Lecture Notes for Chapter 16 Photosynthesis

1. Essential Biochemistry Third Edition Charlotte W. Pratt | Kathleen Cornely Lecture Notes for Chapter 16 Photosynthesis

2. Photosynthesis in Context

3. KEY CONCEPTS: Section 16-1 • Photosynthetic pigments absorb different wavelengths of light to become excited. • Light-harvesting complexes direct light energy to the reaction center.

4. Photosynthesis in green plants takes place in chloroplasts Electron micrograph of a chloroplast from tobacco

5. Chloroplasts contain a variety of light-absorbing groups called pigments or photoreceptors

6. Chloroplasts contain a variety of light-absorbing groups called pigments or photoreceptors

7. Photoreceptors absorb light of different wavelengths

8. Many different things can happen when pigments absorb light

9. Photosynthesis Machinery • The primary reactions of photosynthesis occur at specific chlorophyll molecules called reaction centers. • Some membrane proteins in plants are light-harvesting complexes. • Light-harvesting complexes can also bind pigments and absorb light at various wavelengths.

10. Light-Harvesting Complex Top Viewshowing onlychlorophyll molecules Side View Top View

11. Light-harvesting complexes transfer energy to the reaction center Light-harvesting complex Reaction Center

12. KEY CONCEPTS: Section 16-2 • The P680 reaction center of Photosystem II undergoes photooxidation. • Photosystem II splits water to replace the lost P680 electron and generate O2. • Electrons from Photosystem II travel via plastoquinone, cytochrome b6f, and plastocyanin to Photosystem I. • Photooxidation of P700 in Photosystem I drives cyclic and noncyclic electron flow. • The proton gradient across the thylakoid membrane drives ATP synthesis.

13. Light Reactions • Excitation of the reaction centers drives a series of oxidation-reduction reactions with several net results: • Oxidation of water • Reduction of NADP+ • Generation of a transmembrane proton gradient that powers ATP synthesis • These reactions are the light reactions.

14. The light reactions begin with an integral membrane protein called Photosystem II Protein: gray Pigments: various colors

15. Chlorophyll molecules in Photosystem II funnel energy to reaction centers containing P680 • P680 = pair of chlorophyll molecules • P680 becomes oxidized • Electron transfer occurs across several groups

16. Prosthetic groups in Photosystem II are arranged to facilitate electron transfer Chlorophyll reaction center: green Accessory chlorophyll: yellow Pheophytin: orange Iron atom: red Plastoquinone: blue

17. Photosystem II oxidizes water using a unique cofactor Structure of the Mn4CaO5 clusterMn: purple; Ca: green; O: red

18. The input of solar energy allows an electron to travel a thermodynamically favorable path from water to plastoquinone Let’s see how this is possible by considering reduction potentials…

19. Reduction Potential and Electron Flow in Photosystem II

20. For every O2 molecule evolved, two plastoquinone molecules are reduced to plastoquinol

21. Electrons in Plastoquinol are transferred to another protein complex: cytochrome b6f Each subunit is a different color

22. Plastocyanin and Its Role • Electron flow in cytochrome b6f is probably similar to the Q cycle: • The final electron acceptor is not cytochrome c, but plastocyanin • Plastocyanin uses a copper ion (green) to transfer electrons

23. Production of 1 O2 causes cytochrome b6f to produce 8 lumenal H+

24. A second photooxidation occurs at Photosystem I Protein: gray Prosthetic Groups: various colors

25. The core of each Photosystem I is a pair of chlorophyll molecules: P700

26. Prosthetic Groups in Photosystem I P700: green Accessory chlorophyll: yellow 4Fe-4S clusters: orange Quinones: blue

27. Each electron given up by photooxidized P700 eventually reaches ferredoxin 2Fe-2S cluster: orange

28. Reduced ferredoxin can participate in noncyclic electron flow

29. Reduction potentials for the flow of electrons from H2O to NADP+ is called the Z-scheme of photosynthesis

30. Reduced ferredoxin can also participate in cyclic electron flow

31. Chemiosmosis provides the free energy for ATP synthesis • Chloroplasts and mitochondria use the same mechanism to synthesize ATP • Coupling transmembrane proton pumping to phosphorylation of ADP • In plants, this process is called photophosphorylation

32. KEY CONCEPTS: Section 16-3 • Rubisco catalyzes carbon fixation by adding CO2 to a five-carbon acceptor molecule. • The Calvin cycle shuffles sugars for the net conversion of three CO2 to one glyceraldehyde-3-phosphate. • Light-dependent mechanisms regulate the activity of the Calvin cycle. • Newly synthesized sugars are incorporated into sucrose and polysaccharides.

33. Reactions that utilize the products of the “light reactions” are called “dark reactions” • Dark reactions occur in chloroplast stroma • Dark reactions fix atmospheric CO2

34. The Structure of Spinach Rubisco • 550kD protein • Large subunit: dark colors • Small subunit: light colors

35. Rubisco is not a highly specific enzyme

36. Rubisco is not a highly specific enzyme • Rubisco can also act as an oxygenase • Metabolism of 2-phosphoglycolate involves: • ATP and NADPH consumption • CO2 production • Also called photorespiration

37. The Calvin cycle produces the ribulose-1,5-bisphosphate required for CO2 fixation

38. Initial Reactions of the Calvin Cycle

39. The remaining part of the Calvin Cycle involves isomerization and group transfer reactions

40. The Calvin Cycle recycles 3-C molecules Representations of Glyceraldehyde-3-phosphate

41. Fixing a single CO2 requires 3 ATP and 2 NADPH

42. Calvin cycle products are used to synthesize sucrose

43. Calvin cycle products are used to synthesize starch