Name the two important enzymes of C4 pathway, and explain their role in fixing CO2?

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Q. Name the two important enzymes of C4 pathway, and explain their role in fixing CO2?

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Concept understanding — C4 Pathway

The Problem C4 Plants Solve

Imagine a plant trying to photosynthesise on a hot, dry day. To stop water from escaping, it closes its stomata. But with stomata closed, CO₂ can't enter easily, and the oxygen produced inside the leaf builds up. The Calvin cycle's key enzyme, RuBisCO, is terrible at telling CO₂ apart from O₂. When O₂ levels are high, RuBisCO grabs oxygen instead of carbon dioxide, starting a wasteful process called photorespiration — it burns up energy and releases already-fixed CO₂. The plant is stuck: open stomata and lose water, or close them and starve the Calvin cycle.

C4 plants evolved a clever workaround. They don't try to make RuBisCO better. Instead, they build a CO₂-concentrating pump that delivers a high dose of CO₂ directly to RuBisCO, even when the leaf's air spaces are low on CO₂. This pump is the Hatch-Slack pathway.

The Two-Cell Strategy

C4 plants separate the initial capture of CO₂ from the Calvin cycle into two different types of cells:

Mesophyll cells — the outer, sun-exposed layer. Here, CO₂ is caught quickly and converted into a 4-carbon compound (hence "C4").
Bundle sheath cells — a tightly packed inner layer surrounding the leaf veins. Here, the 4-carbon compound releases its CO₂, creating a high local concentration for the Calvin cycle.

The mesophyll cells act like a trap, and the bundle sheath cells are where the real carbon fixation happens — but only after the trap has concentrated the CO₂.

The Hatch-Slack Pathway Step by Step

Step 1 — Fixation in mesophyll. CO₂ from the air (or from respiration inside the leaf) combines with phosphoenolpyruvate (PEP), a 3-carbon molecule. The enzyme PEP carboxylase catalyses this reaction, producing oxaloacetate (a 4-carbon compound). PEP carboxylase has no affinity for oxygen, so it never wastes time on photorespiration — it just grabs CO₂.

\text{PEP} + \text{CO}_2 \xrightarrow{\text{PEP carboxylase}} \text{oxaloacetate}

Step 2 — Conversion to a transport form. Oxaloacetate is unstable and is quickly reduced to malate (another 4-carbon compound) using NADPH. In some plants, it is converted to aspartate instead — the principle is the same.

Step 3 — Transport to bundle sheath. Malate moves from the mesophyll cell into the bundle sheath cell through plasmodesmata (tiny channels connecting plant cells).

Step 4 — Decarboxylation. Inside the bundle sheath, malate is broken down. It releases CO₂ and leaves behind pyruvate (a 3-carbon compound). The CO₂ now floods the bundle sheath, raising its concentration many times above what the air could provide.

Step 5 — Calvin cycle runs. RuBisCO in the bundle sheath now sees a high CO₂ / low O₂ environment. It fixes CO₂ into 3-phosphoglycerate (3-PGA) without wasting energy on photorespiration.

Step 6 — Pyruvate returns. The leftover pyruvate goes back to the mesophyll cell, where it is converted back into PEP using ATP. The cycle is ready to catch another CO₂.

\text{Overall: } \text{CO}_2 + \text{PEP} \xrightarrow{\text{ATP, NADPH}} \text{3-PGA} + \text{PEP}

Why It Costs More but Works Better …

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The C4 pathway relies on two key enzymes working in two different cell types — PEP carboxylase in the mesophyll for initial CO2 capture, and RuBisCO in the bundle sheath cells for the actual Calvin-cycle fixation.

The C4 pathway achieves its distinctive two-step capture of carbon dioxide through the coordinated action of two enzymes located in two different cell types of the leaf.

The first enzyme, PEP carboxylase (PEPcase), is found in the mesophyll cells, which notably lack RuBisCO altogether. PEP carboxylase catalyses the fixation of atmospheric CO2 onto the 3-carbon acceptor phosphoenol pyruvate (PEP), producing the 4-carbon acid oxaloacetic acid (OAA) as the first stable product of the pathway. This OAA is subsequently converted into other 4-carbon compounds, such as malic acid or aspartic acid, which are then transported into the bundle sheath cells. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

KCET 2025Set C-41 markMCQ
$Q. Read the given statements and choose the correct option: Statement I: In Calvin cycle, Carboxylation is catalysed by PEP Carboxylase Statement II: In Hatch-Slack pathway, Carboxylation is catalysed by RuBP Carboxylase (A) Statement I is true but Statement II is false (B) Statement I is false but Statement II is true (C) Both Statement I and Statement II are false (D) Both Statement I and Statement II are true$
›Reveal solutionSolution

Each statement names the other pathway's enzyme — Calvin uses RuBisCO and Hatch‑Slack uses PEP carboxylase — so both are false.

Step 1 — The correct enzymes

Calvin cycle (C₃ pathway). The primary CO₂ acceptor is the 5‑carbon RuBP (ribulose‑1,5‑bisphosphate). Carboxylation is catalysed by RuBisCO — RuBP carboxylase‑oxygenase — the most abundant enzyme in the world:
$\text{RuBP (5C)} + \text{CO}_2 \xrightarrow{\;\text{RuBisCO}\;} 2 \times \text{3-PGA (3C)}$
The first stable product has 3 carbons, which is why it is the C₃ pathway.

