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Understanding Photosynthesis: Light Reactions and the Calvin Cycle

This comprehensive overview of photosynthesis details the two primary reactions: the light-dependent reactions and the Calvin Cycle. It explains the role of chloroplasts and pigments, such as chlorophyll, in capturing light and converting it into chemical energy. The light reactions occur in the thylakoid membranes, producing ATP and NADPH, while the Calvin Cycle synthesizes glucose from CO2. Key concepts such as electron transport, chemiosmosis, and the importance of autotrophs and heterotrophs are also discussed, providing a solid foundation in understanding photosynthesis.

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Understanding Photosynthesis: Light Reactions and the Calvin Cycle

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  1. Do Now: Structure of Leaves Worksheet Section 29-4, p. 599

  2. Chapter 6: Photosynthesis Section 1: Capturing the Energy in Light

  3. Q.O.D: Briefly describe the two reactions of photosynthesis

  4. Energy and Life Processes • Autotrophs • Photosynthesis • Chemosynthesis • Heterotrophs • Biochemical pathway • Photosynthesis • Cellular respiration

  5. The Chloroplast • Structure: • Double-membrane • Thylakoid discs • Thylakoid space • Grana • Stroma • Chlorophyll and other pigments

  6. The Process • 6CO2 + 6H2O + energy  C6H12O6 + 6O2 • Overview: 2 reactions • Light Reactions • Light energy  Chemical energy • Creates ATP and NADPH (an energy carrier) • Calvin Cycle or Dark Reaction • Takes in CO2 and produces sugars • Uses energy produced in light reaction

  7. Light and Pigments • Light • Visible spectrum of colors • Travels as waves of energy • Wavelengths – shorter = more energy • Colors can be reflected, transmitted or absorbed • Pigments • Compound that absorbs light

  8. 1 m (109 nm) 10–3 nm 103 nm 106 nm 10–5 nm 103 m 1 nm LE 10-6 Gamma rays Micro- waves Radio waves X-rays Infrared UV Visible light 650 750 nm 500 550 600 700 450 380 Shorter wavelength Longer wavelength Higher energy Lower energy

  9. Chloroplast Pigments • Primary pigment: Chlorophyll a • Accessory pigments: • Chlorophyll b • Carotenoids • Carotenes • Xanthophylls

  10. Chlorophyll a LE 10-9a Chlorophyll b Carotenoids Absorption of light by chloroplast pigments 400 700 500 600 Wavelength of light (nm) Absorption spectra

  11. The Light Reaction • Occurs in the thylakoid membranes • In: • Light energy • H2O • Out: • O2 • ATP • NADPH • Stages: Electron transport and Chemiosmosis

  12. Electron Transport Chain • Photosystems • Cluster of pigments + proteins imbedded in membrane of thylakoid • Contain light harvesting complex and reaction center • Transfers energy from light to electrons • Electrons move down electron transport chain • Energy is used to produce ATP and NADPH

  13. Thylakoid Photosystem STROMA Photon LE 10-12 Light-harvesting complexes Reaction center Primary electron acceptor e– Thylakoid membrane Special chlorophyll a molecules Pigment molecules Transfer of energy THYLAKOID SPACE (INTERIOR OF THYLAKOID)

  14. Section 1 The Light Reactions Chapter 6 Light Reactions in Photosynthesis

  15. Steps of Electron Transport • 1: Light energy excites a pair of electrons in 2 chlorophyll a molecules in photosystem II • 2: These electrons leave chlorophyll a (oxidation) and is picked up by the primary electron acceptor (reduction) • 3: Electron transport chain – energy from electrons used to move H+ from stroma into the thylakoid

  16. 4: Light is absorbed by photosystem I, exciting a pair of electrons in chlorophyll a • Electrons are replaced by electrons from photosystem II • 5: These electrons move down another ETC and are picked up by NADP+ to form NADPH • Restoring Photosystem II – replacing lost electrons • Photolysis: Light breaks up water into 4H+, O2 and electrons

  17. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy

  18. Section 1 The Light Reactions Chapter 6 Light Reactions in Photosynthesis

  19. Q.O.D: What is chemiosmosis? What drives this process?

  20. Section 1 The Light Reactions Chapter 6 Converting Light Energy To Chemical Energy

  21. Chemiosmosis • Concentration gradient of H+ ions created by ETC and photolysis • More H+ in thylakoid than in stroma • Creates a potential energy • ATP synthase embedded in thylakoid membrane • As H+ diffuse back into stroma, they flow through the ATP synthase • Movement spins the enzyme, driving the production of ATP from ADP

  22. Light Reaction Reviewed • Reactants: • H2O • Light • Products: • O2 • ATP and NADPH • Looking forward: • ATP and NADPH provide energy for Calvin cycle

  23. Section 1 The Light Reactions Chapter 6 Summary of Processes in Light Reactions

  24. Chapter 6: Photosynthesis Section 2: The Calvin Cycle

  25. Calvin Cycle: An Overview • Light-independent • Location: The stroma • In: • CO2 • ATP • NADPH • Out: • C6H12O6 • ADP + P • NADP+

  26. Steps of the Calvin Cycle • 1: Carbon fixation: CO2 is added to RuBP, a 5-Carbon compound • Enzyme: Rubisco • Intermediate 6-C compound immediately breaks down into 2 3-C compounds (PGA) • 2: PGA is converted to PGAL • a: ATP gives a P to each PGA • b: NADPH gives a proton and energy  PGAL

  27. 3: RuBP is replenished • Most of the PGAL is converted back to RuBP • Allows cycle to continue • One PGAL for every 3 CO2 exits cycle to be incorporated into sugar • Results: • For every 3CO2 entering cycle, we get 6 PGAL. • 1 PGAL exits • 5 PGAL converted back to RuBP • Need 6CO2 for 2 PGAL  sugar

  28. Calvin Cycle To return to the chapter summary click escape or close this document.

  29. Section 2 The Calvin Cycle Chapter 6 The Calvin Cycle

  30. Section 2 The Calvin Cycle Chapter 6 Ongoing Cycle of Photosynthesis

  31. Alternative Pathways • C3 plants • Typical • Alternative pathways • Role of climate • Stomata • Gas exchange • Gas levels

  32. C4 Plants • C4 Pathway • Carbon fixation  4-Carbon compound • Source CO2 for Calvin cycle • Sugar cane, corn, crab grass

  33. LE 10-19 Mesophyll cell Mesophyll cell CO2 PEP carboxylase Photosynthetic cells of C4 plant leaf Bundle- sheath cell The C4 pathway Oxaloacetate (4 C) PEP (3 C) Vein (vascular tissue) ADP Malate (4 C) ATP C4 leaf anatomy Pyruvate (3 C) Bundle-sheath cell CO2 Stoma CALVIN CYCLE Sugar Vascular tissue

  34. CAM Plants • CAM Pathway • Stomata open at night • Fix carbon at night  organic compounds • Day, organic compounds release CO2 to Calvin cycle • Cacti, Pineapple – slower growth

  35. LE 10-20 Sugarcane Pineapple CAM C4 CO2 CO2 Night Mesophyll cell CO2 incorporated into four-carbon organic acids (carbon fixation) Organic acid Organic acid Bundle- sheath cell Day CO2 CO2 Organic acids release CO2 to Calvin cycle CALVIN CYCLE CALVIN CYCLE Sugar Sugar Spatial separation of steps Temporal separation of steps

  36. Rate of Photosynthesis • How it’s measured • O2 • Mass • CO2 • Influenced by: • Light intensity • CO2 • Temperature

  37. Section 1 The Light Reactions Chapter 6 Overview of Photosynthesis

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