DARK REACTION ( Light Independent Phase Reaction )


 

Light Independent Phase (Dark Reaction)

 

The light independent phase (dark reaction) takes its name from the fact that light is not directly required for these reactions to occur. This phase requires the availability of NADPH, ATP (the products of light reaction) and CO₂. In this phase of photosynthesis, NADPH is used to reduce carbon dioxide while ATP is used to incorporate energy. 






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Finally, CO₂ is converted into a phosphorylated triose carbohydrate i.e.. glyceraldehyde-3 phosphate (G3P) which are later on used to make glucose. Dark reaction generally involves a complicated metabolic pathway, the Calvin cycle or C3 pathway. However, in some plants, in addition to Calvin cycle another metabolic pathway is also involved, called C4 pathway. The plants in which only Calvin cycle occurs during dark reaction are called. C3 plants.


Calvin cycle


Calvin cycle term is applied to the series of metabolic reactions in which CO2 is reduced to produce G3P. (These reactions have been explored by Melvin Calvin and co workers at the University of California. Melvin Calvin won the Nobel Prize in 1961 for this work). The Calvin cycle can be divided into three phases, carbon fixation, reduction and regeneration of carbon dioxide acceptor l.e., RuBP.


Carbon fixation

 One of the key substance in this process is a five carbon phosphorylating sugar а called ribulose bisphosphate (RuBP). It is generally referred as CO₂ acceptor because it is capable of combining with carbon dioxide with the help of Ribulose bisphosphate (RuBP) carboxylase/oxygenase also known as RuBisCO. Three intermediate molecules of six carbons are formed during this reaction. 

These molecules are unstable and exist for such a short time that, they cannot be isolated. Each six carbon breaks down to form two molecules of 3- phosphoglycerate (3-PGA), a phosphorous containing compound with three carbon atoms. Since, the carbon of inorganic compound (CO2) becomes the part of organic compound (RuBP) during this phase, hence, it is called carbon fixation. 

As the first stable compound in the Calvin cycle is a three carbon compound (3-PGA) that is why Calvin cycle is also known as C3 pathway.

Reduction

 In this phase six molecules of 3 phosphoglycerate (3-PGA) react with six ATP molecules, a phosphate from each ATP is transferred to each 3-PGA. In this way, 3-PGA molecules are changed into 1,3 Bisphosphoglycerate. These molecules are then reduced by the hydrogen of NADPH and finally glyceraldehyde 3 phosphate (G3P) molecules are produced.

 During this reduction process a phosphate group from each 1,3 Bisphosphoglycerate molecule is also given off. There are total six molecules of G3P are produced in this phase but only one molecule is released from the cycle while rest of the five molecules are used to regenerate the CO₂ acceptor molecules in the next phase.

Regeneration of CO₂ acceptor

Five molecules of G3P from the previous phase are used to regenerate the RuBP (CO₂ acceptor) in this phase. These five molecules each containing three carbon atoms undergo a series of reactions in which three molecules of ribulose phosphate (RuP) each containing five carbon atoms are produced. When three molecules of RuP react with three molecules of ATP, a phosphate group from each ATP is transferred to each RuP. 

Ultimately RuP are converted into RuBP which again participate in the next cycle. The whole process of Calvin cycle indicates that there are three molecules of CO2. six molecules of NADPH (reducing power) and nine molecules of ATP (assimilating power). are used to release just one molecule of G3P form the cycle. 

However, in order to produce a glucose molecule, two molecules of G3P are required.