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.
Both RuBP carboxylase and PEP carboxylase catalyse the primary fixation of CO2, depending on which pathway a plant uses.
In C3 plants, the primary (first) fixation of CO2 happens directly in the Calvin cycle, where RuBP carboxylase (RuBisCO) carboxylates RuBP to form 3-PGA.
In C4 plants, the primary fixation of CO2 instead happens in the mesophyll cells, where PEP carboxylase fixes CO2 onto PEP to form oxaloacetic acid (OAA) — this is the first, or primary, carbon-fixing step in these plants, occurring before RuBisCO is ever involved. …
Primary CO2 fixation is catalysed by RuBP carboxylase in C3 plants and by PEP carboxylase in C4 plants — since the question doesn't specify the plant type, both enzymes are correct.
Not every photosynthetic plant fixes CO2 for the first time using the same enzyme. In C3 plants, carbon fixation begins directly with the Calvin cycle's carboxylation step: CO2 combines with the 5-carbon ribulose 1,5-bisphosphate (RuBP), a reaction catalysed by the enzyme RuBP carboxylase, more fully RuBP carboxylase-oxygenase (RuBisCO), producing two molecules of the 3-carbon acid 3-PGA. In these plants, RuBisCO's action genuinely is the primary — the very first — fixation of atmospheric CO2. …
Method 1 — Identify which enzyme performs the FIRST fixation step, in each pathway separately
Recall that CO2 fixation is not identical across all photosynthetic plants — C3 and C4 plants use different first steps.
For C3 plants: the very first CO2-fixing reaction is the Calvin cycle's carboxylation step, RuBP + CO2 → 2 × 3-PGA, catalysed by RuBP carboxylase (RuBisCO).
For C4 plants: the very first CO2-fixing reaction happens in the mesophyll cells, PEP + CO2 → OAA, catalysed by PEP carboxylase — this happens before RuBisCO is ever involved (RuBisCO only acts later, in the bundle sheath cells). …