The dark reaction of photosynthesis or Calvin cycle


Photosynthesis takes place in two phases, i.e. the light reaction and the dark reaction. The below article deals with the detailed mechanism of the dark reaction of photosynthesis. You will also find the differences between light reaction and the dark reaction of photosynthesis.

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:

  • When photosynthesizing plants are deprived of light the level of PGA (Phospho Glyceric Acid) in increased while the level of RuDP (Rebulose di phosphate) is decreased.
  • When photosynthesizing plants are deprived of CO2 the amount or level of RuDP is increased, whereas the level of PGA is decreased.


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:
  • Carboxylation
  • Reduction
  • Hexose formation
  • Regeneration

Carboxylation

RuMP (Ribose mono phosphate) is the starting compound of C3 cycle. Carboxylation involves the following stages:
  • First of all, ribose monophosphate (RuMP) reacts with ATP in the presence of phosphoribulose kinase enzyme to form ribulose diphosphate (RuDP).
    RuMP + 6ATP = RuDP + 6ADP
  • RuDP (Ribose diphosphate) accepts carbon dioxide in the presence of RuDP-carboxylase (RuBISCO) enzyme to form an unstable 6-carbon compound from which 12 molecules of phosphoglyceric acid (PGA) are formed.
    RuDP + 6 CO2 +5H2O = PGA

Reduction

Reduction takes place in the following steps:
  • The PGA reacts with ATP in the presence of phosphoglycerokinase enzyme to form 1,3 –diphosphoglyceric acid (1,3 DiPGA)
    PGA + 12 ATP = 1,3 DiPGA + 12ADP
  • The 1,3-DiPGA is then reduced to phosphoglyceraldehyde (PGAL) by NADPH + H+.
    1,3-DiPGA + 12 NADPH2 = PGAL + 12NADP + 12H3PO4
  • 5 molecules of PGAL are converted into 5 molecules of dihydroxy acetone phosphate (DiHAP).
    PGAL = DiHAP


Hexose formation

The most important part of the Calvin cycle is the formation of hexose sugar. The hexose sugar is formed under the following steps:
  • Three molecules of PGAL and three molecules of DiHAP, in the presence of aldolase enzyme unite to form 3 molecules of fructose-1,6-diphosphate (Fru-1,6-DP).
    PGAL + DiHAP = Fructose-1,6-diphosphate
  • From fructose-1,6-diphosphate, different types of compounds are synthesized and may be converted into glucose or starch in the following manner:
    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-Phosphate

In 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:
  • The regeneration of ribulose diphosphate is essential to carry on the process of photosynthesis. 2 molecules of fructose-6-phosphate combine with 2 molecules of phosphoglyceraldehyde and break down into 4 carbon compound erythrose-4-phosphate and 5 carbon compound xylulose-5-phosphate.
    Fru-6-P +PGAL = Erythrose-4-P + xylulose-5-P
  • Two molecules of erythrose-4-phosphate combine with two molecules of di-hydroxy acetone phosphate (diHAP) to form 2 molecules of seudoheptulose-1,7-diphosphate from which one phosphate is removed to form seudoheptulose-7-phosphate.
    Ery-4-P + DiHAP = Seudo -1,7-DP
    Seudo-1,7-DP + 2H2O = Seudo-7-P + 2H3PO4
  • Two molecules of seudoheptulose-7-phosphate combine with 2 molecules of phosphoglyceral dehyde (PGAL) to produce two molecules each f ribose-5-phosphate and xylulose-5-phosphate.
    Suedo-7-P +PGAL = Re-5-P + Xyl-5-P
  • Two molecules of ribose-5-phosphate are then converted into its epimer ribulose-5-phosphate in the presence of Isomerase enzyme.
    Ribose-5-P = Ribulose-5-P
  • Four molecules of xylulose-5-phosphate formed by previous reactions are also converted into 4 molecules of ribulose-5-phosphate.
    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 reaction

The main differences between the light reaction and the dark reaction of photosynthesis are as given below:
  1. The light reaction takes place in the presence of light. In other words, light is essential for light reaction whereas light does not require for the dark reaction.
  2. Absorption of light and transport of electrons takes place in the light reaction but absorption of light and transport of electrons does not take place in the dark reaction.
  3. ATP and NADPH2 are formed during light reaction whereas ATP and NADPH2 are not formed during the dark reaction.
  4. Photolysis of water take place and oxygen is released in the light reaction but Photolysis of water does not take place and oxygen is not released in the dark reaction.
  5. The site of light reaction is the grana of chloroplast whereas the site of dark reaction is the stroma of the chloroplast.


Comments

Author: Ryyu29 Jul 2020 Member Level: Bronze   Points : 1

Nice article. The author has written a very brief but quite descriptive account of one of the most amazing phenomena in our nature.

Author: Swati Sharma31 Jul 2020 Member Level: Gold   Points : 1

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.



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