Sakshi Education
Atmospheric carbondioxide is reduced to organic forms through photosynthesis. Autotrophic organisms have the ability to convert physical and chemical sources of energy into carbohydrates in the absence of organic substrates. Most of the external energy is consumed in transforming CO2 to carbohydrate, a reduced state which is used by the cell for all its needs.
The reactions involved in the reduction of CO2 to carbohydrate are catalyzed by the enzymes in stroma, the soluble phase of chloroplast by utilizing the energy of ATP and NADPH. Initially the stroma reactions were considered to be independent of light, as a result they were wrongly referred as Dark reactions. Blackman demonstrated the existence of dark reaction, hence the reaction is also called in his name as Blackman’s Reaction. However, because these stroma-localized reactions depend on the products of photochemical processes, and are directly regulated by light, they are more properly referred to as the Carbon Reactions of Photosynthesis.
Three photosynthetic pathways exist among terrestrial plants, C3, C4, and Crassulacean acid metabolism (CAM). Of all the three pathways, C3 photosynthetic pathway is considered as the most ancient and predominant pathway for carbon fixation occurring in all taxonomic plant groups. The term C3 photosynthesis is based on the fact that the first stable product of photosynthesis is a 3-carbon molecule. Similarly in C4 the first stable photosynthetic product is a 4-carbon molecule.
C4 photosynthesis occurs in the more advanced plant taxa and is common in monocots such as grasses and sedges but not very common in dicots. CAM photosynthesis is named in the honor of the plant family in which this pathway was first documented, occurs in many epiphytes and succulents from very arid regions. However the CAM plants do not represent an appreciable component of global carbon cycle, hence CAM photosynthesis is limited in distribution.
THE CALVIN CYCLE
All photosynthetic eukaryotes, from the most primitive alga to the most advanced angiosperm, reduce CO2 to carbohydrate via the same basic mechanism called as The Calvin Cycle or C3 Cycle or Reductive Pentose Phosphate (RPP) cycle. The calvin cycle was elucidated by Melvin calvin and his colleagues in chlorella, unicellular green algae in 1950s. This remarkable work has made it possible for him to achieve a Nobel Prize in 1961 and the pathway has been named after him as the Calvin Cycle.
In the Calvin Cycle, CO2 and the water from the environment are enzymatically combined with a five-carbon molecule to generate two molecules of a three-carbon compound, 3-Phosphoglycerate, hence the pathway is labeled as C3 pathway. Later the 3-Phosphoglycerate is reduced to carbohydrate by use of ATP and NADPH generated photochemically. The cycle completes by the regeneration of the five-carbon acceptor, Ribulose-1,5-bisphosphate (RuBP).
The Calvin cycle proceeds in three stages (Figure 1.)
1. Carboxylation of the CO2 acceptor ribulose-1,5-bisphosphate, leading to the formation of two 3-carbon molecules, 3-phosphoglycerate, the first stable intermediate of the Calvin cycle.
2. Reduction of 3-phosphoglycerate to a carbohydrate, gyceraldehyde-3-phosphate.
3. Regeneration of the CO2 acceptor ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate.
The carbon in the nature found in the form of CO2 is in the most oxidized (+4) form. The carbon of the first stable intermediate, 3- phosphoglycerate, is more reduced (+3), and it is further reduced in the glyceraldehyde-3-phosphate product (+1). Hence, the pathway is rightly called as reduction of CO2 to carbohydrate.
