Photosynthesis Green plants converts CO2 to organic compound. This process comprises two stages 1. Light reaction, 2. Dark reaction.

Light reaction Phototransduction 2H2O+2NADP+ +8Photons=2NADPH+2H+ +O2. Two photons are required for the transfer of one electrone from H2O to NADP+.

Calvin cycle The light driven synthesis of ATP and NADPH provides energy and reducing power for the fixation of CO2 into trioses, from which all carbon containing compounds of plant cells are synthesized. CO2 assimilated in a cyclic pathway. This cycle is known as photosynthetic carbon reduction cycle.

Carbon fixation reaction CO2+Ribulose1,5-Bisphsphate=2molecules of 3-Phosphoglycerate.

3-phosphor glycerate reduced to triose phosphate.

C3 Plants The plants in which 3 carbon compound is the first intermediate is known as C3 Plants

Rubisco The enzyme that catalyzes covalent attachment of CO2 to 5carbon sugar Ribulose 1,5-bisphosphate carboxylase oxygenase. This name shortend to became Rubisco. Rubisco made upof 8 identical subunits and each subunit contain 1catalytic site and a small subunit.

Triose phosphate isomerase then interconverts glyceraldehyde 3-phosphate and Dihydroxy acetone phosphate. Most of the triose phosphate thus produced is used to regenerate ribulose 1,5-bisphosphate , the rest is either converted to starch in the choloroplast and stored for later use or immediately exported to the cytosol converted to sucrose for transport to growing regions of the plant.

Synthesis of each triose phosphate from CO2requires 1 ATP and 1 NADPH+H+. From 3-phosphate glycerate to fructose 1,6-bisphosphate again use 1ATP and 1NADPH+H+.

Photorespiration Due to Rubisco’s oxygenase activity molecular oxygen competes with CO2 at the active site, and about once in every three or four turn overs. Rubisco catalyzes the condensation of O2 with Ribulose 1,5-bisphosphate to form 3-phoshoglycolate a metabolically useless product.

C4 Plants Mechanism evolved to circumvent the problem of wasteful photorespiration.(Eg., Maize, Sugarcane , Sorghum etc.) In these plants the first intermediate intowhich CO2 is oxaloacetate (4carbon compound). This occurs in cytosol HCO3- is the substrate and the enzyme is phosphoenol pyruvate carboxylase.

Oxaloacetate is either reduced to malate at the expence of NADPH or converted to aspartate by transamination. The malate or aspartate formed in the mesophyll cells then passes in to bundle sheath cells through Plsmodesma Aspartate coverts to oxaloacetate and then to malate.

For each molecule of CO2 assimilation in C4 plants use 5ATP.

To reduce loss of water while CO2 uptake. CAM Plants To reduce loss of water while CO2 uptake. CO2 fixation occurs at night CO2 is fixed to oxaloacete by PEP carboxylase. Oxaloacetate reduced to malate and stored in vaccules During Day stomata closed to reduce water loss. CO2 is released from malate.