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Photosynthesis

Photosynthesis. 6CO 2 + 12H 2 0 C 6 H 12 O 6 + 6O 2 + 6 H 2 O. Light-dependent reactions of photosynthesis Light-independent reactions of photosynthesis C3 versus C4 metabolism. Write an essay on the light-dependent (light) reactions of photosynthesis

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Photosynthesis

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  1. Photosynthesis 6CO2 + 12H20 C6H12O6 + 6O2 + 6 H2O

  2. Light-dependent reactions of photosynthesis Light-independent reactions of photosynthesis C3 versus C4 metabolism Write an essay on the light-dependent (light) reactions of photosynthesis Write an essay on the light-independent (Dark) reactions of photosynthesis Compare and contrast C3 and C4 metabolism Overview

  3. Photosynthesis ADP ATP NADP+ NADPH CO2 Calvin/Benson Cycle (“Dark Reactions”) Light reactions H2O O2 Sucrose

  4. Photosynthesis occurs only in the chloroplasts Vacuole Stroma Grana Chloroplast Palisade cell Thylakoids Thylakoid space PSI PSII

  5. The light reactions of photosynthesis

  6. Plants absorb Light Chlorophyll a Chlorophyll b Abs Carotenoids 400 Green Red Blue 500 600 700 Wavelength of light (nm)

  7. Absorption of light leads to excited electrons • When light energy is absorbed by chlorophyll, an electron is promoted to a higher (more energetic) orbital. This energy can be lost by: • from one orbital to the next with loss of heat • back to the ground state with loss of light (fluorescence) and heat • transfer to another chlorophyll.

  8. Chlorophylls are organised into photosystems to trap light energy Special chlorophyll molecule - primary electron acceptor Light Transfer of light energy from one chlorophyll to another Chlorophylls are organised into photosystems

  9. Cyclic photosphorylation generates ATP Stromal side Fd PQ PQ P700 PSII PSI Thylakoid space side

  10. Non-Cyclic photosphorylation generates ATP and NADPH NADP+ Stromal side NADPH Q Fd PQ PQ P680 P700 PSII PSI H20 2H+ & 2e- & 1/2 O2 Thylakoid space side

  11. H+ gradient generated by: PQ shuttle (H+) Splitting of water (2H+) NADPH production (2H+ removed from stroma) ATP generated by an ATP synthase as H+ ions flow through it Generation of ATP H+ Stroma ATP H+ ADP +Pi H+ H+ H+ H+ H+

  12. Light-independent reactions

  13. The Calvin/Benson cycle Ribulose biphosphate Carbon CO2 Phosphorus 3-phosphoglycerate ATP ADP ADP ATP 1,3 diphospho- glycerate NADPH Sugars NADP+ G-3-P Glyeraldehyde-3-phosphate

  14. C4 metabolism

  15. C3 and C4 metabolism • Inefficient as RUBISCO can act as both a carboxylase and as an oxygenase. • Oxygenase activity leads to loss of carbon that has already been fixed. • Some plants have an alternative pathway where CO2 is first fixed into C4 organic acids (C4 pathway) and then liberated later to undergo the Calvin/Benson cycle (C3 metabolism).

  16. C4 metabolism • 2 types • C3 and C4 metabolism are separated in space (different cells) e.g. sugar cane Characteristic aggregation of cells around the vascular bundles - Krantz morphology • C3 and C4 metabolism are separated in time (same cell) e.g. pineapple.

  17. C3 - note the lack of chloroplasts in the bundle sheath C4 - note the extensive chloroplasts in the bundle sheath Comparison of C3 & C4 leaves

  18. Separation in space CO2 PEP (3C) Mesophyll cell surrounding bundle sheath OAA (4C) ADP ATP Pyruvate (C3) CO2 C3 Bundle sheath cell sugar Vascular tissue

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