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DARK REACTION (BIOSYNTHETIC PHASE) - Coggle Diagram
DARK REACTION (BIOSYNTHETIC PHASE)
It is the use of ATP & NADPH to drive the processes for the synthesis of food
Not directly dependent on light but is dependent on products of light reaction
C3 Pathway (Calvin Cycle)
Carboxylation of RuBP
Primary CO2 acceptor: RuBP (Ribulose bisphosphate)
CO2 is fixed by RuBP to 2 3-PGA in presence of RuBP carboxylase-oxygenase (RuBisCO)
Reduction
2 ATP molecules for phosphorylation & 2 NADPH for reduction per CO2 molecule are used
Fixation of 6 CO2 molecules & 6 turns of the cycle are needed to remove one glucose molecule from the pathway
Regeneration of RuBP
1 ATP for phosphorylation to form RuBP
For every CO2 molecule, 3 ATP molecules & 2 NADPH are required
C4 Pathway (Hatch & Slack Pathway)
Primary CO2 acceptor: Phosphoenol pyruvate (PEP)
Enzyme involved in the fixation is PEP carboxylase
Mesophyll cells lack RuBisCO enzyme
OAA ---> Malic acid ---> Aspartic acid ---> C3 acid
Bundle sheath cells are rich in RubisCO enzyme but lack PEPcase
Photorespiration
RuBisCO has slightly greater affinity for CO2 than O2
This leads to competitive binding
Relative conc. of O2 & CO2 determines which one will bind to the enzyme
In C3 plants, O2 binds to RuBisCO. Hence, CO2 fixation is decreased.
RuBP binds with O2 to form one molecule of phosphoglycerate and phosphoglycolate
This process is absent in C4 plants because they can increase CO2 conc. at the enzyme site.
This occurs when C4 acid from mesophyll is broken down in bundle cells to release CO2 which minimizes oxygenase activity of RuBisCO.