Question

Which one of the answer is correct with respect to the acceptor molecule of CO₂ in C₃ and C₄ pathway. Respectively they are
1. Rubisco and PEP
2. Phosphoglycerate and Oxalo Acetate
3. Ribulose biphosphate and Phosphoenolpyruvate
4. RUBP and RUBISCO

Answer 1

Correct Answer - Option 3 : Ribulose biphosphate and Phosphoenolpyruvate

Explanation:

• Different Plants use different photosynthetic processes for atmospheric carbon fixation.
• The three main processes are:
• C3 cycle - 
  • ​In the C3 pathway or Calvin cycle, the primary CO2 acceptor molecule is ribulose bisphosphate (RuBP). 
  • This reaction is catalysed by the enzyme RuBP carboxylase which results in the formation of two molecules of 3-PGA.
• C4 cycle - 
  • The primary CO2 acceptor is 3-carbon molecule phosphoenolpyruvate (PEP) and is present in the mesophyll cells.
  • The enzyme responsible for this fixation is PEP carboxylase or PEPcase
• CAM pathway - 
  • The primary CO₂ acceptor is PEP at night, RuBP in the day. 
  • The enzyme responsible for this fixation is PEPcase at night, Rubisco at daytime.

Points	C3 Pathway	C4 Pathway	CAM Pathway
Initial CO2 acceptor	Ribulose-1, 5-bisphosphate (RuBP)	Phosphoenolpyruvate (PEP)	PEP at night, RuBP in the day
CO2-fixing enzyme	Ribulose-1, 5-bisphosphate  carboxylase/oxygenase(RuBisCo)	Phosphoenolpyruvate carboxylase (PEPcase) then Rubisco	PEPcase at night, Rubisco at daytime
First stable product of CO2 fixation	3-phosphoglycerate (3-PGA)	Oxaloacetate (OAA) in C4 cycle	OAA at night, 3-PGA at daytime