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Question
Describe the calvin cycle with ray diagram.
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Solution
Calvin cycle is also known as C3 cycle. The Calvin cycle can be divided into four major steps which are as follows -
(A) Carboxylation
(B) Reduction of phosphoglyceric acid
(C) Synthesis of hexose sugar
(D) Regeneration of ribulose bisphosphate
(A) Carboxylation - In this process, 5C compound ribulose 1, 5 biphosphate (RuDP or RuBP), primary acceptor of CO2 is photosynthesis, combines with CO2 to form a 6C- compound which is quite unstable and splits into 2 molecules of 3C- compound, 3 phosphoglyceric acid. This reaction is catalyzed by the enzyme ribulose diphosphate carboxylase (RuDP carboxylase). This enzyme is abundantly found in all the photosynthetic cells. 3C- compound phosphoglyceric acid is the first stable product (compound) of Calvin cycle.
(B) Reduction of phosphoglyceric acid - 12 molecules of 3 phosphoglyceric acids combine with 12 molecules of ATP (produced in the light reaction) to form 12 molecules of 1, 3- di phosphoglyceric acid. This reaction takes place in the presence of enzyme phosphoglycerol kinase
(C) Synthesis of hexose sugar - 3 molecules of phosphoglyceraldehyde combine with 3 molecules of dihydroxyacetone phosphate (formed in the reaction number 4) to form 3 molecules of fructose 1, 6- diphosphate. The reaction is catalyzed by enzyme aldolase.
3 molecules of fructose 1, 6- diphosphate forms 3 molecules of fructose -6- phosphate by giving out phosphoric acid in the presence of phosphatase enzyme.
(D) Regeneration of ribulose diphosphate -2 molecules of 3- phosphoglyceraldehyde reacts with 2 molecules of fructose -6- phosphate to produce 3 molecules of xylulose -5- phosphate and 2 molecules of erythrose -4 phosphate. The reaction is catalyzed by enzyme transketolase.
Finally, all the 6 molecules of ribulose -5- phosphate get phosphorylated by 6 molecules of ATP in the presence of enzyme phospho- ribulose kinase and regenerate 6 molecules of ribulose 1, 5- diphosphate. In this way, all the 6 molecules of ribulose 1, 5-diphosphate are regenerated and then again enter the cycle.
(A) Carboxylation
(B) Reduction of phosphoglyceric acid
(C) Synthesis of hexose sugar
(D) Regeneration of ribulose bisphosphate
(A) Carboxylation - In this process, 5C compound ribulose 1, 5 biphosphate (RuDP or RuBP), primary acceptor of CO2 is photosynthesis, combines with CO2 to form a 6C- compound which is quite unstable and splits into 2 molecules of 3C- compound, 3 phosphoglyceric acid. This reaction is catalyzed by the enzyme ribulose diphosphate carboxylase (RuDP carboxylase). This enzyme is abundantly found in all the photosynthetic cells. 3C- compound phosphoglyceric acid is the first stable product (compound) of Calvin cycle.
(B) Reduction of phosphoglyceric acid - 12 molecules of 3 phosphoglyceric acids combine with 12 molecules of ATP (produced in the light reaction) to form 12 molecules of 1, 3- di phosphoglyceric acid. This reaction takes place in the presence of enzyme phosphoglycerol kinase
(C) Synthesis of hexose sugar - 3 molecules of phosphoglyceraldehyde combine with 3 molecules of dihydroxyacetone phosphate (formed in the reaction number 4) to form 3 molecules of fructose 1, 6- diphosphate. The reaction is catalyzed by enzyme aldolase.
3 molecules of fructose 1, 6- diphosphate forms 3 molecules of fructose -6- phosphate by giving out phosphoric acid in the presence of phosphatase enzyme.
(D) Regeneration of ribulose diphosphate -2 molecules of 3- phosphoglyceraldehyde reacts with 2 molecules of fructose -6- phosphate to produce 3 molecules of xylulose -5- phosphate and 2 molecules of erythrose -4 phosphate. The reaction is catalyzed by enzyme transketolase.
Finally, all the 6 molecules of ribulose -5- phosphate get phosphorylated by 6 molecules of ATP in the presence of enzyme phospho- ribulose kinase and regenerate 6 molecules of ribulose 1, 5- diphosphate. In this way, all the 6 molecules of ribulose 1, 5-diphosphate are regenerated and then again enter the cycle.
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