1. Photosynthesis 6CO 2 + 12H 2 0 C 6 H 12 O 6 + 6O 2 + 6 H 2 O

2. Overview 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

3. Photosynthesis Light reactions Calvin/Benson Cycle (Dark Reactions) ADP ATP NADP+ NADPH CO 2 Sucrose H 2 O O 2

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

5. The light reactions of photosynthesis

6. Plants absorb Light Wavelength of light (nm) 400 500600700 GreenRed Blue Abs Chlorophyll a Chlorophyll b Carotenoids

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

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

10. Non-Cyclic photosphorylation generates ATP and NADPH PQ PSII PSI Stromal side Thylakoid space side Fd NADP+ NADPH Q P700P680 H 2 0 2H+ & 2e- & 1/2 O 2

11. Generation of ATP 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 ADP +Pi ATP H+ Stroma

12. Light-independent reactions

13. The Calvin/Benson cycle Ribulose biphosphate CO 2 Carbon Phosphorus 3-phosphoglycerate ATP ADP 1,3 diphospho- glycerate NADPH NADP+ Glyeraldehyde-3-phosphate G-3-P Sugars ATP ADP

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 CO 2 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. Comparison of C3 & C4 leaves C3 - note the lack of chloroplasts in the bundle sheath C4 - note the extensive chloroplasts in the bundle sheath

18. Separation in space Vascular tissue Bundle sheath cell Mesophyll cell surrounding bundle sheath CO 2 PEP (3C) C3 CO 2 OAA (4C) Pyruvate (C3) ATP ADP sugar