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Chapter 7: Photosynthesis (Outline)

Chapter 7: Photosynthesis (Outline)

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Chapter 7: Photosynthesis (Outline)

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  1. Chapter 7: Photosynthesis (Outline) • Photosynthetic Organisms • Flowering Plants as Photosynthesizers • Photosynthesis • Light Reactions • Noncyclic Pathway • Calvin Cycle Reactions • Carbon Dioxide Fixation • Reduction of Carbon Dioxide • Regeneration of RuBP • C4 • CAM

  2. Photosynthetic Organisms • Organisms cannot live without energy • Almost all the energy that organisms used comes from the solar energy • Photosynthesis: • A process that captures solar energy and transforms it into chemical energy (carbohydrate) • Occurrs in higher plants, algae, mosses, photosynthetic protists and some bacteria (cyanobacteria)

  3. Photosynthetic Organisms • Autotrophs – organisms having the ability to synthesize carbohydrates • Heterotrophs – are consumers that use carbohydrates produced by photosynthesis

  4. Flowering Plants & Photosynthesis • Photosynthesis takes place in the green portions of plants • Leaf of flowering plant contains mesophyll tissue with chloroplasts • Specialized to carry on photosynthesis • CO2 enters leaf through stomata • Diffuses into chloroplasts in mesophyll cells • In the stroma, CO2 is combined with H2O to form C6H12O6 (sugar) • Chlorophyll is capable of absorbing solar energy, which drives photosynthesis

  5. Photosynthesis • The process of photosynthesis is an example of a redox reaction (i.e. movement of electrons from one molecule to another) • In living things, electrons are accompanied by hydrogen ions (H+ + e) • Oxidation is the loss of hydrogen atoms; Reduction is the gain of hydrogen atoms

  6. Photosynthesis • Carbon dioxide reduction requires energy and hydrogen atoms • Solar energy will be used to generate ATP needed to reduce CO2 to sugar • Electrons needed to reduce CO2 will be carried by a coenzyme (NADP+) • NADP+ is the active redox coenzyme of photosynthesis • When NADP+ is reduced, it accepts 2 e- and H+ • When NADP+ is oxidized, it gives up its electrons

  7. Photosynthetic Reactions • In the 1930’s, C. B. van Niel studying photosynthesis in bacteria discovered that CO2 was not split in the process of carbohydrate synthesis • Researchers later confirmed using the isotope oxygen (18O) that O2 given off from photosynthesis comes from waternot CO2 • When water splits, O2 is released and the hydrogen atoms (2 e- + H+) are taken up by NADPH

  8. Photosynthetic Reactions • Light Reactions: • Chlorophyll present in thylakoid membranes absorbs solar energy • This energizes electrons • Electrons move down electron transport chain • Pumps H+ into thylakoids • Used to makeATP out of ADP and NADPH out of NADP+ • Calvin Cycle Reactions: • In the stroma, CO2 is reduced to a carbohydrate • Reduction requires the ATP and NADPH produced in the light reactions

  9. Photosynthesis Overview

  10. Photosynthetic Pigments • Pigments: • Molecules that absorb some colors in rainbow (visible light) more than others, why? • Colors least absorbed reflected/transmitted most • Absorption Spectra • Graph showing relative absorption of the various colors of the rainbow • Chlorophylls a and b(main pigments) are green because they absorb much of the reds and blues of white light • Carotenoids are accessory pigments

  11. Phosynthetic Pigments and Photosynthesis

  12. Spectrophotometer • An instrument which measures the amount of light of a specified wavelength which passes through a medium • Amount of light absorbed at each wavelength is plotted on a graph, resulting in an absorption spectra

  13. Light Reactions • Thelight reactions utilize two photosystems: • Photosystem I (PSI) • Photosystem II (PSII) • A photosystem consists of • Pigment complex (molecules of chlorophylls a & b the carotenoids) and electron acceptor molecules thathelp collect solar energy like an antenna • Occur in the thylakoid membranes

  14. The Noncyclic Electron Pathway • Uses two photosystems, PS IandPS II • Energy enters the system when PS II becomes excited by light • Causes an electron to be ejected from the reaction center (chlorophyll a) • Electron travels down electron transport chain to PS I • PS II takes replacement electron from H2O, which splits, releasing O2 and H+ ions • The H+ ions concentrate in thylakoid chambers • Which causes ATP production, how?

