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.
Grasses (many of them C4 plants) avoid photorespiratory losses via the C4 or Hatch-and-Slack pathway, which fixes CO2 twice across mesophyll and bundle sheath cells (Kranz anatomy) to concentrate CO2 around RuBisCO and suppress its oxygenase activity. …
Step 1. Photorespiration is triggered whenever CO2 is scarce and O2 relatively abundant at RuBisCO's active site, causing RuBisCO's oxygenase activity to dominate over its productive carboxylase activity, wasting fixed carbon and yielding no ATP.
Step 2. Many grasses (sugarcane, maize, sorghum and other tropical/sub-tropical grasses) are C4 plants, meaning they use the Hatch and Slack (C4) pathway rather than relying on the C3 cycle alone.
Step 3. In the C4 pathway's first stage, mesophyll cells fix CO2 using PEP carboxylase (an enzyme with very high CO2 affinity and, unlike RuBisCO, complete insensitivity to oxygen) onto PEP, forming oxaloacetic acid, which is converted to malic acid and transported into bundle sheath cells.
Step 4. In the second stage, the bundle sheath cells decarboxylate this malic acid, releasing a locally concentrated burst of CO2 exactly where RuBisCO and the Calvin cycle are located (Kranz anatomy physically rings bundle sheath cells with mesophyll cells to support this arrangement). …
Naming CAM instead of C4 - CAM separates CO2 capture and use in time (night/day), not across two cell types, and is found in succulents, not grasses. …