Q.PEP is primary CO2 acceptor in
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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.
PEP+CO2PEP carboxylaseoxaloacetate
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.
Overall: CO2+PEPATP, NADPH3-PGA+PEP
The PEP is regenerated; the net gain is one CO₂ fixed into the Calvin cycle.
Why It Costs More but Works Better …
Phosphoenol pyruvate (PEP) is the primary CO2 acceptor in C4 plants.
- In the Hatch and Slack pathway followed by C4 plants, the primary CO2 acceptor is the 3-carbon molecule PEP, present in the mesophyll cells.
- The enzyme PEP carboxylase (PEPcase) catalyses this fixation, forming the 4-carbon acid oxaloacetic acid (OAA) as the first stable product. …
PEP is the primary carbon dioxide acceptor specifically in C4 plants, where it is fixed by PEP carboxylase in the mesophyll cells to form the 4-carbon acid oxaloacetic acid.
Plants differ in the identity of the first stable product formed when they fix carbon dioxide, and this difference defines the two main pathways of CO2 assimilation. In plants following the C4 pathway — plants adapted to dry tropical regions — the primary CO2 acceptor is phosphoenol pyruvate (PEP), a 3-carbon molecule present in the mesophyll cells. The enzyme responsible for this fixation is PEP carboxylase (PEPcase), and it is notable that the mesophyll cells of C4 plants lack the enzyme RuBisCO altogether. Fixation of CO2 onto PEP produces oxaloacetic acid (OAA), the 4-carbon acid that gives this pathway its name. …
Method 1 — Match the CO2 acceptor to its pathway
- Recall that C3 plants use RuBP (5-carbon) as their CO2 acceptor, fixed by RuBisCO.
- Recall that C4 plants use PEP (3-carbon) as their CO2 acceptor, fixed by PEP carboxylase (PEPcase), forming the 4-carbon acid OAA — this is what gives the C4 pathway its name. …
- KCET 2025Set C-41 markMCQQ.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:
RuBP (5C)+CO2RuBisCO2×3-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):
PEP (3C)+CO2PEP carboxylaseOAA (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 …
- KCET 2021Set C-31 markMCQQ.The enzyme that is not found is C3 plants is (A) ATP synthase (B) RUBP carboxylase (C) NADP reductase (D) PEP carboxylase
›Reveal solutionSolution
C3 plants fix CO2 directly via the Calvin cycle using RuBisCO, so they lack the C4 pathway enzyme PEP carboxylase. The correct option is (D).
The key to this question lies in understanding the difference between C3 and C4 photosynthetic pathways. In C3 plants, the first stable product of carbon fixation is a 3-carbon compound (3-phosphoglycerate), and the enzyme that captures CO2 is RuBisCO (RUBP carboxylase). In C4 plants, CO2 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 C4 pathway — it is not found in C3 plants. Let’s check each option:
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ATP synthase — This enzyme synthesises ATP during both photosynthesis and respiration. It is present in all plants, including C3 plants (in thylakoid membranes, mitochondria). So this is found.
-
RUBP carboxylase (RuBisCO) — This is the primary CO2-fixing enzyme in the Calvin cycle. It is abundant in C3 plants (and also present in C4 plants, but only in bundle sheath cells). So this is definitely found. …
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- KCET 2020Set A-11 markMCQQ.In C4 plants, C3 cycle takes place in (A) Mesophyll cells (B) Bulliform cells (C) Companion cells (D) Bundle sheath cells
›Reveal solutionSolution
In Kranz anatomy the initial CO2 fixation is in the mesophyll but the Calvin (C3) cycle is confined to the bundle sheath cells.
Step 1 — The concept: division of labour in C4 plants.
C4 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 C4 pathway.
Step 2 — What happens in the mesophyll.
- CO2 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 "C4".
- OAA is converted to malic acid/aspartic acid and shipped to the bundle sheath.
Note: mesophyll cells of C4 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 C4 acid is decarboxylated, releasing CO2 and a 3-carbon compound (pyruvate, which returns to the mesophyll to regenerate PEP).
- This decarboxylation creates a high local CO2 concentration around RuBisCO.
- RuBisCO — present only in the bundle sheath — now fixes that CO2 into the Calvin cycle (C3 cycle). …
- KCET 2018Set A-11 markMCQQ.In the maize plant, CO2 fixation occurs in both mesophyll and bundle sheath cells. The enzymes involved in these cells for the process respectively are, (A) RuBisCO and PEP Kinase (B) PEP Kinase and Pepsin (C) RuBisCO and PEP Carboxylase (D) PEP Carboxylase and RuBisCO
›Reveal solutionSolution
Maize follows the C4 (Hatch–Slack) pathway with Kranz anatomy: PEP carboxylase fixes CO2 in the mesophyll, RuBisCO fixes it (again) in the bundle sheath.
Step 1 — Recognise maize as a C4 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 C4 photosynthesis.
Step 2 — CO2 fixation in the mesophyll (primary fixation).
In the mesophyll cytoplasm, atmospheric CO2 is fixed onto the 3-carbon acceptor phosphoenolpyruvate (PEP) by the enzyme PEP carboxylase (PEPcase), forming the 4-carbon oxaloacetic acid (OAA) — hence "C4".
PEP (3C)+CO2PEP carboxylaseOAA (4C)
Crucially, mesophyll cells of a C4 plant have no RuBisCO.
Step 3 — CO2 fixation in the bundle sheath (Calvin cycle).
OAA is converted to malic acid, which travels to the bundle sheath cells and is decarboxylated, releasing CO2. This CO2 enters the Calvin cycle, where it is fixed onto RuBP by RuBisCO. …
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