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- 1. PHOTOSYNTHESIS BY ASMAT ALI LECRURER IN BOTANY GC PESHAWAR
- 2. Photosynthesis • An anabolic, endergonic, carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose). 6CO2 + 6H2O C6H12O6 + 6O2 glucose SUN photons
- 3. Plants • Autotrophs: self-producers. • Location: 1.Leaves a. stoma b. mesophyll cells Stoma Mesophyll Cell Chloroplast
- 4. Stomata (stoma) • Pores in a plant’s cuticle through which water and gases are exchanged between the plant and the atmosphere. Guard Cell Guard Cell Carbon Dioxide (CO2) Oxygen (O2)
- 6. Chloroplast • Organelle where photosynthesis takes place. Granum Thylakoid Stroma Outer Membrane Inner Membrane
- 8. Question: • Why are plants green?
- 9. Chlorophyll Molecules • Located in the thylakoid membranes. • Chlorophyll have Mg+ in the center. • Chlorophyll pigments harvest energy (photons) by absorbing certain wavelengths (blue-420 nm and red-660 nm are most important). • Plants are green because the green wavelength is reflected, not absorbed.
- 10. Wavelength of Light (nm) 400 500 600 700 Short wave Long wave (more energy) (less energy)
- 11. Absorption of Chlorophyll wavelength Absorption violet blue green yellow orange red
- 12. Question: • During the fall, what causes the leaves to change colors?
- 13. Fall Colors • In addition to the chlorophyll pigments, there are other pigments present. • During the fall, the green chlorophyll pigments are greatly reduced revealing the other pigments. • Carotenoids are pigments that are either red or yellow.
- 14. Breakdown of Photosynthesis • Two main parts (reactions). 1. Light Reaction or Light Dependent Reaction Produces energy from solar power (photons) in the form of ATP and NADPH.
- 15. Breakdown of Photosynthesis 2. Calvin Cycle or Light Independent Reaction or Carbon Fixation or C3 Fixation Uses energy (ATP and NADPH) from light rxn to make sugar (glucose).
- 16. 1. Light Reaction (Electron Flow) • Occurs in the Thylakoid membranes • During the light reaction, there are two possible routes for electron flow. A. Cyclic Electron Flow B. Noncyclic Electron Flow
- 17. A. Cyclic Electron Flow • Occurs in the thylakoid membrane. • Uses Photosystem I only • P700 reaction center- chlorophyll a • Uses Electron Transport Chain (ETC) • Generates ATP only ADP + ATP P
- 18. A. Cyclic Electron Flow P700 Primary Electron Acceptor e- e- e- e- ATP produced by ETC Photosystem I Accessory Pigments SUN Photons
- 19. B. Noncyclic Electron Flow • Occurs in the thylakoid membrane • Uses PS II and PS I • P680 rxn center (PSII) - chlorophyll a • P700 rxn center (PS I) - chlorophyll a • Uses Electron Transport Chain (ETC) • Generates O2, ATP and NADPH
- 20. B. Noncyclic Electron Flow P700 Photosystem I P680 Photosystem II Primary Electron Acceptor Primary Electron Acceptor ETC Enzyme Reaction H2O 1/2O2 + 2H+ ATP NADPH Photon 2e- 2e- 2e- 2e- 2e- SUN Photon
- 21. B. Noncyclic Electron Flow • ADP + ATP • NADP+ + H NADPH • Oxygen comes from the splitting of H2O, not CO2 H2O 1/2 O2 + 2H+ (Reduced) P (Reduced) (Oxidized)
- 22. Calvin Cycle • Carbon Fixation (light independent rxn). • C3 plants (80% of plants on earth). • Occurs in the stroma. • Uses ATP and NADPH from light rxn. • Uses CO2. • To produce glucose: it takes 6 turns and uses 18 ATP and 12 NADPH.
- 24. Calvin Cycle (C3 fixation) 6CO2 6C-C-C-C-C-C 6C-C-C 6C-C-C 6C-C-C-C-C 12PGA RuBP 12G3P (unstable) 6NADPH 6NADPH 6ATP 6ATP 6ATP C-C-C-C-C-C Glucose (6C) (36C) (36C) (36C) (30C) (30C) (6C) 6C-C-C 6C-C-C C3 glucose
- 25. Review: Photosynthesis uses light energy to make food molecules Light Chloroplast Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma LIGHT REACTIONS CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds • A summary of the chemical processes of photosynthesis
- 26. Photorespiration • Occurs on hot, dry, bright days. • Stomates close. • Fixation of O2 instead of CO2. • Produces 2-C molecules instead of 3-C sugar molecules. • Produces no sugar molecules or no ATP. • Photorespiration is estimated to reduce photosynthetic efficiency by 25%
- 28. Strategy for Preventing Photorespiration Fix CO2 in an environment shielded from O2 Use an enzyme that does not react with O2 Avoid RUBISCO PLAN
- 29. Strategy (cont.) SOLUTION CO2 fixation occurs in Mesophyll cells CO2 fixing enzyme is not RUBISCO PEP carboxylase will not react with O2 RUBISCO never changed Instead plant anatomy changed CO2 fixing enzyme is PEP carboxylase
- 30. Photorespiration • Because of photorespiration: Plants have special adaptations to limit the effect of photorespiration. 1. C4 plants 2. CAM plants
- 31. C4 Plants • Hot, moist environments. • 15% of plants (grasses, corn, sugarcane). • Divides photosynthesis spatially. • Light rxn - mesophyll cells. • Calvin cycle - bundle sheath cells.
- 32. Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells. Mesophyll cells Bundle sheath cells
- 33. C4 Pathway • In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells.
- 34. C4 Pathway • CO2 is fixed into a 4- carbon intermediate • Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate
- 35. C4 Pathway • The 4 carbon intermediate is “smuggled” into the bundle sheath cell • The bundle sheath cell is not very permeable to CO2 • CO2 is released from the 4C malate goes through the Calvin Cycle C3 Pathway
- 36. How does the C4 Pathway limit photorespiration? • Bundle sheath cells are far from the surface– less O2 access • PEP Carboxylase doesn’t have an affinity for O2 allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)
- 37. CAM Plants • Hot, dry environments. • 5% of plants (cactus and ice plants). • Stomates closed during day. • Stomates open during the night. • Light rxn - occurs during the day. • Calvin Cycle - occurs when CO2 is present.
- 38. CAM Pathway • Fix CO2 at night and store as a 4 carbon molecule • Keep stomates closed during day to prevent water loss • Same general process as C4 Pathway
- 39. How does the CAM Pathway limit photorespiration? • Collects CO2 at night so that it can be more concentrated during the day • Plant can still do the calvin cycle during the day without losing water
- 40. Summary of C4 Photosynthesis • C4 Pathway – Separates by space (different locations) • CAM Pathway – Separates reactions by time (night versus day)
- 41. • CO2 directly • RuBP recipient • RUBISCO open • O2 can interfere • Photorespiration likely • CO2 indirectly • PEP recipient • RUBISCO shielded • O2 cannot interfere • No photorespriation C3 vs C4 Plants A Lesson in Photoefficiency C3 C4
- 42. • soybean • wheat • rice • sugar beet • alfalfa • spinach • tobacco • sunflower • corn • sorghum • sugar cane • millet • crab grass • Bermuda grass • pigweed C3 C4 C3 and C4 Plants