1. Chapter Twenty-Two
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?
Prokaryotes: in granules bonded to the plasma membrane
Eukaryotes: in chloroplasts
Chloroplast
Inner, outer, and thylakoid membranes
Grana, which consist of stacks of thylakoid disks
Trapping of light and production of O2 take place in thylakoid disks
Light reactions take place in thylakoid disks
Dark reactions take place in the stroma

4. Photosynthesis in Eukaryotes

5. Light Dependent and Light-Independent Reactions of Photosynthesis

6. Chlorophyll
Structure similar to the heme group of Mb, Hb, and the cytochromes
It is based on tetrapyrrole ring of porphoryns

7. Chlorophyll (Cont’d)
Absorb red ( nm) and blue ( 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

8. Visible Spectra of Chlorophylls and Accessory Pigments

10. 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

11. 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 photophosphorylation
H2O + NADP+ ---> NADPH + H+ + O2
ADP + Pi ---> ATP
The light reactions are accomplished by two distinct photosystems; photosystem I and photosystem II

12. Light Reactions Photosystem I (PSI) is the reduction of NADP+ to NADPH
Photosystem II (PSII) is the 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

13. The Z Scheme of Photosynthesis

14. 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+
Mn complex

15. Cyclic Electron Transport in PSI can be Coupled to ATP Production

16. The Calvin Cycle

17. 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 and 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

18. The C4 Pathway