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  1. 1. Photosynthesis Photosynthesis Presentation by: Mr M Dlamini 201221083 1
  2. 2. Photosynthesis Photosynthesis, the biochemical process by which plants capture energy from sunlight and store it in carbohydrates.
  3. 3. Autotrophic organisms use an inorganic form of carbon, e.g. carbon dioxide, to make up complex organic compounds, with 1) light energy from two sources: 2) chemicals Rhizobium in root nodules
  4. 4. Photosynthetic Organisms 4
  5. 5. Photosynthesis Photosynthesis is more common and important because: 1. It provides a source of complex organic molecules for heterotrophic organisms. 2. It releases oxygen for use by aerobic organisms.
  6. 6. Photosynthesis Photosynthesis Photosynthesis takes place in the green portions of plants Leaf of flowering plant contains mesophyll tissue Cells containing chloroplasts Specialized to carry on photosynthesis CO2 enters leaf through stomata Diffuses into chloroplasts in mesophyll cells In stroma, CO2 combined with H2O to form C6H12O6 (sugar) Energy supplied by light 6
  7. 7. Leaves and Photosynthesis 7
  8. 8. Photosynthetic Reactions: Overview Light Reaction: Photosynthesis 8 Chlorophyll absorbs solar energy This energizes electrons Electrons move down electron transport chain - Pumps H+ into thylakoids - Used to make ATP out of ADP and NADPH out of NADP Calvin Cycle Reaction CO2 is reduced to a carbohydrate Reduction requires the ATP and NADPH produced above
  9. 9. Photosynthesis Light Dependent Reactions A. Light absorption 1. As chlorophyll absorbs light its electrons are raised to a higher energy level by photons at certain wavelengths 2. The electrons at higher energy levels are said to be excited electrons 3. The excited electrons cause the chlorophyll to become photoactivated 4. Photoactivation is the activation of a particular pigment’s electrons (It is caused by absorbing energy from photons.)
  10. 10. Photosynthesis Overview 10
  11. 11. Photosynthesis 11 Light Reactions: The Noncyclic Electron Pathway Takes place in thylakoid membrane Uses two photosystems, PS-I and PS-II PS II captures light energy Causes an electron to be ejected from the reaction center (chlorophyll a) Electron travels down electron transport chain to PS I Replaced with an electron from water Which causes H+ to concentrate in thylakoid chambers Which causes ATP production PS I captures light energy and ejects an electron Transferred permanently to a molecule of NADP+ Causes NADPH production
  12. 12. Light Reactions: Noncyclic Electron Pathway 12
  13. 13. Photosynthesis 13 Light Reactions: The Cyclic Electron Pathway Uses only photosystem I (PS-I) Begins when PS I complex absorbs solar energy Electron ejected from reaction center Travels down electron transport chain Causes H+ to concentrate in thylakoid chambers Which causes ATP production Electron returns to PS-I (cyclic) Pathway only results in ATP production
  14. 14. Light Reactions: Cyclic Electron Pathway 14
  15. 15. Chapter 7 15 Light-Independent Reactions NADPH and ATP from lightdependent reactions used to power glucose synthesis Light not directly necessary for lightindependent reactions if ATP & NADPH available Light-independent reactions called the Calvin-Benson Cycle or C3 Cycle
  16. 16. The C3 Cycle Chapter 7 16 6 CO2 used to synthesize 1 glucose (C6H12O6) Carbon dioxide is captured and linked to ribulose bisphosphate (RuBP) ATP and NADPH from light dependent reactions used to power C3 reactions