Reactions of the calvin cycle
The Carboxylation of Ribulose Bisphosphate
CO2 enters the Calvin cycle by reacting with ribulose-1,5-bisphosphate yielding two molecules of 3-phosphoglycerate for every molecule of CO2 . This reaction is catalyzed by the chloroplast enzyme ribulose bisphosphate carboxylase/oxygenase referred to as rubisco. Initially carboxylation of CO2 to ribulose-1,5-bisphosphate yields an unstable, enzyme bound intermediate. This intermediate is hydrolyzed to yield two molecules of the 3-carbon stable compound, 3-phosphoglycerate
Ribulose-1,5-bisphosphate
6 Ribulose-1,5- + 6 CO2 + 6 H2O ? 12 (3-phosphoglycerate) +12 H+ bisphosphate carboxylase/oxygenase
Fig. 1. Stages of the Calvin Cycle
Reductive phase
Six molecules of 3-PGA are phosphorylated by 6 molecules of ATP (produced in the light reaction) to yield 6 molecules of 1,3-bisphospho glyceric acid and 6 molecules of ADP. This reaction is catalyzed by the enzyme, Kinase.
3-Phosphoglycerate kinase
6 (3-Phosphoglycerate) + 6 ATP ? 6 (1,3 -bisphosphoglycerate) + 6 ADP
Six molecules of 1, 3 bisphosphoglyceric acid are reduced with the use of 6 molecules of NADPH2 (produced in light reaction) to form 6 molecules of glyceraldehydes-3-phosphate. This
reaction is catalyzed by the enzyme, triose phosphate dehydrogenase.
glyceraldehyde-3-phosphate
6 (1,3-Bisphospho + 6 NADPH + 6 H+ ? 6 glyceraldehyde-3- + 6 NADP+
glycerate) dehydrogenase -Phosphate + 6 Pi.
Fig. 2. The Calvin Cycle
The carboxylation of three molecules of ribulose-1,5-bisphosphate leads to the net synthesis of one molecule of glyceraldehyde-3-phosphate and the regeneration of the three molecules of starting material. This process starts and ends with three molecules of ribulose-1,5-bisphosphate, reflecting the cyclic nature of the pathway
Five-sixths of the triose phosphate is used in the regeneration of the ribulose-1,5-bisphosphate, for continuous happening of the calvin cycle and one-sixth is exported to the cytosol for the synthesis of sucrose or other metabolites that are converted to starch in the chloroplast.
Regeneration of Ribulose-1,5-Bisphosphate
For continued uptake of CO2, the CO2 acceptor, ribulose-1,5-bisphosphatemust be constantly regenerated.
4) Two molecules of glyceraldehyde-3-phosphate are isomerized to dihydroxyacetone- 3-phosphate by triose phosphate isomerase.
Triose phosphate isomerase
2 Glyceraldehyde-3-phosphate ? 2 dihydroxyacetone-3-phosphate
5) Dihydroxyacetone-3-phosphate then undergoes aldol condensation with a third molecule of glyceraldehyde- 3-phosphate and forms fructose-1,6-bisphosphate, This reaction is catalyzed by aldolase
Aldolase
Glyceraldehyde- + dihydroxyacetone- ? fructose-1,6-bisphosphate
3-phosphate 3-phosphate
6) Fructose-1,6-bisphosphate is hydrolyzed to fructose-6-phosphate by Fructose-1,6-bisphosphatase .
Fructose-1,6
Fructose-1,6-bisphosphate + H2O ? fructose-6-phosphate + Pi
-bisphosphatase
7) Glyceraldehyde-3-phosphate (fourth molecule) reacts with fructose-6-phosphate in the presence of enzyme transketolase to form erythrose 4 phosphate (4C sugar) and xylulose 5 phosphate (5C sugar).
Transketolase
Fructose- + glyceraldehyde- ? erythrose- + xylulose-
6-phosphate 3-phosphate 4-phosphate 5-phosphate
8) Erythrose-4-phosphate then combines with a triose phosphate (dihydroxyacetone- 3-phosphate) (second molecule) to yield the seven-carbon sugar sedoheptulose-1,7-bisphosphate. This reaction is catalyzed by aldolase.
Aldolase
Erythrose- + dihydroxyacetone- ? sedoheptulose-1,7-bisphosphate
4-phosphate 3-phosphate
9) This seven-carbon bisphosphate, sedoheptulose-1,7-bisphosphate is then hydrolyzed by Sedoheptulose-1,7,bisphosphatase yielding sedoheptulose- 7-phosphate.
