 |
| C3 and C4 Pathway (Dark Reaction) |
|
|
| ATP and NADPH2, formed during the earlier phase are essential for the assimilation of CO2 to carbohydrates. This assimilation reaction takes place in the stroma, where all the necessary enzymes are present. Collectively, these reactions are known as carbon reactions or dark reactions. |
| |
| The Calvin Cycle : The Three Carbon Pathway |
| |
| The C3 type of carbon reactions occurs in the strome of the chloroplast. It is also called Calvin cycle after its discoverer, Melvin Calvin. |
| |
| a) In the first step CO2 is accepted by a 5- carbon molecule, ribulose -1,5 biphosphate (RuBP) and 2 molecules of 3-carbon compound that is 3- phosphoglycerate (PGA) are formed. This reaction is catalysed by an enzyme called Rubisco-Ribulose biphosphate carboxylase oxygenase. Formation of PGA is called carboxylation. |
| |
| b) The 3 - carbon compound formed is the first stable product of this pathway and hence the name C3 pathway. |
| |
| c) After carboxylation, reduction of PGA occurs by utilising ATP and NADPH2 formed during photochemical reactions. Reduction results in the formation of glyceraldehyde-3-phosphate. |
| |
| d) These 3 - carbon molecules, also called triose phosphates are diverted from the Calvin cycle and act as precursors for the synthesis of sucrose and starch. |
| |
 |
| |
| The Calvin cycle proceeds in three stages: (1) carboxylation, during which CO2 combines with ribulose-1,5-bisphosphate (2) reduction,
during which carbohydrate is formed at the expense of the photochemically made ATP and NADPH; and (3) regeneration during which the CO2 acceptor ribulose 1,5-bisphosphate is formed again so that the cycle continues |
| |
| For the cycle to continue on its own regeneration of the initial 5- carbon acceptor molecule i.e., RuBP takes place, from glyceraldehyde 3- phosphate using an ATP molecule. |
| |
