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Photosynthesis

Photosynthesis. Photosynthesis. Critical to life on Earth Producers fuel consumers. Solar Energy. Light is a portion (small) of the electromagnetic spectrum Gamma waves have very short lengths Radio waves have very long lengths Visible light ranges from violet (short) to red (longer)

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Photosynthesis

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  1. Photosynthesis

  2. Photosynthesis • Critical to life on Earth • Producers fuel consumers

  3. Solar Energy • Light is a portion (small) of the electromagnetic spectrum • Gamma waves have very short lengths • Radio waves have very long lengths • Visible light ranges from violet (short) to red (longer) • Short wavelengths have more energy per photon; long have too little energy to excite biological molecules

  4. Solar energy • Photosynthesis is based on visible light with moderate energy levels (Fig. 9-3, pp 170) • This light energy stimulates electrons in atoms • The electrons are boosted to higher energy shells • Such electrons may be picked up by electron carriers and used to do work

  5. Photosynthetic Pigments • Chlorophyll is the pigment that absorbs light energy • Chlorophyll appears green because it absorbs all visible wavelengths except green • Chlorophyll a and b are the most important of the photosynthetic pigments in plants • Accessory pigments include carotenoids

  6. Chlorophyll Structure • Hydrocarbon tail secures to thylakoid membrane • Light absorbed by ring structure

  7. Chloroplast Structure

  8. Chloroplast Structure • Thylakoids: stacks of membranes (grana) • Present in prokaryotes, • Thylakoids are extensions of the plasma membrane in pros. • In euks, thylakoids found in chloroplasts • Stroma: fluid within the chloroplast, outside the thylakoids

  9. Photosystems • Collections of chlorophyll and antenna pigment molecules • Donate electrons when stimulated by light energy (photons) • Energy is transferred from molecule to molecule until triggering the “reaction center”

  10. water-splitting complex thylakoid compartment H2O 2H + 1/2O2 P680 P700 pool of electron transporters acceptor acceptor PHOTOSYSTEM II (light green) PHOTOSYSTEM I (light green) stroma Fig. 7.9, p. 119

  11. Photosystems

  12. Glycolysis • Term means sugar splitting • 1 Glucose molecule -->two 3-carbon pyruvic acid molecules • Occurs in the cytoplasm, in aerobic or anaerobic conditions

  13. A Detour to Oxidation-reduction Reactions • In reduction rxns, substances gain e- • Oxygen participates in reduction rxns • In oxidation rxns substances loose e- • Glucose participates in the oxidation rxn • E- are transferred to electron acceptors, reducing them • NADP+ - nicotinamide adenine dinucleotide phosphate • Chemical energy is released and used to form ATP from ADP and Pi

  14. The Reactions • Photosynthesis involves the formation of energy rich organic molecules • Summary: Sunlight + 12 H2O + 6 CO2 --> 6 O2 + C6H12O6 + 6 H2O

  15. The Reactions • Two stages • Light dependent reactions and • Light independent (dark) reactions

  16. The Reactions (overall)

  17. The Reactions • Light dependent rxns • Occur only in presence of light • Chlorophyll absorbs energy and e- are excited and flow from the chlorophyll molecule • Excited e- produce ATP, water is split • O2 from water returns to the atmosphere; H+ is used to combine with NADP+ and NADP+ is reduced to NADPH, • *No CO2 is used nor is any glucose produced by this point.*

  18. Light Dependent Reactions • Light energy is absorbed by photosystem pigments • P680 of PS II emits e- • e- pass to a series of acceptors in a series of redox rxns • e- are replaced in PS II by splitting of water • Water is split into H+, e- and O2 • ATP is produced • e- are ultimately passed to PS I

  19. The Reactions

  20. Cyclic Electron Flow

  21. Chemiosmosis

  22. Another look:

  23. Chemiosmosis • Proton gradient produced within the thylakoid • ATP synthetase - proton channels • H+ diffuses through channels in ATPase, ATP produced • Function of ATP production is to fuel the next steps (light independent reactions)

  24. The Reactions • Light independent rxns • ATP and NADPH participate in rxns in which CO2 is used to make sugars (primarily glucose)

  25. Light Independent Reactions • Calvin cycle • Occurs in the stroma • Does not need light directly • But occurs only in light (why?)

  26. Light Independent Reactions • CO2 combines with RuBP • 3PG reduced to G3P using ATP, NADPH+ • 1/6 G3P used for glucose • Rest regenerates RuBP

  27. Light Independent Reactions • Requirements • ATP and NADPH from previous rxns • CO2 • Ribulose phosphate, a five carbon sugar • Enzymes to catalyse each step

  28. Light Independent Reactions Rubisco (enzyme) combines RuBP and Co2 • RuBP splits into PGA (Phosphoglycerate, 3 carbon ) • PGA converted to PGAL (phosphoglyceraldehyde) with energy from ATP and hydrogen from NADPH • Some of the PGAL forms glucose • Remaining PGAL molecules are changed to RP and reenter the cycle • Glucose is subsequently changed to sucrose (transport form of sugar)

  29. Light Independent Reactions

  30. A Problem • Photorespiration • When CO2 levels are low, rubisco binds RuBP to oxygen and ultimately the compounds produced are degraded into CO2 and H2O • This represents a loss to the plant. Why? • Photorespiration occurs when plants are water stressed and close their stomates (CO2 levels drop and O2 levels rise) • Photoresp because: occurs in daylight, requires O2, produces CO2 and H2O like aerobic resp.

  31. Alternative photosynthetic pathways • C4 pathways • If you live in a warm climate there is an advantage to avoiding photoresp., you can be more efficient. • Many tropical plants produce a 4-carbon cmpd in mesophyll cells first

  32. C4 pathways • This 4-C cmpd is transported in bundle sheath cells of the leaf • This molecule then proceeds through the Clavin cycle • The effect here is to concentrate CO2 in the bundle sheath above what diffusion from the atmosphere will provide • Rubisco doesn’t have a chance to bind to O2

  33. C4 pathways • Therefore photorespiration is negligible • and these plants are more efficient in warm, dry climates • Corn and sugar cane are examples of C-4 plants

  34. Alternative photosynthetic pathways • CAM plants • Family Crassulaceae, cactaceae, Bromeliaceae • These are warm climate families • Since the day is hot and dry there is an advantage in being able to keep stomates closed and prevent water loss • But, this also causes the levels of CO2 to drop and O2 to rise leading to increased photoresp.

  35. CAM plants • Plants fix CO2 during the night, when stomates are open for gas exchange • Store carbon cmpds in vacuoles for processing during the day • Storage cmpds are decarboxylated during daylight producing CO2 within leaf tissues • During the day, the cmpds are processed in C3 pathways without the need for gas exchange through the stomates.

  36. C4 - CAM Comparison

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