Sedoheptulose-1,7,bisphosphatase
Sedoheptulose-1,7-bisphosphate + H2O ? sedoheptulose-7-phosphate + Pi
10) sedoheptulose- 7-phosphate combines with Glyceraldehyde-3-phosphate (fifth molecule) in the presence of Transketolase forming ribose-5-phosphate, and xylulose-5-phosphate.
Transketolase
Sedoheptulose- + glyceraldehyde- ? ribose- + xylulose-5-phosphate
7-phosphate -3-phosphate 5-phosphate
11) The two molecules of xylulose-5-phosphate are converted to two molecules of ribulose-5-phosphate sugars by a ribulose-5-phosphate epimerase. The third molecule of ribulose-5-phosphate is formed from ribose-5-phosphate by ribose-5-phosphate isomerase.
Ribulose-5-phosphate epimerase
2 Xylulose-5-phosphate ? 2 ribulose-5-phosphate
Ribose-5-phosphate isomerase
Ribose-5-phosphate ? ribulose-5-phosphate
12) Finally, ribulose-5-phosphate kinase catalyzes the phosphorylation of ribulose-5-phosphate with ATP, thus regenerating the three needed molecules of the initial CO2 acceptor, ribulose-1,5-bisphosphate.
Ribulose-5-phosphate kinase
3 Ribulose-5-phosphate + 3 ATP ? 3 ribulose-1,5-bisphosphate +3 ADP + 3 H+
Energy Input
6 CO2 + 11 H2O + 12 NADPH + 18 ATP ? Fructose-6-phosphate + 12 NADP+ + 6 H+ + 18 ADP + 17 Pi
In order to synthesize 1 molecule of hexose, 6 molecules of CO2 are fixed at the expense of 18 ATP and 12 NADPH. In other words, the Calvin cycle consumes two molecules of NADPH and three molecules of ATP for every molecule of CO2 fixed into carbohydrate.
The reactions involved in the reduction of CO2 to carbohydrate are catalyzed by the enzymes in stroma, the soluble phase of chloroplast by utilizing the energy of ATP and NADPH. Initially the stroma reactions were considered to be independent of light, as a result they were wrongly referred as Dark reactions. Blackman demonstrated the existence of dark reaction, hence the reaction is also called in his name as Blackman’s Reaction. However, because these stroma-localized reactions depend on the products of photochemical processes, and are directly regulated by light, they are more properly referred to as the Carbon Reactions of Photosynthesis.
Three photosynthetic pathways exist among terrestrial plants, C3, C4, and Crassulacean acid metabolism (CAM). Of all the three pathways, C3 photosynthetic pathway is considered as the most ancient and predominant pathway for carbon fixation occurring in all taxonomic plant groups. The term C3 photosynthesis is based on the fact that the first stable product of photosynthesis is a 3-carbon molecule. Similarly in C4 the first stable photosynthetic product is a 4-carbon molecule.
C4 photosynthesis occurs in the more advanced plant taxa and is common in monocots such as grasses and sedges but not very common in dicots. CAM photosynthesis is named in the honor of the plant family in which this pathway was first documented, occurs in many epiphytes and succulents from very arid regions. However the CAM plants do not represent an appreciable component of global carbon cycle, hence CAM photosynthesis is limited in distribution.
THE CALVIN CYCLE
All photosynthetic eukaryotes, from the most primitive alga to the most advanced angiosperm, reduce CO2 to carbohydrate via the same basic mechanism called as The Calvin Cycle or C3 Cycle or Reductive Pentose Phosphate (RPP) cycle. The calvin cycle was elucidated by Melvin calvin and his colleagues in chlorella, unicellular green algae in 1950s. This remarkable work has made it possible for him to achieve a Nobel Prize in 1961 and the pathway has been named after him as the Calvin Cycle.
