Biosynthetic Phase or Photosynthetic Carbon Reduction Cycle (PCR) :
1. Biosynthetic Phase of Photosynthesis utilises assimilatory powers (ATP and NADPH ` + H^(+)`) produced during light reaction. They are used to fix and reduce carbon dioxide into carbohydrates.
2. This reaction does not require light. Therefore, it is named Dark reactin. Riblose `1.5` bis phosphate (RUBP) act s acceptro molecular of carbon dioxide and fix the `CO_(2)` by RUBISCO enzyme.
3. The first product of the pathway is a 3-carbon compound (Pospho Glyeric Acid) and so It is also called as `C_(3)` Cycle or Calvin Cycle. It takes place in the stroma of the cloroplast. It was discovered Melin calvin.
4. Dark reaction is temperature dependent and so it is also called Thermo-Chemical Reaction. It consists of three phases.
(a) Phase 1-Carboxylation (Fixation)
(i)The acceptor molecule ribulose 1,5 Bisposphate (RUBP) a 5 carbon compound with the help of RUBP carboxylase oxygenase (RUBISCO) enzyme accepts one molecule of carbon dioxide to form an unstable 6 carbon compound.
(ii) This 6C compound is broken down into two molecules of 3-carbon compound phospho glyceric acid (PGA).
`RIBP+CO_(2) overset("Rubisco")to` 2 molecules PGA .
(or)
(b) Phase 2-Glycolytic Reversal/Reduction
(i). Phospho glyceric acid is phosphorylatd by ATP and produces 1,3 his phospho glyceric acid by PGA kinase.
ii. 1,3 bis phospho glyceric acid is reduced to glycerldehyde 3 Phosphate (G-3-P) by using the reducing power `NADPH+H^(+)`.
iii. Glyceraldehyde 3 phosphate is convered acetone phosphate (DHAP).
1,3 bisphoshoglyceric acid 1,3 phoshoglyceric acid
Glyceraldehyde-3Phosphate.
(C) Phase 3- Regeneration
i. Regeneration of RIBP involves the formation of several intermediate compounds of 6-carbon, 5-carbon, 4-carbon and 7-carbon skelton.
ii. Fixatin of one carbon dioxide requires 3 ATPs and `3NADPH+H^(+)`, and for the fixation of `6CO_(2)`requires 18 ATPs and `12 NADPH+H^(+)` during `C_(3)` cycle. One 6 carbon compound is the net gain to form hexose sugar.
Overall equation for dark reaction :
`6CO_(2)+18ATP+12NADPH+H^(+)C_(6)H_(12)O_(6)+`
`6H_(2)+18ADP+18Pi+12NADP^(+)`
