The main objective of light reaction is the synthesis of assimilatory powers like ATP and NADPH. Here assimilatory powers are synthesised during the electron transport reactions. This synthesis of assimilatory power is called photophosphorylation. It is of two different types as; cyclic and non cyclic
Here PS I gets excited by illumination and the excited electrons move through a series of electron carries and return back to PS I. During this ATP molecules are formed. PS II will never involve in the reaction.
Non cyclic photophosphorylation
Here both the PS I and PS II are get excited by absorbing light. Electrons from PS II move through different electron carriers and finally reach the PS I. Electron from PS I finally reaches NADP+. The electron path is not cyclic and is zig zag. So it is known as Z scheme. PS II is associated with water splitting complex. Spliting of water results in the release of electron, proton and 02. Electrons are accepted by PS II, H+ ions move to NADP and NADPH is formed. 02 is released into the atmosphere. Spliting of water takes place in light reaction is known as photolysis of water. As a result of non cyclic electron transport ATP, NADPH and 02 are formed. Schematic representation of both cyclic and non cyclic photophosphorylation can be represented as follows.
Difference between Cyclic and Non-Cyclic Photophosphorylation.
ATP and NADPH produced during the light reaction are used during this light independent
biochemical enzymatic reaction to reduce C02 into carbohydrates.
6RuBP + 6CO2 + 18ATP + 12NADPH ----------> C6H1206 + 6RuBP + 18ADP + 18 Pi + 12NADP
This phase is called Calvin cycle as it was traced by Calvin. It is also called C3 cycle as the first stable product is a 3 -carbon compound called phosphoglyceric acid. It is called dark reaction, as it is independent of light.
Dark reaction occurs in three major steps in stroma as (1) Carboxylation, (2) Reduction and (3) Regeneration of RuBP.
1.Carboxylation - Ribulose bisphosphate (5 carbon) combines with C02 and forms a stable 3 carbon, phospho glyceric acid in presence of RuBisCO enzyme..
2. Reduction (Glycolytic reversal) - PGA first reacts with ATP to form 1,3 diphosphoglyceric acid and then it is reduced to glyceraldehyde phosphate (PGAL) by NADPH. 12mol. PGA + 12 ATP ----------> 12 mol 1,3 di PGA + 12 ADP + 12 P
3. Regeneration of RuBP - All the molecules of RuBP entering in the Calvin cycle had to be regenerated at the end of the cycle. Out of the 12 mol of glyceraldehyde phosphate, 10 molecules are utilized for this along with 6 molecules of ATP. The other 2 mol of PGAL released from Calvin cycle combine to form fructose 1, 6-biphosphate and then gets converted to glucose, the ultimate product of photosynthesis.