  15. The Noncyclic Electron Pathway • PS I captures light energy and ejects the energized electron (e-) from the reaction center • Transferred permanently by electron acceptors to a molecule of NADP+ • Causes NADPH production NADP+ + 2 e- + H+ • NADPH and ATP produced are used by the Calvin cycle reaction (reduction of CO2) in the stroma

  16. Organization of theThylakoid Membrane • PS II: • Pigment complex and electron-acceptors • Adjacent to an enzyme that oxidizes water • Oxygen is released as a gas (by-product) • Electron transport chain: • Consists of Pq (plastoquinone) and cytochrome complexes • Carries electrons from PS II to PS I • Pumps 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. Organization of a Thylakoid

  18. ATP Production • Thylakoid space acts as a reservoir for H+ ions • Each time water is oxidized, two H+ remain in the thylakoid space • Electrons yield energy • Used to pump H+ across thylakoid membrane • Move from stroma into the thylakoid space • Flow of H+ back across thylakoid membrane • Energizes ATP synthase • Enzymatically produces ATP from ADP + Pi • This method of producing ATP is called chemiosmosis

  19. 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 phases: • Carbon dioxide fixation • Carbon dioxide reduction • RuBP Regeneration

  20. Calvin Cycle Reactions:Phase 1: Carbon Dioxide Fixation • CO2 enters the stroma of the chloroplast via the stomata of the leaves • CO2 is attached to 5-carbon RuBP molecule • Result in an intermediate 6-carbon molecule • This splits into two 3-carbon molecules (3PG) • Reaction is accelerated by RuBP Carboxylase (Rubisco) • CO2 now “fixed” because it is part of a carbohydrate

  21. Calvin Cycle Reactions:Phase 2: Carbon Dioxide Reduction • ATP phosphorylates each 3PG molecule and creates 1,3-bisphosphoglycerate (BPG) • BPG is then reduced by NADPH to glyceraldehyde-3-phosphate (G3P) • NADPH and some ATP used for the reduction reactions are produced in light reactions

  22. Calvin Cycle Reactions:Phase 3: Regeneration of RuBP • RuBP used in CO2 fixation must be replaced • Every three turns of Calvin Cycle, • FiveG3P molecules are used • To remakethreeRuBP molecules • 5 X 3 (carbons in G3P) = 3 X 5 (carbons in RuBP)

  23. The Calvin Cycle

  24. CO2 + RUBP Rubisco 2 3PG C3 Plants • C3 plants include more than 95% percent of the plant species on earth (e.g. trees) • Called C3 because the CO2 is first incorporated into a 3-carbon compound • In hot dry conditions, the photosynthetic efficiency of C3 plants suffers because of photorespiration • Stomata must close to avoid wilting • CO2 decreases and O2 increases • O2 starts combining with RuBP instead of CO2

  25. C4 Photosynthesis: C4 Plants • Use a supplementary method of CO2 uptake forming a 4-carbon molecule instead of the two 3-carbon molecules • In C4 plants • CO2 is inserted into a 3-carbon molecule called phosphoenolpyruvate (PEP) forming • Oxaloacetate, a C4 molecule in the mesophyll cells • Oxaloacetate is converted into malate, which is transported into the bundle sheath cells • Here the 4-carbon molecule is broken down into CO2, which enters the Calvin cycle to form sugars & starch

  26. Chloroplast distribution inC4 vs. C3 Plants

  27. CO2 Fixation in C4 vs. C3 Plants • In hot and dry climates • C4 plants avoid photorespiration as PEP in mesophyll cells does not bind to O2 • Net productivity of C4 is about 2-3 times of C3 plants • In cool, moist conditions, CO2 fixation in C3 is three times more efficient than in C4

  28. CAM Photosynthesis • Called CAM after the plant family in which it was first found (Crassulaceae) • CAM plants partition carbon fixation by time • Crassulacean-Acid Metabolism • During the night through their open stomata • CAM plants use PEPCase to fix CO2 • Forms C4 molecules (malate) • Stored in large vacuoles in mesophyll cells • During daylight • NADPH and ATP are available • Stomata closed for water conservation • C4 molecules release CO2 to Calvin (C3) cycle

  29. CO2 Fixation in a CAM Plant • Adaptive Value: Better water use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are lower (no sunlight, lower temperatures, lower wind speeds, etc.)