What is the light reaction and the dark reaction?
4.1.9 Light Independent Reaction or Dark Reaction
The light-independent reaction was discovered by Melvin Calvin and coworkers (1950) at the University of California. He was awarded the Nobel Prize in 1961 for his work. Therefore, this cycle is also called the Calvin cycle.
They used a radioactive isotope of C11 in CO,. Light-independent reactions do not need the direct energy from sunlight. It may occur during day time but it is called a dark reaction, to differentiate it from the light reaction. Calvin cycle occurs in the stroma of chloroplast by a series of reactions in which CO, is fixed into carbohydrate (CH, O)n in the absence of light.
The Calvin cycle is completed in three stages:
- Carbon fixation
- Reduction
- Regeneration of ribulose bi-phosphate.
Carbon Fixation:
It is the first step of a dark reaction in which CO, from the air, combines with pre-existing five-carbon phosphorylating sugar known as ribulose biphosphate (RuBP). As a result, an unstable 6-carbon intermediate compound is formed.
The enzyme that speeds up this reaction is called RuBP carboxylase; also known as Rubisco. The six-carbon intermediate molecule exists for such a brief time that it cannot be isolated and thus named as an intermediate compound.
Formation of PGA:
The unstable intermediate compound splits into two molecules of three carbon-containing phoshoglyceric acid (PGA). It is the first identifiable product in a dark reaction Therefore, the Calvin cycle is also called the C, Cycle.
CO2+RuBP(C) (intermediate compound) →→→→ 2 PGA(C) The carbon that was part of the CO, molecule is now a part of an organic molecule. This is called CO, fixation.
Reduction nation of PGAL or G3P):
In this step, the product of light reaction that is ATP and NADPH, are used. Each phosphoglyceric acid (PGA) molecule receives energy from ATP and H from NADPH, forming 3-carbon phosphoglyceraldehyde (PGAL).
In this step, water is also formed. In the reduction process fixed carbon is reduced into a 3-carbon sugar molecule of PGAL.
PGA + ATP + NADPH, →→→→→→ PGAL+ADP + Pi + NADP+ H2O ADP and Pi and NADP return back to light reaction where ADP is converted into ATP and NADP is reduced into NADPH.
Regeneration of RuBP
For every turn of the Calvin cycle, five molecules of PGAL are used to reform three molecules of RuBP, so that the cycle can continue. It also uses ATP of light reaction.
5 PGAL + ATP →→→ADP+ Pi+3 RuBP
Thus out of every six molecules of PGAL formed in the reduction stage, only one molecule leaves the cycle, which is to be used by plants for making glucose and other organic compounds.
Use of PGAL From PGAL 3C, 4C, 5C, 6C, and 7C compounds are produced, all are
interconvertible. Two PGAL molecules from the Calvin cycle are converted into glucose phosphate within the chloroplast. Glucose phosphate is then converted to starch. Fixed carbons leave the chloroplast as dihydroxyacetone phosphate (DHAP). It is formed from PGAL.
In the cytoplasm, DHAP can be used to synthesize the six-carbon sugars, glucose, and fructose, which are then joined to form sucrose, and amino acids (with the addition of nitrogen). Other compounds like organic acids that as Glucose are also used to synthesize cellulose.
Glucose is readily converted into fatty acids and glycerols appear quite rapidly in the cell during photosynthesis. Glucose accumulates more than other compounds, so it was observed more readily by early investigators in chemical analysis. Other compounds can be seen by methods.
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