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.
Fixation of CO2 onto PEP in the mesophyll cells of C4 plants produces oxaloacetate (OAA), a 4-carbon acid, as the first stable product.
Not all plants form the same first product when they fix carbon dioxide. In plants following the C4 pathway, the primary CO2 acceptor is the 3-carbon molecule phosphoenol pyruvate (PEP), present in the mesophyll cells, and the enzyme catalysing its fixation is PEP carboxylase. When CO2 combines with PEP under the action of this enzyme, the resulting stable product is oxaloacetic acid (OAA) — a 4-carbon organic acid, which is why CO2 assimilation of this kind is called the C4 pathway. …