Hatch‑Slack pathway (C₄). In the mesophyll cells the primary CO₂ acceptor is the 3‑carbon PEP (phosphoenol pyruvate), and carboxylation is catalysed by PEP carboxylase (PEPcase):
$\text{PEP (3C)} + \text{CO}_2 \xrightarrow{\;\text{PEP carboxylase}\;} \text{OAA (4C)}$
The first stable product, oxaloacetic acid, has 4 carbons — hence C₄. Crucially, mesophyll cells of a C₄ plant have no RuBisCO; RuBisCO is confined to the bundle‑sheath cells, where the CO₂ released from the C₄ acid finally enters the Calvin cycle.

Step 2 — Judge the statements …
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KCET 2021Set C-31 markMCQ
$_3$
›Reveal solutionSolution
C $_3$ plants fix CO $_2$ directly via the Calvin cycle using RuBisCO, so they lack the C $_4$ pathway enzyme PEP carboxylase. The correct option is (D).

The key to this question lies in understanding the difference between C $_3$ and C $_4$ photosynthetic pathways. In C $_3$ plants, the first stable product of carbon fixation is a 3-carbon compound (3-phosphoglycerate), and the enzyme that captures CO $_2$ is RuBisCO (RUBP carboxylase). In C $_4$ plants, CO $_2$ is first fixed into a 4-carbon compound in mesophyll cells using PEP carboxylase, before being shuttled to bundle sheath cells for the Calvin cycle.

So, PEP carboxylase is a hallmark of the C $_4$ pathway — it is not found in C $_3$ plants. Let’s check each option:

ATP synthase — This enzyme synthesises ATP during both photosynthesis and respiration. It is present in all plants, including C $_3$ plants (in thylakoid membranes, mitochondria). So this is found.

RUBP carboxylase (RuBisCO) — This is the primary CO $_2$ -fixing enzyme in the Calvin cycle. It is abundant in C $_3$ plants (and also present in C $_4$ plants, but only in bundle sheath cells). So this is definitely found. …
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KCET 2020Set A-11 markMCQ
$C_4$
›Reveal solutionSolution
In Kranz anatomy the initial $CO_2$ fixation is in the mesophyll but the Calvin ( $C_3$ ) cycle is confined to the bundle sheath cells.

Step 1 — The concept: division of labour in $C_4$ plants.

$C_4$ plants (maize, sugarcane, sorghum, Amaranthus) show Kranz anatomy: the vascular bundle is ringed by large, thick-walled bundle sheath cells, which are themselves surrounded by mesophyll cells. Photosynthesis is split between these two cell types — this is the whole point of the $C_4$ pathway.

Step 2 — What happens in the mesophyll.

$CO_2$ is fixed by PEP carboxylase (PEPcase) onto phosphoenolpyruvate (PEP, a 3-carbon acceptor).

The first stable product is a 4-carbon acid — oxaloacetic acid (OAA) — hence " $C_4$ ".

OAA is converted to malic acid/aspartic acid and shipped to the bundle sheath.

Note: mesophyll cells of $C_4$ plants lack RuBisCO, so the Calvin cycle cannot occur there. This directly rules out option (A).

Step 3 — What happens in the bundle sheath.

The $C_4$ acid is decarboxylated, releasing $CO_2$ and a 3-carbon compound (pyruvate, which returns to the mesophyll to regenerate PEP).

This decarboxylation creates a high local $CO_2$ concentration around RuBisCO.

RuBisCO — present only in the bundle sheath — now fixes that $CO_2$ into the Calvin cycle ( $C_3$ cycle). …
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KCET 2018Set A-11 markMCQ
$\mathrm{CO}_2$
›Reveal solutionSolution

Maize follows the C $_4$ (Hatch–Slack) pathway with Kranz anatomy: PEP carboxylase fixes CO $_2$ in the mesophyll, RuBisCO fixes it (again) in the bundle sheath.

Step 1 — Recognise maize as a C $_4$ plant.

Maize (along with sugarcane, sorghum, Amaranthus) shows Kranz anatomy: large, thick-walled bundle sheath cells with no intercellular spaces, wrapped around the vascular bundles, surrounded by mesophyll cells. This division of labour between two cell types is the structural basis of C $_4$ photosynthesis.

Step 2 — CO $_2$ fixation in the mesophyll (primary fixation).

In the mesophyll cytoplasm, atmospheric CO $_2$ is fixed onto the 3-carbon acceptor phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase (PEPcase), forming the 4-carbon oxaloacetic acid (OAA) — hence "C $_4$ ".
$\mathrm{PEP\ (3C)} + \mathrm{CO_2} \xrightarrow{\text{PEP carboxylase}} \mathrm{OAA\ (4C)}$
Crucially, mesophyll cells of a C $_4$ plant have no RuBisCO.

Step 3 — CO $_2$ fixation in the bundle sheath (Calvin cycle).

OAA is converted to malic acid, which travels to the bundle sheath cells and is decarboxylated, releasing CO $_2$ . This CO $_2$ enters the Calvin cycle, where it is fixed onto RuBP by RuBisCO. …
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Q.Name the two important enzymes of C4 pathway, and explain their role in fixing CO2?

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