C4 Pathway and Kranz Anatomy: Why Maize Beats Heat (NEET)
Maize and sugarcane keep growing happily under a scorching noon sun, while a wheat or rice plant in the same heat struggles to photosynthesise efficiently. Both use the Calvin cycle, so why does one cope so much better? The answer is a clever piece of anatomy plus a second carbon-fixing step, called the C4 pathway. This concept matters for NEET only, and it is a favourite of the Plant Physiology chapter.
Quick Summary
In C4 plants the first stable product of CO₂ fixation is a 4-carbon acid, oxaloacetic acid (OAA), not the 3-carbon PGA.
CO₂ is first fixed in mesophyll cells by the enzyme PEP carboxylase (PEPcase), which has no affinity for O₂.
The 4-carbon acid moves to bundle sheath cells, releases CO₂ there, and the Calvin cycle runs only in the bundle sheath.
This concentrates CO₂ around RuBisCO, so photorespiration is almost absent.
Cost: 5 ATP per CO₂ fixed, against 3 ATP in C3 plants. NADPH need stays at 2 per CO₂.
Start from the problem
You already know the Calvin cycle: the enzyme RuBisCO attaches CO₂ to a 5-carbon sugar, RuBP, and the product splits into two molecules of the 3-carbon PGA. The trouble is that RuBisCO is not picky. It can also bind O₂, which starts photorespiration, a wasteful process that burns fixed carbon without making ATP or sugar. When a leaf is hot and bright, stomata partly close to save water, CO₂ inside the leaf falls, and O₂ from the light reaction builds up. The O₂ to CO₂ ratio rises, and RuBisCO wastes more and more effort.
C4 plants solve this not by changing RuBisCO but by changing its surroundings: they build a CO₂ pump so that RuBisCO sits in a CO₂-rich environment where O₂ cannot compete.
Kranz anatomy: the structure behind the pump
"Kranz" is the German word for wreath. In a C4 leaf, large bundle sheath cells form a ring around each vascular bundle, and the mesophyll cells surround that ring. The bundle sheath cells have thick walls that limit gas exchange, many chloroplasts, and no intercellular air spaces between them. So CO₂ delivered into them cannot leak back out easily. Mesophyll cells are the ones in contact with air spaces and incoming CO₂.
That is the whole idea: two cell types, two jobs. Mesophyll cells capture CO₂; bundle sheath cells hand it to RuBisCO.
The pathway, step by step
In mesophyll cells: CO₂ (as bicarbonate) is combined with PEP, phosphoenolpyruvate, a 3-carbon compound. The enzyme PEP carboxylase does this. The product is OAA, a 4-carbon acid (3 + 1 = 4, hence C4).
OAA is converted into other 4-carbon acids such as malate or aspartate.
Transport: these acids move into the bundle sheath cells.
In bundle sheath cells: the 4-carbon acid is broken down, releasing CO₂ and leaving a 3-carbon molecule (pyruvate).
The released CO₂ enters the normal Calvin cycle, fixed by RuBisCO to make sugar.
The 3-carbon pyruvate goes back to the mesophyll and is converted to PEP using ATP, so the pump can run again.
Notice what is special: PEPcase does the first capture, and it does not react with O₂ at all. RuBisCO works only in the bundle sheath, where CO₂ has been pumped up to a high level. That is why photorespiration is negligible in C4 plants.
Worked example: the energy price of the pump
Question: compare the ATP needed to make one glucose (6 carbons) in C3 and C4 plants, given 3 ATP and 2 NADPH per CO₂ in C3, and 5 ATP and 2 NADPH per CO₂ in C4.
One glucose needs 6 CO₂ fixed.
C3: ATP = 6 × 3 = 18; NADPH = 6 × 2 = 12.
C4: ATP = 6 × 5 = 30; NADPH = 6 × 2 = 12.
Extra ATP in C4 = 30 − 18 = 12 ATP per glucose. The extra 2 ATP per CO₂ is spent regenerating PEP from pyruvate.
So C4 plants pay more ATP but save the far bigger loss from photorespiration. Under high light and temperature this trade is a clear win. Under cool, dim conditions it is not, which is why C3 plants do fine in cooler climates.
Why C4 plants do better in heat
Higher optimum temperature: C4 plants respond to roughly 30–45 °C, while C3 plants peak at lower temperatures.
They show no saturation at high light intensity, whereas C3 plants saturate at lower light.
Their rate of photosynthesis is high and they have greater productivity of biomass.
Typical examples: maize, sugarcane, sorghum.
Mistakes students actually make
Thinking C4 plants skip the Calvin cycle. They do not. The Calvin cycle still happens, only in the bundle sheath cells.
Mixing up the two enzymes. PEPcase works in mesophyll and fixes CO₂ first. RuBisCO works in bundle sheath. Questions often swap them.
Saying the first product is PGA. In C4 plants the first stable product is OAA; PGA appears only later in the Calvin cycle.
Thinking the cycle needs less energy. It needs more ATP (5 per CO₂), not less.
Calling the pathway the Hatch and Slack pathway and then forgetting it is the same thing as C4. They are two names for one pathway.
How NEET asks it
Direct recall: "First stable product of CO₂ fixation in maize is…" (answer: OAA).
Match the column: PEPcase with mesophyll, RuBisCO with bundle sheath.
Statement-based: "C4 plants lack photorespiration" or "Kranz anatomy shows two types of chloroplast-containing cells", where you judge true or false.
Numerical-style: ATP and NADPH needed to make one glucose, like the example above.
Pro Tip: When you meet any C4 question, ask "where is this happening, mesophyll or bundle sheath?" before anything else. Almost every wrong option is a correct fact placed in the wrong cell. Treat it as a spatial map: capture in mesophyll, Calvin cycle in bundle sheath. Learn the map and the details follow. Platforms such as Sukriti NeetJee often teach it exactly this way, as a two-room factory, because the picture sticks better than a list.
Frequently Asked Questions
Why is PEP carboxylase better than RuBisCO at the first capture?
PEPcase has a high affinity for CO₂ (as bicarbonate) and does not bind O₂ at all. So it can pick up CO₂ even when the leaf has little of it, without any wasteful side reaction.
Do C4 plants have photorespiration?
Almost none. Because CO₂ is concentrated in the bundle sheath cells, RuBisCO rarely meets O₂, and the oxygenase reaction is largely suppressed.
Why does the C4 pathway need 5 ATP per CO₂?
Three ATP are used in the Calvin cycle as in C3 plants. The extra two ATP are used to regenerate PEP from pyruvate in the mesophyll, which keeps the CO₂ pump going.
Which cells in a C4 leaf have chloroplasts?
Both mesophyll and bundle sheath cells have chloroplasts, which is why the leaf is said to show dimorphic chloroplasts. This is one of the features of Kranz anatomy.
Is every C4 plant a grass?
No. Many important C4 plants such as maize, sugarcane and sorghum are grasses, but the pathway is found in other plants too. For NEET, remember maize, sugarcane and sorghum as the standard examples.
Over to you
Stuck on a C3 versus C4 comparison? Ask in the comments and we will work through it together. And if this cleared things up, share it with a fellow NEET aspirant.
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