In C₄ plants, the primary CO₂ acceptor is:

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AIIMS BSc NURSING 2023 Memory-Based Paper

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Answer 1

The correct answer is Phosphoenol pyruvate

Concept:

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 CO₂acceptor 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

Explanation:

Phosphoenolpyruvate (PEP):

PEP is the primary CO₂ acceptor in C4 plants, occurring in the mesophyll cells.
The enzyme PEP carboxylase catalyzes the fixation of CO₂ to PEP, forming oxaloacetate, which is then converted to malate or aspartate.
This initial fixation step is highly efficient, even at low CO₂ concentrations, contributing to the high photosynthetic efficiency of C4 plants.