PEP and PEPcase: Primary CO2 Acceptor in C4 Plants
Biology · Photosynthesis in Higher Plants · NEET
In C4 plants the primary CO2 acceptor is PEP (phosphoenolpyruvate), a 3-carbon molecule present in the mesophyll cells. The enzyme PEP carboxylase (PEPcase) fixes CO2 onto PEP. Memory hook: "PEP has 3 carbons but the C4 pathway is named after its 4-carbon PRODUCT, not its 3-carbon acceptor."
In C4 mesophyll cells, PEPcase adds CO2 to the 3-carbon PEP to form the 4-carbon OAA, which then moves to the bundle sheath cells for the Calvin cycle.
Your doubts, answered
Is PEP the primary CO2 acceptor in C3 or C4 plants?
PEP is the primary CO2 acceptor only in C4 plants. In C3 plants the primary acceptor is the 5-carbon RuBP, fixed by RuBisCO. NEET 2017 asked exactly this and the answer is C4 plants. Do not tick 'C3 and C4 both' - that is the trap option.
How many carbons does PEP have if the pathway is called C4?
PEP itself has only 3 carbons. The pathway is named 'C4' because when CO2 is added to PEP, the FIRST STABLE PRODUCT is the 4-carbon oxaloacetic acid (OAA). So the acceptor is 3-carbon PEP, but the product is 4-carbon OAA. Students lose marks by assuming the acceptor is 4-carbon.
Which enzyme fixes CO2 onto PEP in C4 plants?
The enzyme is PEP carboxylase, also written as PEPcase. It is present in the mesophyll cells. It has a very high affinity for CO2 and, unlike RuBisCO, it has no oxygenase activity, so it does not do photorespiration. NEET 2026 tested that the Calvin-cycle carboxylation enzyme is RuBisCO, not PEPcase - keep the two enzymes separate.
Where in the C4 leaf does PEP fix CO2?
PEP fixation happens in the MESOPHYLL cells, where PEPcase is located. The mesophyll cells of C4 plants LACK RuBisCO. RuBisCO and the actual Calvin cycle work only in the bundle sheath cells. NEET 2022 confirmed both these statements are correct.
Is OAA the acceptor or the product in the C4 pathway?
OAA is the PRODUCT, not the acceptor. PEP is the acceptor; adding CO2 to PEP gives OAA as the first stable product. NEET 2019 gave OAA as a tempting wrong option for 'primary CO2 acceptor' - the correct answer there is PEP.
⚠️ The NEET trap ✗ PEP is a 4-carbon molecule because the pathway is called C4. ✓ PEP is a 3-carbon molecule. The pathway is named C4 after its first stable PRODUCT, the 4-carbon OAA, not after the acceptor. 🧠 Acceptor = 3C (PEP). First product = 4C (OAA). The pathway name copies the PRODUCT.
Real NEET questions
NEET 2017
Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in:
A · C3 plants
B · C4 plants ✓
C · C2 plants
D · C3 and C4 plants
Solution: In the C4 (Hatch and Slack) pathway, the primary CO2 acceptor is the 3-carbon PEP present in mesophyll cells, where PEP carboxylase fixes CO2 to form OAA. In C3 plants the acceptor is the 5-carbon RuBP, not PEP. So PEP is the primary CO2 acceptor in C4 plants. 'C3 and C4' is a trap.
NEET 2019
In Hatch and Slack pathway, the primary CO2 acceptor is:
A · Oxaloacetic acid
B · Phosphoglyceric acid
C · Phosphoenol pyruvate ✓
D · RuBisCO
Solution: Hatch and Slack pathway is the C4 pathway. The primary CO2 acceptor is the 3-carbon PEP in the mesophyll cells; PEPcase fixes CO2 to form the 4-carbon OAA. OAA is the first product (not the acceptor), and RuBisCO is the C3 enzyme, so the acceptor is PEP.
NEET 2022
Statement I: The primary CO2 acceptor in C4 plants is phosphoenolpyruvate and is found in the mesophyll cells. Statement II: Mesophyll cells of C4 plants lack RuBisCO enzyme. Choose the correct answer:
A · Both Statement I and Statement II are correct ✓
B · Both Statement I and Statement II are incorrect
C · Statement I is correct but Statement II is incorrect
D · Statement I is incorrect but Statement II is correct
Solution: Statement I is correct: PEP (a 3-carbon molecule) is the primary CO2 acceptor and is present in mesophyll cells. Statement II is also correct: NCERT states mesophyll cells lack RuBisCO, which is confined to the bundle sheath cells. Hence both statements are correct.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The primary CO2 acceptor in C4 plants is phosphoenolpyruvate (PEP), a 3-carbon molecule present in the mesophyll cells.
What is the full form of PEP and PEPcase?
PEP stands for phosphoenolpyruvate. PEPcase stands for PEP carboxylase, the enzyme that fixes CO2 onto PEP in the mesophyll cells of C4 plants.
How many carbons are in PEP?
PEP has 3 carbons. After CO2 is added, the first stable product OAA has 4 carbons, which is why the pathway is called C4.
Do mesophyll cells of C4 plants have RuBisCO?
No. Mesophyll cells of C4 plants lack RuBisCO. They only have PEPcase. RuBisCO and the Calvin cycle operate in the bundle sheath cells.
Why is PEPcase better than RuBisCO for CO2 fixation?
PEPcase has a very high affinity for CO2 and no oxygenase activity, so it can fix CO2 even at low concentrations and does not cause photorespiration, unlike RuBisCO.