  17. 17. Organization of the Thylakoid Membrane Photosynthesis PS II: Pigment complex and electron-acceptors Adjacent to an enzyme that oxidizes water Oxygen is released as a gas Electron transport chain: Consists of cytochrome complexes Carries electrons between PS II and PS I Also pump H+ from the stroma into thylakoid space PS I: Pigment complex and electron acceptors Adjacent to enzyme that reduces NADP+ to NADPH ATP synthase complex: Has a channel for H+ flow Which drives ATP synthase to join ADP and Pi 17
  18. 18. Photosynthesis C3 Cycle Has Three Parts 2. Synthesis of Glyceraldehyde 3Phosphate (G3P) Energy is donated by ATP and NADPH Phosphoglyceric acid (PGA) molecules are converted into glyceraldehyde 3-Phophate (G3P) molecules … Ch 18 ap ter 7
  19. 19. Photosynthesis C3 Cycle Has Three Parts 3. Regeneration of Ribulose bisphosphate (RuBP) 10 of 12 G3P molecules converted into 6 RuBP molecules 2 of 12 G3P molecules used to synthesize 1 glucose ATP energy used for these reactions Ch 19 ap ter 7
  20. 20. Photosynthesis A Summary of Photosynthesis Lightdependent reactions occur in thylakoids Lightindependent reactions (c3 cycle) occur in stroma Ch 20 ap ter 7
  21. 21. Organization of a Thylakoid 21
  22. 22. Photosynthesis 22 Calvin Cycle Reactions: Overview of C3 Photosynthesis A cyclical series of reactions Utilizes atmospheric carbon dioxide to produce carbohydrates Known as C3 photosynthesis Involves three stages: -Carbon dioxide fixation -Carbon dioxide reduction -RuBP Regeneration
  23. 23. Calvin Cycle Reactions: Carbon Dioxide Fixation Photosynthesis 23 CO2 is attached to 5-carbon RuBP molecule Result in a 6-carbon molecule This splits into two 3-carbon molecules (3PG) Reaction accelerated by RuBP Carboxylase (Rubisco) CO2 now “fixed” because it is part of a carbohydrate
  24. 24. The Calvin Cycle: Fixation of CO2 24
  25. 25. Calvin Cycle Reactions: Carbon Dioxide Reduction Photosynthesis 3PG reduced to BPG BPG then reduced to G3P Utilizes NADPH and some ATP produced in light reactions 25
  26. 26. The Calvin Cycle Reduction of CO2 InLine Figure p125 26
  27. 27. Calvin Cycle Reactions: Regeneration of RuBP Photosynthesis 27 RuBP used in CO2 fixation must be replaced Every three turns of Calvin Cycle, Five G3P (a 3-carbon molecule) used To remake three RuBP (a 5-carbon molecule) 5X3=3X5
  28. 28. The Calvin Cycle Regeneration of RuBP 28
  29. 29. Photosynthesis Importance of Calvin Cycle G3P (glyceraldehyde-3-phosphate) can be converted to many other molecules The hydrocarbon skeleton of G3P can form Fatty acids and glycerol to make plant oils Glucose phosphate (simple sugar) Fructose (which with glucose = sucrose) Starch and cellulose Amino acids 29
  30. 30. Photosynthesis 30 C4 Photosynthesis In hot, dry climates Stomata must close to avoid wilting CO2 decreases and O2 increases O2 starts combining with RuBP instead of CO2 Photorespiration, a problem solve in C4 plants In C4 plants Fix CO2 to PEP a C3 molecule The result is oxaloacetate, a C4 molecule In hot & dry climates - Avoid photorespiration - Net productivity about 2-3 times C3 plants In cool, moist, can’t compete with C3
  31. 31. Chloroplast distribution in C4 vs. C3 Plants 31
  32. 32. CO2 Fixation in C4 vs. C3 Plants 32
  33. 33. Photosynthesis CAM Photosynthesis Crassulacean-Acid Metabolism CAM plants partition carbon fixation by time - During the night  CAM plants fix CO2  Forms C4 molecules,  Stored in large vacuoles - During daylight  NADPH and ATP are available  Stomata closed for water conservation  C4 molecules release CO2 to Calvin cycle 33
  34. 34. CO2 Fixation in a CAM Plant 34
  35. 35. Reference list 35 thesis-29183537&user_login=mazz4