Photosynthesis

2. Photosynthesis • Photosynthetic organisms carry out the reaction 6CO2 + 6H2O → C6H12O6 + 6O2 • The equation represents two processes • Light Reactions: NADPH and ATP are produced • Dark Reactions: ATP and NADPH provide the energy and reducing power for the fixation of CO2

3. Where is the Site of Photosynthesis? • Site of photosynthesis • Prokaryotes: in granules bonded to the plasma membrane • Eukaryotes: in chloroplasts • Chloroplast • Inner, outer, and _______________ membranes • _________, which consist of stacks of thylakoid disks • Trapping of light and production of O2 take place in thylakoid disks • Light reactions take place in ______________ disks • Dark reactions take place in the ____________

4. Photosynthesis in Eukaryotes

5. Chlorophyll • Structure similar to the heme group of Mb, Hb, and the cytochromes • Based on tetrapyrrole ring of porphoryns

6. Chlorophyll • Absorb red (600-700 nm) & blue (400-500 nm) light • Accessory pigments absorb light and transfer energy to chlorophylls (Chl) • Chlorophylls arranged in photosynthetic units • Antennae chlorophylls gather light • Harvested light energy passed to specialized Chl molecules at a reaction center • Several hundred light-harvesting antennae Chl for each Chl at a reaction center • Chemical reactions of photosynthesis begin at reaction centers

7. Visible Spectra of Chlorophylls and Accessory Pigments

8. Summary • In eukaryotes, photosynthesis takes place in chloroplasts. The light reactions take place in the thylakoid membrane, a third membrane in chloroplasts in addition to the inner and outer membrane • The dark reactions of photosynthesis take place in the stroma, in between the thylakoid membrane and the inner membrane of the chloroplast • The absorption of light by chlorophyll supplies the energy required for the reactions of photosynthesis. All types of chlorophylls have a tetrapyrrole ring structure similar to that of the porphyrins of heme, but they also have differences that affect the wavelength of light they absorb • This property allows more wavelengths of sunlight to be absorbed than would be the case with a single kind of chlorophyll

9. Photosynthesis I and II and Light Reactions • In the light reactions of photosynthesis, H2O is oxidized to O2 and NADP+ is reduced to NADPH • This series of redox reactions is coupled to the phosphorylation of ADP to ATP in a process called _______________________________ H2O + NADP+ → NADPH + H+ + O2 ADP + Pi → ATP • Two distinct photosystems of the light reactions: photosystem I and photosystem II

10. Light Reactions • Photosystem I(PSI), reduction of NADP+ to NADPH • Photosystem II(PSII): oxidation of H2O to O2 • The reaction is endergonic (G˚’=+220 kJ mol-1) • The reaction is driven by the light energy absorbed by the chlorophylls of the two photosystems

11. The Z Scheme of Photosynthesis

12. The Oxygen Evolving Process • Involves photosystem II through a system of five oxidation states S0-S4 • Thenet reaction of photosystems I and II is 2H2O + 2NADP+ → O2 + 2NADPH + 2H+

13. Cyclic Electron Transport in PSI Coupled to ATP Production

14. The Structure of Photosynthetic Reaction Centers Most extensively studied is in Rhodopseudomonas • A reaction center contains a pair of bacteriochlorophyll molecules embedded in a protein complex that is, in turn, an integral part of the photosynthetic membrane • Absorption of light raises it to a higher energy level • An excited electron is passed to pheophytin, then to menaquinone, and then to ubiquinone (next screen) • A cytochrome molecule transfers an electron to the reaction center; the cytochrome molecule now has a positive charge • The excited electron is passed to menaquinone and then to ubiquinone • The charge separation represents stored energy

15. Quinone Electron Acceptors

16. The Structure of Photosynthetic Reaction Centers • Accessory pigments have specific positions close to the special pair of chlorophylls • The first of the accessory pigments is pheophytin • Pheophytin is structurally similar to chlorophyll • The next electron acceptor is menaquinone (QA) • Menaquinone is structurally similar to coenzyme Q • The last electron acceptor is coenzyme Q (QB)

