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Introduction to Photosynthesis

Introduction to Photosynthesis. Chapter 10 p. 181-188. Autotrophs: Producers of the Biosphere. Autotroph: “self-feeding”; produce own organic molecules from CO 2 & inorg. molec. in environment i.e.: plants, algae, some bacteria

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Introduction to Photosynthesis

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  1. Introduction to Photosynthesis Chapter 10 p. 181-188

  2. Autotrophs: Producers of the Biosphere • Autotroph: “self-feeding”; produce own organic molecules from CO2 & inorg. molec. in environment • i.e.: plants, algae, some bacteria • Heterotroph: “feeds on others”; must consume other orgs to obtain nutrients, O2, & energy • i.e.: animals, fungi, most bacteria

  3. Chloroplasts: Sites of Photosynthesis • Plants appear green due to pigment chlorophyllinside thylakoid spaceof chloroplasts • Found in mesophylltissue of leaves • 30-40 per cell • Stomata: pores through which CO2 enters & O2 leaves

  4. Stomata

  5. Chloroplast Structure

  6. Tracking Atoms through Photosynthesis • O2 given off by plants comes from H2O; NOT CO2 • Chloroplast splits H2O → 2H + O • C.B. van Niel: proved 2H from split H2O goes to glucose; O released as atmospheric O2

  7. Photosynthesis is a Redox Reaction • Unlike cell respir., photosynthesis is endergonic • Energy comes from sun • Reverses direction of e- flow from H2O → CO2 (oxidized) • CO2 is reduced to glucose • CO2 + H2O + energy → C6H12O6 + O2

  8. The 2 Stages of Photosynthesis • 1) Light Reactions (“photo”) • e- + H+ transferred to NADP+ (cousin of NAD+) • O2 given off as byproduct • Produces 1 ATP (“photophosphorylation”) • Occurs in the thylakoid

  9. The 2 Stages of Photosynthesis • 2) Calvin Cycle (“synthesis”) • CO2 incorporated into organic molecules already present (“carbon fixation”) • “Fixed” C is reduced to glucose (add e-) • Powered by NADPH & ATP from light rxns • Occurs during day in most plants; relies on light rxns • Occurs in stroma

  10. Photosynthesis Overview

  11. Nature of Sunlight • Light: electromagnetic energy (“radiation”) • Travels in waves; distance between called wavelength • Also acts as photons:particles of light energy • Electromagnetic Spectrum: entire radiation spectrum • Visible light = 380-750nm • Amt energy inversely proportional to wavelength • Purple photon > red photon

  12. Photosynthetic Pigments • Pigment: substance that absorbs visible light at different wavelengths • Reflected/transmitted wavelength is color we see • Spectrophotometer: measures absorbed wavelengths

  13. Photosynthetic Pigments • Chlorophyll a: main photosynthetic pigment • Absorbs red & blue photons; reflects green • Only pigment directly involved in light rxns • Other pigments (Chlorophyll B & Carotenoids) transfer photons to chlorophyll a & provide photoprotection

  14. Excitation of Chlorophyll by Light • When molecules absorb light energy (photons), e- “jump” to next orbital • Ground state → excited state • Specific to wavelength • Unstable e- will “fall” back quickly, releasing energy (heat) • Fluorescence: energy released as light

  15. Reactions of Photosynthesis Chapter 10 p. 189-198

  16. Photosystems • Consists of 3 sections: • 1) Light-Harvesting Complex: • contain all 3 types pigments; • ↑ surface area to absorb more light • 2) Reaction-Center: at center; receives energy from light-harvesting complex & becomes excited • Contains special chlorophyll a molecules whose e-’s move to higher energy level • 3) Primary Electron Acceptor: receives e-s from excited chlorophyll a molecules & “catches” them • e-’s then enter into Noncyclic Electron Flow

  17. Types of Photosystems • Photosystem II:absorbs 680nm best (“P680”) • P700 & P680 identical, but surrounded by diff. proteins • Work together to make ATP & NADPH for Calvin Cycle • Photosystem I:Reaction center chlorophyll a absorbs 700nm best (“P700”)

  18. Noncyclic Electron Flow • Predominant route for e-s • Steps: • 1) Photots. II absorbs light, P680 excited • 2) e-s captured by Primary e- Acceptor • P680 now has e- “hole” • 3) e-s replaced in P680 by split H2O molecule; O2 released inside thylakoid • 4) ETC takes e-s from Primary e- Acceptor to Photosystem I • Composed of plastoquinone (Pq), 2 cytochromes, & plastocyanin (Pc)

  19. Noncyclic Electron Flow • 5) As e-s “fall” down chain, energy is harvested to make ATP by chemiosmosis outside thylakoid • “Noncyclic Photophosphorylation” • 6) Final e- acceptor is P700 • P700 e-’s excited by light energy are captured by Primary e- Acceptor • Fills “hole” created by Primary e- Acceptor of Photo II • 7) Primary e- Acceptor passes e-s to 2nd ETC → ferredoxin (Fd) • 8) NADP+ reductase transfers e-’s from Fd to NADP+→ makes NADPH in stroma

  20. Summary of Noncyclic Electron Flow • P680 → Primary e- Acceptor → 1st ETC → P700 → Primary e- Acceptor → Fd → NADP+ reductase → NADPH

  21. Cyclic Electron Flow • Calvin cycle uses more ATP than NADPH • If ATP runs low, chloroplast switches to cyclic • Involves Photosystem I (P700) only • Fd takes e-s to cytochrome complex of 1st ETC & returns them to P700 • No NADPH produced; no O2 released

  22. Calvin Cycle • C enters as CO2, leaves as sugar (G3P) • Cycle must turn 3 x’s to make glucose • Must “fix” 3 C’s into org. molecules • Occurs in 3 phases: • 1) C Fixation:3C’s from 3CO2 are incorporated into RuBP, catalyzed by rubisco • 2) Reduction:e-’sfrom NADPH reduce 6 1,3 biphosphate → 6 G3P (↑ energy) • Spends 6 ATP • 3) Regeneration of RuBP:G3P rearranged → RuBP (can pick up CO2 again) • Spends 3 ATP

  23. Calvin Cycle - Summarized • For each turn of Calvin: • In:Out: • 9 ATP 9 ADP • 6 NADPH 6 NADP+ • 3 CO2 1 G3P (will • become glucose)

  24. Alternate Methods of C Fixation • In hot, dry climates, stomata remain closed to prevent H2O loss • Also prevents CO2 in & O2 out • Result is Photorespiration

  25. Photorespiration • Most plants are called C3 because C fixation creates a 3-C compound • Closed stomata ↓ [CO2] inside leaf, and ↑ [O2] • O2 will be picked up by rubisco (instead of CO2) • Photorespiration: uses light (“photo”) to consume O2 (“respiration”) • No ATP produced; no sugar made • May be ancient evolutionary adaptation

  26. C4 Plants • C fixed by PEP carboxylase to form 4-C compound (oxaloacetate → malate) • PEP carbox. has ↑↑ affinity for CO2; can “fix” CO2 when rubisco can’t • 4-C cmpnd (malate) enters Bundle Sheath cells where CO2 breaks off & enters Calvin • Keeps CO2 levels ↑ for rubisco • Minimizes photorespiration & ↑ sugar production

  27. CAM Plants • Water-storing plants (cacti, pineapple, etc.) close stomata during day, open at night • Store org. molec. until day when light rxns can provide ATP & NADPH

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