The C4 pathway, also called the Hatch and Slack cycle, fixes CO2 twice. First, in the mesophyll cells, the enzyme PEP carboxylase joins CO2 to a 3-carbon molecule (PEP) to form the 4-carbon OAA — this is why it is called "C4". This C4 acid moves to the bundle sheath cells and releases CO2 for the normal Calvin cycle. Memory hook: "C4 = fix CO2 4 times better — PEP catches it, OAA carries it, bundle sheath uses it."
CO2 is fixed twice: first by PEP carboxylase in the mesophyll (giving 4-carbon OAA), then released in the bundle sheath cells where RuBisCO runs the Calvin cycle without photorespiration.
Your doubts, answered
Why is it called the C4 pathway?
Because the first stable product formed when CO2 is fixed is a 4-carbon compound, oxaloacetic acid (OAA). In the C3 pathway the first product is a 3-carbon compound (3-PGA). NEET often asks the number of carbons in the first product, so remember: C4 = 4-carbon OAA first.
What is the primary CO2 acceptor in the C4 (Hatch and Slack) pathway?
The primary CO2 acceptor is phosphoenolpyruvate (PEP), a 3-carbon molecule present in the mesophyll cells. Do not confuse the ACCEPTOR (PEP, 3C) with the first PRODUCT (OAA, 4C). In C3 plants the acceptor is RuBP (5C).
Which enzyme fixes CO2 in the mesophyll cells of C4 plants?
PEP carboxylase (PEPcase). It has a very high affinity for CO2 and no oxygenase activity, so it never wastes energy on photorespiration. The mesophyll cells of C4 plants lack RuBisCO — RuBisCO is present only in the bundle sheath cells.
Does the C4 pathway replace the Calvin cycle?
No. The C4 pathway is only a CO2-pumping stage. The 4-carbon acid carries CO2 into the bundle sheath cells, releases it there, and the released CO2 then enters the normal Calvin (C3) cycle. So the Calvin cycle is still the main biosynthetic pathway even in C4 plants.
Why does CO2 get fixed twice in C4 plants?
The first fixation (in mesophyll by PEPcase) is a trick to concentrate CO2. The C4 acid moves to the bundle sheath and breaks down, releasing CO2 in high amount around RuBisCO. This high CO2 makes RuBisCO act as a carboxylase, not an oxygenase, so photorespiration is avoided. The second fixation is the real Calvin-cycle fixation.
⚠️ The NEET trap ✗ The first stable product of the C4 / Hatch-Slack pathway is 3-PGA (phosphoglyceric acid). ✓ The first stable product of the C4 pathway is OAA (oxaloacetic acid), a 4-carbon acid. 3-PGA is the first product of the C3 (Calvin) pathway. 🧠 Match the number: C4 -> 4-carbon OAA, C3 -> 3-carbon PGA. The name tells you the answer.
Real NEET questions
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 the mesophyll cells, where PEP carboxylase fixes CO2 to form OAA. In C3 plants the acceptor is the 5-carbon RuBP. So PEP is the primary acceptor in C4 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 primary CO2 acceptor is the 3-carbon phosphoenolpyruvate (PEP) in the mesophyll cells; PEP carboxylase 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.
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 Hatch-Slack pathway, PEP carboxylase fixes CO2 onto PEP in the mesophyll cells to give the 4-carbon oxaloacetic acid (OAA) as the first stable product. PGA is the first product only in C3 plants.
Solved Photosynthesis in Higher Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It was worked out by M.D. Hatch and C.R. Slack, which is why it is also called the Hatch and Slack pathway.
Give examples of C4 plants.
Maize (corn), sugarcane, sorghum and amaranthus are common C4 plants. They mostly grow in hot, dry, tropical regions.
What special leaf anatomy do C4 plants have?
They show Kranz anatomy: the vascular bundles are surrounded by large bundle sheath cells that are rich in chloroplasts and thick-walled. This anatomy houses the CO2-concentrating machinery.
Is 3-PGA ever formed in C4 plants?
Yes, but only later inside the bundle sheath cells during the Calvin cycle. The FIRST stable product of the C4 pathway itself is OAA. 3-PGA is not the first C4 product.
Why are C4 plants more efficient at high temperature?
Because they concentrate CO2 in the bundle sheath cells, RuBisCO acts as a carboxylase and photorespiration does not occur. This gives higher photosynthesis, better water and nitrogen use, and greater productivity at high temperatures.