17. Summary Photosynthesis consists of two processes. The light reactions are electron transfer processes, in which water is oxidized to produce oxygen and NADP+ is reduced to produce NADPH The path of electrons in the light reactions: • The first is the transfer of electrons from water to the reaction-center chlorophyll of PSII • Next is the transfer of electrons from the excited-state chlorophyll of PSII to an electron transport chain consisting of accessory pigments and cytochromes, with energy provided by absorption of a photon of light. The components of this electron transport chain resemble those of the miochondrial electron transport chain; they pass the electrons to the reaction-center chlorophyll of PSI • The third and last part of the path of the electrons is their transfer from the excited-state chlorophyll of PSI to the ultimate electron NADP+, producing NADPH. Again, energy is provided by absorption of a photon of light

18. Photosynthesis and ATP Production • A proton gradient across the inner mitochondrial membrane drives ___________________________ • The proton gradient is created • By the splitting of H2O which releases H+ into the thylakoid space • By electron transport from Photosystem II to Photosystem I • When Photosystem I reduces NADP+ by using H+ in the stroma to produce NADPH • The flow of H+ back to the stroma through ATP synthase provides the energy for ADP + Pi → ATP

19. Photosynthesis and ATP Production • Chloroplasts can phosphorylate ADP in the dark if they are provided with ________________________ • The production of ATP does not require _____________; the proton gradient produced by the pH gradient supplies the energy Summary: • The mechanism of ATP in chloroplasts closely resembles the process that takes place in mitochondria. • The structures of ATP synthase in the chloroplasts and mitochondria are similar Fig 22.12 ATP is synthesized in the dark by a proton gradient

20. Components of the ETC of the Thylakoid Membrane

21. Evolutionary Implications of Photosynthesis • Photosynthetic prokaryotes other than cyanobacteria have only one photosystem and do not produce oxygen • Anaerobic photosynthesis is not as efficient as photosynthesis linked to oxygen, but the anaerobic version of the process appears to be evolutionary • The ultimate source of electrons for these organisms is not H2O, but some more easily oxidized substance,one of which is H2S • The H-acceptor may also be NO2- or NO3 - --> NH3 • There are two possible pathways for electron transport to occur in an anaerobic organism: • Cyclic • Noncyclic

22. Evolutionary Implications of Photosynthesis

23. Summary: • When photosynthesis first evolved, it was most likely to have been carried out by organisms that used compounds other than water as the primary electron source. • Cyanobacteria were the first organisms to use water as the source of electrons, giving rise to the present oxygen-containing atmosphere

24. The Dark Reaction of Photosynthesis Fix CO2 • CO2 fixation takes place in the _______________ • The actual reaction pathway has features in common with __________________ and some in common with the _____________ ______________ ______________ • The overall reaction is called the Calvin cycle after Melvin Calvin, Nobel Prize for Chemistry in 1961

25. The Calvin Cycle

26. The Calvin Cycle • The first reaction is the carboxylation of six molecules of ribulose-1,5-bisphosphate • This carboxylation is the actual fixation step • Each carboxylation product splits to give two molecules of 3-phosphoglycerate (12 total) • Two molecules of 3-phosphoglycerate are converted to glucose • Ten 3-phosphoglycerates are used to regenerate six molecules of ______________________________

27. The Calvin Cycle • The reaction of ___________________________ with CO2 produces the 3-phosphoglycerate

28. The Calvin Cycle The cycle can be divided into four stages: • _______________: production of glyceraldehyde-3-phosphate and dihydroxyacetone phosphate • _______________: many reshuffling reactions are like those of the pentose phosphate pathway and involve transaldolase and transketolase reactions • _______________: conversion of both ribose 5-phosphate and xylulose-5-phosphate to ribose-5-phosphate • _______________: in the final step, ribulose-5-phosphate is phosphorylated to ribulose-1,5-bisphosphate

29. The Calvin Cycle Series of Reactions

30. Summary • In the dark reactions of photosynthesis, the fixation of CO2 takes place when the key intermediate ____________________________ reacts with CO2 to produce two molecules of 3-phosphoglycerate. The reaction is catalyzed by RUBISCO. • The remainder of the dark reaction is the regeneration of ribulose-1,5-bisphosphate in the Calvin Cycle

31. CO2 Fixation in Tropical Plants • An alternative pathway for CO2 fixation in tropical plants, known as the Hatch-Slack pathway, also called a C4 pathway • CO2 enters the outer (mesophyll) cells and reacts with phosphoenolpyruvate to give oxaloacetate & Pi • Oxaloacetate is reduced to malate • Malate is transported to inner (bundle-sheath) cells where it is oxidized and decarboxylated to pyruvate • CO2 is then passed to the Calvin cycle where it reacts with ribulose-1,5-bisphosphate

32. The C4 Pathway