In the Calvin Cycle, CO2 and the water from the environment are enzymatically combined with a five-carbon molecule to generate two molecules of a three-carbon compound, 3-Phosphoglycerate, hence the pathway is labeled as C3 pathway. Later the 3-Phosphoglycerate is reduced to carbohydrate by use of ATP and NADPH generated photochemically. The cycle completes by the regeneration of the five-carbon acceptor, Ribulose-1,5-bisphosphate (RuBP).
The Calvin cycle proceeds in three stages (Figure 1.)
1. Carboxylation of the CO2 acceptor ribulose-1,5-bisphosphate, leading to the formation of two 3-carbon molecules, 3-phosphoglycerate, the first stable intermediate of the Calvin cycle.
2. Reduction of 3-phosphoglycerate to a carbohydrate, gyceraldehyde-3-phosphate.
3. Regeneration of the CO2 acceptor ribulose-1,5-bisphosphate from glyceraldehyde-3-phosphate.
The carbon in the nature found in the form of CO2 is in the most oxidized (+4) form. The carbon of the first stable intermediate, 3- phosphoglycerate, is more reduced (+3), and it is further reduced in the glyceraldehyde-3-phosphate product (+1). Hence, the pathway is rightly called as reduction of CO2 to carbohydrate.
Reactions of the calvin cycle
The Carboxylation of Ribulose Bisphosphate
CO2 enters the Calvin cycle by reacting with ribulose-1,5-bisphosphate yielding two molecules of 3-phosphoglycerate for every molecule of CO2 . This reaction is catalyzed by the chloroplast enzyme ribulose bisphosphate carboxylase/oxygenase referred to as rubisco. Initially carboxylation of CO2 to ribulose-1,5-bisphosphate yields an unstable, enzyme bound intermediate. This intermediate is hydrolyzed to yield two molecules of the 3-carbon stable compound, 3-phosphoglycerate
Ribulose-1,5-bisphosphate
6 Ribulose-1,5- + 6 CO2 + 6 H2O ? 12 (3-phosphoglycerate) +12 H+ bisphosphate carboxylase/oxygenase
Fig. 1. Stages of the Calvin Cycle
Reductive phase
Six molecules of 3-PGA are phosphorylated by 6 molecules of ATP (produced in the light reaction) to yield 6 molecules of 1,3-bisphospho glyceric acid and 6 molecules of ADP. This reaction is catalyzed by the enzyme, Kinase.
3-Phosphoglycerate kinase
6 (3-Phosphoglycerate) + 6 ATP ? 6 (1,3 -bisphosphoglycerate) + 6 ADP
Six molecules of 1, 3 bisphosphoglyceric acid are reduced with the use of 6 molecules of NADPH2 (produced in light reaction) to form 6 molecules of glyceraldehydes-3-phosphate. This
reaction is catalyzed by the enzyme, triose phosphate dehydrogenase.
glyceraldehyde-3-phosphate
6 (1,3-Bisphospho + 6 NADPH + 6 H+ ? 6 glyceraldehyde-3- + 6 NADP+
glycerate) dehydrogenase -Phosphate + 6 Pi.
Fig. 2. The Calvin Cycle
The carboxylation of three molecules of ribulose-1,5-bisphosphate leads to the net synthesis of one molecule of glyceraldehyde-3-phosphate and the regeneration of the three molecules of starting material. This process starts and ends with three molecules of ribulose-1,5-bisphosphate, reflecting the cyclic nature of the pathway
Five-sixths of the triose phosphate is used in the regeneration of the ribulose-1,5-bisphosphate, for continuous happening of the calvin cycle and one-sixth is exported to the cytosol for the synthesis of sucrose or other metabolites that are converted to starch in the chloroplast.
Regeneration of Ribulose-1,5-Bisphosphate
For continued uptake of CO2, the CO2 acceptor, ribulose-1,5-bisphosphatemust be constantly regenerated.
