Chapter Twenty-Two Photosynthesis

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

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

Photosynthesis in Eukaryotes

Light Dependent and Light-Independent Reactions of Photosynthesis

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

Chlorophyll (Cont’d) Absorb red (600 - 700 nm) and 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

Visible Spectra of Chlorophylls and Accessory Pigments

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

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

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

The Z Scheme of Photosynthesis

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

Cyclic Electron Transport in PSI can be Coupled to ATP Production

The Calvin Cycle

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

The C4 Pathway