Introduction The mechanism of photosynthesis takes place in two phases, which are the light reaction (Hill reaction) and the dark reaction. The dark reaction of photosynthesis was discovered by Calvin Benson and Bassham (1946) in Chlorella. So, the reduction of carbon dioxide is also known as Calvin cycle. The Calvin described the Calvin cycle on the basis of the following two observations:
The main reason of first observation is, in the absence of light the formation of NADP + H+ and ATP is inhibited which is necessary for the transformation of PGA. Therefore, the amount of PGA is increased. The main reason of second observation is, in the absence of carbon di oxide, RuDP cannot be converted into PGA. But PGA present before this reaction transformed into RuDP with the help of ATP and NADP + H.
Mechanism of Calvin Cycle
The dark reaction or Calvin cycle of carbon di oxide reduction takes place in the following cyclic sequences:
CarboxylationRuMP (Ribose mono phosphate) is the starting compound of C3 cycle. Carboxylation involves the following stages:
RuMP + 6ATP = RuDP + 6ADP
RuDP + 6 CO2 +5H2O = PGA
ReductionReduction takes place in the following steps:
PGA + 12 ATP = 1,3 DiPGA + 12ADP
1,3-DiPGA + 12 NADPH2 = PGAL + 12NADP + 12H3PO4
PGAL = DiHAP
Hexose formationThe most important part of the Calvin cycle is the formation of hexose sugar. The hexose sugar is formed under the following steps:
PGAL + DiHAP = Fructose-1,6-diphosphate
Fructose-1,6-DP + H2O = Fru-6-P + H3PO4
Fru-6-P = Fructose-1-phosphate
Fructose-1-phosphate = Glucose-1-phosphate
Glucose-1-phosphate = Starch
Regeneration of Ribose-5-PhosphateIn the last phase of Calvin cycle, the ribose-5-phosphate is regenerated which begins the cycle again. The regeneration takes place in the following steps:
Fru-6-P +PGAL = Erythrose-4-P + xylulose-5-P
Ery-4-P + DiHAP = Seudo -1,7-DP
Seudo-1,7-DP + 2H2O = Seudo-7-P + 2H3PO4
Suedo-7-P +PGAL = Re-5-P + Xyl-5-P
Ribose-5-P = Ribulose-5-P
Xylulose-5-phosphate = Ribulose-5-phosphate
Thus, 6 molecules of ribulose-5-phosphate are regenerated in the reaction which ultimately forms RuDP utilizing ATP. The overall reaction perform in the dark phase of photosynthesis may be summarized as below:
6 CO2 +12 NADPH2 + 18 ATP + 11 H2O = Fru-6-P + 12 NADP+ + 18 ADP + 17 Pi
Differences between Light reaction and Dark reactionThe main differences between the light reaction and the dark reaction of photosynthesis are as given below:
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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Nice article. The author has written a very brief but quite descriptive account of one of the most amazing phenomena in our nature.
We all know about the process of photosynthesis that it is so important for all the living things on this planet directly or indirectly. I was in class II when I learned about the photosynthesis process first time. But the way the author has explained the same is really appreciable.