In the C4 pathway, the first stable product of CO2 fixation is OAA (oxaloacetic acid), a 4-carbon organic acid. It is made in the mesophyll cells when the enzyme PEP carboxylase adds CO2 to the primary acceptor PEP (a 3-carbon molecule). Memory hook: "C4 = 4 carbons in OAA", and remember PEP + CO2 -> OAA, so the acceptor is PEP but the first product is OAA.
In C4 plants, PEP (3C) is the primary CO2 acceptor in mesophyll cells; PEP carboxylase adds CO2 to form OAA (4C), the first stable product. The C4 acid then moves to bundle sheath cells for the Calvin cycle.
Your doubts, answered
Is OAA the first product or the primary CO2 acceptor in C4 plants?
OAA is the first stable PRODUCT, not the acceptor. The primary CO2 acceptor is PEP (phosphoenolpyruvate), a 3-carbon molecule. When PEP takes up CO2, it forms the 4-carbon acid OAA. So the order is: acceptor = PEP, product = OAA. NEET often swaps these two to trick you.
Why is the C4 pathway called the C4 pathway?
It is named C4 because the first stable product formed after CO2 fixation is a 4-carbon (C4) acid, OAA. This is the same logic as C3: in C3 plants the first product is the 3-carbon acid PGA. The name always comes from the number of carbons in the first product, not in the acceptor.
What is the difference between PEP and OAA in the C4 pathway?
PEP (phosphoenolpyruvate) is the 3-carbon primary CO2 acceptor. OAA (oxaloacetic acid) is the 4-carbon first stable product formed when PEP fixes CO2. In short: PEP is the starting acceptor and OAA is the immediate 4-carbon product of carboxylation.
Which enzyme makes OAA in C4 plants?
PEP carboxylase (PEPcase) makes OAA. It is present in the mesophyll cells and adds CO2 to PEP to form OAA. Note: mesophyll cells of C4 plants do NOT have RuBisCO; RuBisCO is only in the bundle sheath cells, where the Calvin cycle runs.
Where in the leaf is OAA formed?
OAA is formed in the mesophyll cells of C4 plants, because that is where PEP and PEP carboxylase are located. OAA is later converted to other 4-carbon acids (like malic acid) that move into the bundle sheath cells, where CO2 is released for the Calvin cycle.
Is OAA the first stable product in C3 or C4 plants?
OAA is the first stable product in C4 plants only. In C3 plants the first stable product is 3-PGA (3-phosphoglyceric acid), a 3-carbon acid. Do not mix these: C3 -> PGA (3C), C4 -> OAA (4C).
⚠️ The NEET trap ✗ OAA is the primary CO2 acceptor in the C4 (Hatch and Slack) pathway. ✓ OAA is the first stable PRODUCT of CO2 fixation in C4 plants. The primary CO2 acceptor is PEP; PEP + CO2 -> OAA using PEP carboxylase. 🧠 Acceptor comes first and reacts (PEP), product forms after (OAA). NEET repeatedly swaps 'acceptor' and 'first product' in the same option list.
Real NEET questions
2021
The first stable product of CO2 fixation in sorghum is:
A · Succinic acid
B · Phosphoglyceric acid
C · Pyruvic acid
D · Oxaloacetic acid ✓
Solution: Sorghum is a C4 plant. In the C4 pathway the first stable product of CO2 fixation is the 4-carbon acid oxaloacetic acid (OAA), formed when PEP carboxylase adds CO2 to PEP in the mesophyll cells. Phosphoglyceric acid (PGA) is the first product only in C3 plants.
2019
In Hatch and Slack pathway, the primary CO2 acceptor is:
A · Oxaloacetic acid
B · Phosphoglyceric acid
C · Phosphoenol pyruvate ✓
D · Rubisco
Solution: The Hatch and Slack pathway is the C4 pathway. Its primary CO2 acceptor is PEP (phosphoenolpyruvate), a 3-carbon molecule. OAA is the first stable PRODUCT, not the acceptor, so this question tests whether you can tell the acceptor (PEP) apart from the product (OAA).
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: Both statements are correct. PEP is the primary CO2 acceptor and it is present in the mesophyll cells, where PEP carboxylase forms OAA. Mesophyll cells of C4 plants truly lack RuBisCO; RuBisCO and the Calvin cycle are confined to the bundle sheath cells.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
OAA (oxaloacetic acid) has 4 carbon atoms. That 4-carbon count is exactly why the pathway is named the C4 pathway.
What is the primary CO2 acceptor in the C4 pathway?
The primary CO2 acceptor is PEP (phosphoenolpyruvate), a 3-carbon molecule. PEP fixes CO2 to form the 4-carbon product OAA.
What enzyme forms OAA in C4 plants?
PEP carboxylase (PEPcase), present in the mesophyll cells, adds CO2 to PEP to form OAA.
Is OAA formed in C3 plants too?
No. In C3 plants the first stable product is 3-PGA (3 carbons). OAA as a first stable product is a feature of C4 plants only.
What happens to OAA after it is formed?
OAA is converted to other 4-carbon acids such as malic acid in the mesophyll cells. These acids move into the bundle sheath cells, where they release CO2 for the Calvin cycle.
Why is knowing OAA important for NEET?
NEET frequently asks about the first stable product versus the primary acceptor in C4 plants. Confusing OAA (product) with PEP (acceptor) is one of the most common mistakes, so this single fact protects easy marks.
