Dark Reaction of Photosynthesis


 
 
Concept Explanation
 

Dark Reaction of Photosynthesis

The process of photosynthesis can be divided into two phases : Light phase and Dark phase

Dark phase of photosynthesis:

This phase does not require direct sunlight, but it depends on the products of the light reaction, i.e., ATP and NADPH beside CO_2 and water that drive the processes leading to the synthesis of food more accurately the sugars (glucose is first product of photosynthesis).

1. C3 Pathway : - This pathway is followed by the plans when first product of CO_2 fixation is a C_3 acid, i.e., PGA.

2. C4 Pathway : - This pathway is followed or shown by the plants in which first product of CO_2- fixation is a C_4 acid, .i.e., OAA.

This is a cycle biochemical pathway of reduction of CO_2  or photosynthetic carbon, cycle which was  discovered by Calvin. The Calvin cycle runs in all photosynthetic plants, no matter they show C_3C_4 or any other pathway, It occurs in stroma of the chloroplast.

Primary Acceptor of CO2 in C3 Pathway

After a long research and conducting many experiments it was concluded by the scientists that in C_3 pathway, the acceptor molecule is a 5-carbon S ketose sugar, i.e., Ribulose 5-Bisphosphate (5 RUBP). Calvin or C_3  cycle has following three major steps Calvin or C3 cycle has many steps which are known as glycotic reversal or formation of sugar and takes place between reduction and regeneration. There are three major steps as follows

Carboxylation :It is the most crucial step of the Calvin cycle. In this fixation of CO_2  molecule takes place in the form of carboxylation of RUBP(5C). This reaction is catalysed by the enzyme RUBP carboxylase. This finally leads to the formation of two molecules of 3 Phosphoglyceric Acid (3PGA). As the RUBP carboxylase enzyme also has an activity of oxygenation.

Thus, it is more commonly known as RuBP carboxylase - oxygenase or RuBisCO.

Reduction :Afrer the carboxylation reaction, reduction of PGA takes place through a series of reactions leading to the formation of glucose. In this step, the ATP (as energy source) and NADPH (hydrogen atom carrier) are utilised. It is to be noted that 2 molecules of ATP and 2 molecules of NADPH urilised in this step for phosphorylation and for the eduction of CO_2 respectively.

Regeneration :For the continuous and uninterrupted functioning of the Calvin cycle, there must be a regular supply of ATP, NADPH and also sufficient amount of RuBP is required.

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