-6a, p.111

  19. Photosynthesis SUNLIGHT O2 H2O CO2 NADPH, ATP light-dependant reactions light-independant reactions NADP+, ADP sugars CHLOROPLAST Fig. 7-6c, p.111

  20. Reactants 12H2O 6CO2 Products 6O2 C6H12O6 6H2O Where Atoms End Up

  21. Two Stages of Photosynthesis sunlight water uptake carbon dioxide uptake ATP ADP + Pi LIGHT-DEPENDENT REACTIONS LIGHT-INDEPENDENT REACTIONS NADPH NADP+ glucose P oxygen release new water

  22. Arrangement of Photosystems water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 acceptor acceptor pool of electron carriers PHOTOSYSTEM II stroma PHOTOSYSTEM I

  23. Light-Dependent Reactions • Pigments absorb light energy, give up e-, which enter electron transfer chains • Water molecules split, ATP and NADH form, and oxygen is released • Pigments that gave up electrons get replacements

  24. Light-Dependent Reactions photon Photosystem Light-Harvesting Complex Fig. 7-7, p.112

  25. LIGHT- HARVESTING COMPLEX sunlight PHOTOSYSTEM II PHOTOSYSTEM I H+ NADPH e- e- e- e- e- e- NADP + + H+ e- thylakoid compartment H2O H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ O2 H+ thylakoid membrane stroma ATP ADP + Pi cross-section through a disk-shaped fold in the thylakoid membrane H+ Fig. 7-8, p.113

  26. Pigments in a Photosystem reaction center

  27. Photosystem Function: Reaction Center • Energy is reduced to level that can be captured by molecule of chlorophyll a • This molecule (P700 or P680) is the reaction center of a photosystem • Reaction center accepts energy and donates electron to acceptor molecule

  28. Electron Transfer Chain • Adjacent to photosystem • Acceptor molecule donates electrons from reaction center • As electrons pass along chain, energy they release is used to produce ATP

  29. Cyclic Electron Flow • Electrons • are donated by P700 in photosystem I to acceptor molecule • flow through electron transfer chain and back to P700 • Electron flow drives ATP formation • No NADPH is formed

  30. Cyclic Electron Flow e– electron acceptor Electron flow through transfer chain sets up conditions for ATP formation at other membrane sites. electron transfer chain e– e– ATP e–

  31. Noncyclic Electron Flow • Two-step pathway for light absorption and electron excitation • Uses two photosystems: type I and type II • Produces ATP and NADPH • Involves photolysis - splitting of water

  32. Machinery of Noncyclic Electron Flow H2O second electron transfer chain photolysis e– e– ATP SYNTHASE first electron transfer chain NADPH NADP+ ATP ADP + Pi PHOTOSYSTEM II PHOTOSYSTEM I

  33. Energy Changes second transfer chain e– NADPH e– first transfer chain Potential to transfer energy (volts) e– e– (Photosystem I) (Photosystem II) 1/2O2 + 2H+ H2O

  34. PHOTOSYSTEM I p700* H+ e- photon p700 Higher energy Cyclic Pathway of ATP Formation Fig. 7-9a, p.114

  35. PHOTOSYSTEM I NADPH p700* PHOTOSYSTEM II NADH+ p680* e- p700 photon p680 2H2O Noncyclic Pathway of ATP and NADPH Formation 4H+ + O2 Fig. 7-9b, p.114

  36. Chemiosmotic Model of ATP Formation • Electrical and H+ concentration gradients are created between thylakoid compartment and stroma • H+ flows down gradients into stroma through ATP synthesis • Flow of ions drives formation of ATP

  37. Chemiosmotic Model for ATP Formation H+ is shunted across membrane by some components of the first electron transfer chain Gradients propel H+ through ATP synthases; ATP forms by phosphate-group transfer Photolysis in the thylakoid compartment splits water H2O e– acceptor ATP SYNTHASE ATP ADP + Pi PHOTOSYSTEM II

  38. PHOTOSYNTHESIS QUIZ 1 • 1. Give the equation for photosynthesis. • 2. Give the 2 major reactions occurring in photosynthesis. • 3. What reactant goes into the light reactions? What product? • 4. What reactant goes into the Calvin cycle? What product? • 5. Within the chloroplast, where do the light reactions occur? • 6. Within the chloroplast, where does the Calvin cycle occur? • 7. What does noncyclic electron flow produce (besides ATP) that that cyclic electron flow does not produce? • 8. Why doesn’t the overall action spectrum of photosynthesis exactly match the absorption spectrum for chlorophyll? • 9. Which photosystem acts first in noncyclic electron flow? • ****BONUS**** 1. What are Pq, Pc, and Fd? 2. Explain what chemiosmosis is.

  39. Light-Independent Reactions • Synthesis part of photosynthesis • Can proceed in the dark • Take place in the stroma • Calvin-Benson cycle

  40. Calvin-Benson Cycle • Overall reactants • Carbon dioxide • ATP • NADPH • Overall products • Glucose • ADP • NADP+ Reaction pathway is cyclic and RuBP (ribulose bisphosphate) is regenerated

  41. 6 CO2 (from the air) Calvin- Benson Cycle CARBON FIXATION 6 6 RuBP unstable intermediate 12 PGA 6 ADP 12 ATP 6 ATP 12 NADPH 4 Pi 12 ADP 12 Pi 12 NADP+ 10 PGAL 12 PGAL 2 PGAL Pi P glucose

  42. Calvin- Benson Cycle THESE REACTIONS PROCEED IN THE CHLOROPLAST’S STROMA Fig. 7-10a, p.115

  43. Calvin- Benson Cycle 6CO2 ATP 12 PGA 6 RuBP 12 6 ADP 12 ADP + 12 Pi Calvin-Benson cycle ATP 12 NADPH 4 Pi 12 NADP+ 12 PGAL 10 PGAL 1 Pi phosphorylated glucose 1 Fig. 7-10b, p.115

  44. The C3 Pathway • In Calvin-Benson cycle, the first stable intermediate is a three-carbon PGA • Because the first intermediate has three carbons, the pathway is called the C3 pathway

  45. Photorespiration in C3 Plants • On hot, dry days stomata close • Inside leaf • Oxygen levels rise • Carbon dioxide levels drop • Rubisco attaches RuBP to oxygen instead of carbon dioxide • Only one PGAL forms instead of two

  46. C3 Plants Fig. 7-11a1, p.116

  47. C3 Plants upper epidermis palisade mesophyll spongy mesophyll lower epidermis stoma leaf vein air space Basswood leaf, cross-section. Fig. 7-11a2, p.116

  48. C3 Plants Stomata closed: CO2 can’t get in; O2 can’t get out 6 PGA + 6 glycolate Rubisco fixes oxygen, not carbon, in mesophyll cells in leaf RuBP Calvin-Benson Cycle 5 PGAL 6 PGAL CO2 + water 1 PGAL Twelve turns of the cycle, not just six, to make one 6-carbon sugar Fig. 7-11a3, p.117

  49. C4 Plants • Carbon dioxide is fixed twice • In mesophyll cells, carbon dioxide is fixed to form four-carbon oxaloacetate • Oxaloacetate is transferred to bundle-sheath cells • Carbon dioxide is released and fixed again in Calvin-Benson cycle

  50. C4 Plants Fig. 7-11b1, p.117

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