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?
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
• The net 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 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