Dark Reaction of Photosynthesis : Calvin cycle/C3 cycle
The dark reaction or light independent reaction of photosynthesis reduces carbon dioxide to glucose. These reactions take place in the stroma of chloroplast. Although this process does not require light, it depends upon the products of light reaction of photosynthesis.
The energy required for the reduction of carbon dioxide is thus derived from ATP and hydrogen from NADPH2 formed in the light reaction of photosynthesis. Hence, dark reaction of photosynthesis requires the assimilatory power (i.e., 8 ATP and 12 NADPH2).
The dark reaction of photosynthesis occurs through one of the following two cycles under different conditions.
- Calvin cycle or C3 cycle and
- Hatch and Slack cycle or C4 cycle.
Calvin cycle or C3 cycle
Melvin Calvin and his co-workers (1946-1953) worked out the mechanism of dark reaction in chlorella by using (C14 O2), which is popularly known as C3 pathway or Calvin cycle. They were successful in tracing various intermediate products of photosynthesis by using radioactive carbon, paper chromatography and auto-radiography. We can study the various reactions of Calvin cycle under following three headings:
- Carboxylation.
- Glycolytic reversal
- Regeneration of RuBP.
Carboxylation
Addition of carbon dioxide to a compound is termed as carboxylation. In this step of dark reaction of photosynthesis, ribulose 1, 5 diphosphate, (also known as ribulose biphosphate) is a phosphorylated 5 carbon sugar and acts as carbon dioxide acceptor. It combines with carbon dioxide. The enzyme, RUBISCO, acts as a catalyst in this reaction. As a result 2 molecules of 3-phosphoglyceric acid is formed.
Glycolytic reversal
In this phase of dark reaction of photosynthesis, phosphoglyceric acid forms phosphoglyceral dehyde by utilizing ATP molecules. Phosphoglyceral dehyde converts into hexose sugar. Thus, the entire process is just a reverse of glycolysis in which hexose sugars are first oxidised into phosphoglyceral dehyde and then to carbon dioxide and water, releasing ATP.
The step involves utilization of 2 molecules of ATP for phosphorylation. It also requires 2 molecules of NADPH for reduction of per carbon dioxide molecule fixed. Glucose is a six carbon compound, thus it requires six turns of Calvin cycle to synthesise its one molecule.
Following are the major steps of glycolytic reversal:
- Formation of diphosphoglyceric acid:
12 molecules of phosphoglyceric acid forms 12 molecules of diphosphoglyceric acid by combining with 12 molecules of ATP.
12 phosphoglyceric acid+12 ATP→12 diphosphoglyceric acid +12 ADP
- Formation of phosphoglyceraldehde:
Reduction of diphosphoglyceric acid to glyceraldehyde-3-phosphate occurs. In this reduction process hydrogen comes from NADPH formed during the light reaction.
12 diphosphoglyceric acid+12 NADPH2 → 12 phosphoglyceral dehyde +12 NADP
- Formation of dihydroxy acetone phosphate:
Out of 12 molecules of phosphoglyceraldehyde, 5 molecules transform into its isomer dihydroxy acetone phosphate.
5 phosphoglyceraldehyde → 5 dihydroxy acetone phosphate.
- Production of fructose 1,6 diphosphate:
3 molecules of dihydroxy acetone phosphate combines with 3 molecules of phosphoglyceraldehyde. Thus it forms 3 molecules of fructose 1,6 diphosphate.
3 dihydroxy acetone phosphate+3 phosphoglyceraldehyde→3 fructose 1,6 diphosphate.
- Formation of fructose 6 phosphate:
Fructose 1,6 diphosphate converts into fructose 6 phosphate by giving out one molecule of phosphate.
3 fructose 1,6 diphosphate → 3 fructose 6 phosphate + H2PO4
- Production of hexose sugar
One molecule of fructose 6 phosphate forms one molecule of hexose sugar.
