Unit: Photosynthesis

1. Unit: Photosynthesis Light-Dependent Reactions Light-Independent Reactions Carbon Fixation

2. Where does photosynthesis occur? Chloroplasts • about 0.5 million per m2 electron micrograph • TEM

3. Where does photosynthesis occur? Leaf cells • mesophyll – the interior of leaf cells • stomata – microscopic pores that facilitate intake of CO2 and release of O2 image • LM

4. Chemical Pathways of Photosynthesis Research • first thought that oxygen was a by-product from the metabolism of carbon dioxide • 1930s – research at Stanford University by C. B. van Niel disproved this theory Evidence • chemical evidence illustrates that a H source is required by all organisms that undergo some form of photosynthetic series of pathways • sulfur bacteria: CO2 + 2H2S  [CH2O] + H2O + 2S • plants: CO2 + 2H2O  [CH2O] + H2O + O2

5. Chemical Pathways of Photosynthesis Light Dependent Reactions • chemiosmosis processes provide the chemical energy to bond a phosphate group (Pi) to ADP - photophosphorylation

6. Chemical Pathways of Photosynthesis Light Dependent Reactions • chemiosmosis processes provide the chemical energy to bond a phosphate group (Pi) to ADP - photophosphorylation Redox • NADP+ is reduced to carry electrons Reduction of an electron acceptor • NADP+ + 2e- + H+ NADPH

7. Chemical Pathways of Photosynthesis Light Independent Reactions • carbon fixation – chemical addition of carbon atoms from CO2 into organic compounds • these “dark” reactions are also referred to as the Calvin Cycle, named after Melvin Calvin (1940s)

8. Chemical Pathways of Photosynthesis Light Independent Reactions • carbon fixation – chemical addition of carbon atoms from CO2 into organic compounds Redox • CO2 is reduced by the e- and H+ from NADPH • CH2O units are produced

9. Chemical Pathways of Photosynthesis Photosystem II to Photosystem I

10. Chemical Pathways of Photosynthesis Photosystem II to Photosystem I

11. Chemical Pathways of Photosynthesis Photosystem II to Photosystem I

12. Chemical Pathways of Photosynthesis Photosystem II to Photosystem I

13. Chemical Pathways of Photosynthesis Photosystem II to Photosystem I

14. Chemical Pathways of Photosynthesis Calvin Cycle: Phase 1 Carbon fixation • CO2 is added to a 5-C molecule, RuBP • the enzyme ribulosebiphosphate carboxylase (Rubisco) catalyzes this reaction RuBP • ribulosebiphosphate

15. Chemical Pathways of Photosynthesis Calvin Cycle: Phase 2 Reduction PGA is: • phosphorylated by ATP • then reduced by NADPH • 1 molecule of G3P is available to form sugars • 5 molecules of G3P are recycled to form RuBP

16. Chemical Pathways of Photosynthesis Calvin Cycle: Phase 3 Regeneration of RuBP • G3P is phosphorylated by ATP molecules to form RuBP • RuBP acts as a CO2 acceptor • the process begins anew