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.
PEP carboxylase is the enzyme not found in C3 plants.
C3 plants carry out CO2 fixation directly through the Calvin cycle, using RuBP carboxylase (RuBisCO) as the fixing enzyme — this enzyme is common to all photosynthetic plants, C3 or C4.
NADP reductase and ATP synthase are both components of the light reaction machinery, which every photosynthetic plant, including C3 plants, possesses. …
Of the four enzymes listed, PEP carboxylase is unique to the C4 pathway and so is absent from C3 plants, while the other three are used by all photosynthetic plants.
C3 plants carry out carbon fixation entirely through the Calvin cycle: CO2 is added directly to the 5-carbon RuBP by the enzyme RuBP carboxylase (RuBisCO), forming two molecules of the 3-carbon acid PGA. This same Calvin cycle, and the same RuBisCO enzyme, is used by every photosynthetic plant — C3 or C4 — since the Calvin pathway is universal to photosynthetic plants, regardless of which route the plant uses to first capture CO2.
NADP reductase and ATP synthase both belong to the machinery of the light reaction, situated on the thylakoid membrane: NADP reductase (on the stroma side) reduces NADP+ to NADPH, and ATP synthase (with its CF0 and CF1 parts) synthesises ATP using the proton gradient. Since every photosynthetic plant carries out light reactions, both enzymes are present in C3 plants as well. …