3 fructose 6 phosphate → 1 glucose + 2 fructose 6 phosphate
Regeneration of RuBP:
In the first step of Calvin cycle, ribulose diphosphate accepts carbon dioxide and it thus enters the cycle. Later, once glucose is formed, this ribulose 5 diphosphate must be regenerated so that it is once again available to accept carbon dioxide to manufacture hexose sugar.
The regeneration steps require ATP for phosphorylation to form RuBP. Hence for every carbon dioxide entering the Calvin cycle, it requires 3 molecules of ATP and also 2 molecules of NADPH . It requires 6 turns of the cycle to make 1 molecule of glucose.
Following is the net reaction of C3 dark fixation of carbondioxide:
6 molecules of RuBP+ 6 molecules of CO2+18 ATP+12 NADPH→6 molecules of RuBP + 1 molecule of glucose+18 ADP+ 18 P+12 NADP+
Fig: Calvin cycle (Dark reaction of photosynthesis)
Following are the steps involved in calvin cycle/ c3 pathway :
Carboxylation and Glycolytic Reversal
1: 6 molecules of ribulose 1-5 disphosphate combines with 6 molecules of carbon dioxide to form an unstable 6 carbon compound β-keto acid. Thus it immediately splits into 12 molecules of 3 carbon compound 3-phosphoglyceric acid. The reaction takes place in the presence of enzyme, carboxydismutase.
2: 12 molecules of 3-phosphoglyceric acid reacts with 12 molecules of ATP and thus produces 12 molecules of 1, 3-diphosphoglyceric acid. It takes place in the presence of enzyme, phosphoglycerokinase.
3: 12 molecules of NADPH2 and 12 molecules of H2O reduces 12 molecules of 1-3 diphosphoglyceric acid to 3-phosphoglyceraldehyde . The reaction takes place in the presence of enzyme, phosphoglyceraldehyde dehydrogenase.
4: 3-phosphoglyceraldehyde participates in 4 kinds of reaction. Out of 12 molecules, 5 molecules of 3-phosphoglyceraldehyde converts into dihydroxyacetone phosphate. It takes place in the presence of enzyme triosephosphate isomerase.
5: Out of 5 molecules of dihydroxyacetone, 3 molecules condense with 3 molecules of 3-phosphoglyceraldehyde . As a result a 6 carbon compound fructose 1-6 diphosphate is formed.
6: Fructose 1-6 diphosphate converts into fructose-6-phosphate by losing a molecule of phosphate in the form of H3PO4. The reaction thus occurs in the presence of enzyme, phosphatase and water.
7: Out of 3 molecules of fructose 6 phosphate, 1 molecule converts into fructose or glucose and thus it is released as final product of photosynthesis.
Regeneration of RuBP
8: 2 molecules of fructose 6 phosphate combines with 2 molecules of 3-phosphoglyceraldehyde. As a result it gives 2 molecules of a 4 carbon compound. Erythrose 4-phosphate and 2 molecules of 5 carbon compound xylulose 5-phohsphate in the presence of enzyme, transketolase.
9: 2 molecules of dihydroxyacetone phosphate reacts with 2 molecules of erythrose 4-phosphate. Thus it forms 2 molecules of a 7 carbon compound, sedoheptulose-1, 7 diphosphate with the help of enzyme transaldolase.
10: Sedoheptulose-1, 7 diphosphate converts into sedoheptulose 7-phosphate in the presence of enzyme, phosphatase and water.
11: 2 molecules of sedoheptulose 7-phosphate reacts with remaining 2 molecules of 3 -phosphoglyceraldehyde. As a result it forms 2 molecules of xylulose 5-phosphate and 2 molecules of ribose 5-phosphate. It requires enzyme transketolase.
12: Isomerisation of 2 molecules of ribose 5-phosphate to 2 molecules of ribulose 5-phosphate takes place in the presence of enzyme, phosphoribose isomerase.
13: Isomerisation of all the 4 molecules of xylulose 5-phosphate to ribulose 5-phosphate takes place in the presence of phosphoribulose epimerase.
14: All the 6 molecules of ribulose 5-phosphate converts into ribulose 1, 5-diphosphate with the help of an enzyme phosphoribulokinase. It also requires ATP.