4) Two molecules of glyceraldehyde-3-phosphate are isomerized to dihydroxyacetone- 3-phosphate by triose phosphate isomerase.
Triose phosphate isomerase
2 Glyceraldehyde-3-phosphate ? 2 dihydroxyacetone-3-phosphate
5) Dihydroxyacetone-3-phosphate then undergoes aldol condensation with a third molecule of glyceraldehyde- 3-phosphate and forms fructose-1,6-bisphosphate, This reaction is catalyzed by aldolase
Aldolase
Glyceraldehyde- + dihydroxyacetone- ? fructose-1,6-bisphosphate
3-phosphate 3-phosphate
6) Fructose-1,6-bisphosphate is hydrolyzed to fructose-6-phosphate by Fructose-1,6-bisphosphatase .
Fructose-1,6
Fructose-1,6-bisphosphate + H2O ? fructose-6-phosphate + Pi
-bisphosphatase
7) Glyceraldehyde-3-phosphate (fourth molecule) reacts with fructose-6-phosphate in the presence of enzyme transketolase to form erythrose 4 phosphate (4C sugar) and xylulose 5 phosphate (5C sugar).
Transketolase
Fructose- + glyceraldehyde- ? erythrose- + xylulose-
6-phosphate 3-phosphate 4-phosphate 5-phosphate
8) Erythrose-4-phosphate then combines with a triose phosphate (dihydroxyacetone- 3-phosphate) (second molecule) to yield the seven-carbon sugar sedoheptulose-1,7-bisphosphate. This reaction is catalyzed by aldolase.
Aldolase
Erythrose- + dihydroxyacetone- ? sedoheptulose-1,7-bisphosphate
4-phosphate 3-phosphate
9) This seven-carbon bisphosphate, sedoheptulose-1,7-bisphosphate is then hydrolyzed by Sedoheptulose-1,7,bisphosphatase yielding sedoheptulose- 7-phosphate.
Sedoheptulose-1,7,bisphosphatase
Sedoheptulose-1,7-bisphosphate + H2O ? sedoheptulose-7-phosphate + Pi
10) sedoheptulose- 7-phosphate combines with Glyceraldehyde-3-phosphate (fifth molecule) in the presence of Transketolase forming ribose-5-phosphate, and xylulose-5-phosphate.
Transketolase
Sedoheptulose- + glyceraldehyde- ? ribose- + xylulose-5-phosphate
7-phosphate -3-phosphate 5-phosphate
11) The two molecules of xylulose-5-phosphate are converted to two molecules of ribulose-5-phosphate sugars by a ribulose-5-phosphate epimerase. The third molecule of ribulose-5-phosphate is formed from ribose-5-phosphate by ribose-5-phosphate isomerase.
Ribulose-5-phosphate epimerase
2 Xylulose-5-phosphate ? 2 ribulose-5-phosphate
Ribose-5-phosphate isomerase
Ribose-5-phosphate ? ribulose-5-phosphate
12) Finally, ribulose-5-phosphate kinase catalyzes the phosphorylation of ribulose-5-phosphate with ATP, thus regenerating the three needed molecules of the initial CO2 acceptor, ribulose-1,5-bisphosphate.
Ribulose-5-phosphate kinase
3 Ribulose-5-phosphate + 3 ATP ? 3 ribulose-1,5-bisphosphate +3 ADP + 3 H+
Energy Input
6 CO2 + 11 H2O + 12 NADPH + 18 ATP ? Fructose-6-phosphate + 12 NADP+ + 6 H+ + 18 ADP + 17 Pi
In order to synthesize 1 molecule of hexose, 6 molecules of CO2 are fixed at the expense of 18 ATP and 12 NADPH. In other words, the Calvin cycle consumes two molecules of NADPH and three molecules of ATP for every molecule of CO2 fixed into carbohydrate.
Published date : 30 May 2014 06:24PM
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