C₄ Cycle / Hatch–Slack Pathway
A specialized CO₂-concentrating mechanism adapted to high light, elevated temperatures, and dry environments.
In ordinary C₃ photosynthesis, atmospheric CO₂ is directly fixed by RuBisCO. In C₄ plants, CO₂ is first fixed into a 4-carbon dicarboxylic acid in mesophyll cells, transported to the bundle sheath region, and decarboxylated to saturate RuBisCO with high CO₂.
The very first stable products formed after initial CO₂ fixation are 4-carbon dicarboxylic acids:
- Oxaloacetate (OAA)
- Malate
- Aspartate
Experimentally established in 1966 by Marshall D. Hatch and C. Roger Slack using 14CO₂-labeling tracer techniques in sugarcane, discovering that CO₂ was initially incorporated into a C₄ compound before release to the Calvin cycle.
Most classical C₄ plants exhibit Kranz anatomy (German Kranz = wreath). The vascular bundle is surrounded by an inner concentric layer of bundle-sheath cells, which is further enveloped by mesophyll cells:
Atmospheric CO₂ enters through stomata and is rapidly converted to bicarbonate by Carbonic Anhydrase (CA) because PEPC requires HCO₃⁻ as substrate.
Catalysed by PEP carboxylase (PEPC). The 3-carbon PEP captures inorganic carbon to produce the 4-carbon dicarboxylic acid Oxaloacetate (OAA).
Unstable OAA is rapidly converted into Malate (via malate dehydrogenase) or Aspartate (via aspartate aminotransferase) and transported via plasmodesmata into bundle-sheath cells.
The C₄ acid undergoes decarboxylation, generating a high local concentration of CO₂ and releasing a 3-carbon residual compound (pyruvate or PEP).
RuBisCO fixes concentrated CO₂ into the Calvin–Benson cycle to yield sugars. Meanwhile, the C₃ compound returns to the mesophyll where PPDK (Pyruvate phosphate dikinase) regenerates PEP.
Subtypes are classified based on the principal C₄ acid decarboxylating enzyme rather than PEPC:
| Feature | NADP-ME Type | NAD-ME Type | PCK (PEP-CK) Type |
|---|---|---|---|
| Major Decarboxylase | NADP-malic enzyme | NAD-malic enzyme | PEP carboxykinase |
| Transported C₄ Acid | Malate | Aspartate | Aspartate |
| Decarboxylation Site | Bundle-sheath Chloroplast | Bundle-sheath Mitochondrion | Bundle-sheath Cytosol |
| Decarboxylation Product | Pyruvate + CO₂ | Pyruvate + CO₂ | PEP + CO₂ |
| PPDK Role | Essential for PEP regeneration | Essential for PEP regeneration | Less central to immediate product |
| Typical Examples | Maize, Sugarcane, Sorghum | Amaranthaceae, Millets | Specific grasses & dicots |
| Character | C₃ Plants | C₄ Plants |
|---|---|---|
| Initial CO₂ Acceptor | RuBP (5C) | PEP (3C) |
| Initial Carboxylase | RuBisCO | PEP carboxylase (PEPC) |
| First Stable Product | 3-PGA (3-carbon) | Oxaloacetate / OAA (4-carbon) |
| Photosynthetic Cells | Mesophyll only | Mesophyll + Bundle Sheath |
| Kranz Anatomy | Absent | Usually present in classical types |
| CO₂ Pump Mechanism | Absent | Present (concentrating mechanism) |
| Photorespiration Rate | Significant (up to 25–40% loss) | Suppressed / Negligible |
| Water-Use Efficiency (WUE) | Moderate / Lower | High (maintains fixation with lower conductance) |
| High-Temp Performance | Lower (photorespiration increases) | Superior & Optimal |
| Energy Cost per Triose Phosphate | 9 ATP + 6 NADPH (for 3 CO₂) | ~12 ATP + 6 NADPH (costs ~1 extra ATP / CO₂) |
| RuBisCO Localization | Mesophyll chloroplasts | Bundle-sheath chloroplasts |
RuBisCO has dual affinity (carboxylase vs oxygenase). High local CO₂ in bundle sheath suppresses competitive O₂ binding and eliminates photorespiratory carbon waste.
PEPC's high affinity for HCO₃⁻ enables rapid carbon fixation even when stomata are partially closed, drastically reducing transpirational water loss.
Maintains high photosynthetic capacity in tropical conditions where C₃ plants experience heavy photorespiratory drag and stomatal stress.
| Enzyme | Primary Biological Role |
|---|---|
| Carbonic Anhydrase (CA) | Catalyses rapid hydration: CO₂ + H₂O ⇌ HCO₃⁻ + H⁺ |
| PEP Carboxylase (PEPC) | Initial carboxylation: PEP + HCO₃⁻ → Oxaloacetate (OAA) |
| PEPC Kinase (PPCK) | Reversible light/dark phosphorylation regulating PEPC catalytic activity |
| Malate Dehydrogenase | Interconverts OAA and Malate |
| Aspartate Aminotransferase | Interconverts OAA and Aspartate via transamination |
| NADP / NAD-Malic Enzyme | Decarboxylates Malate → CO₂ + Pyruvate |
| PEP Carboxykinase (PCK) | Decarboxylates C₄ intermediate → CO₂ + PEP |
| Pyruvate Phosphate Dikinase (PPDK) | Regenerates PEP: Pyruvate + ATP + Pi → PEP + AMP + PPi |
| RuBisCO | Primary assimilator of Calvin cycle, fixing released CO₂ onto RuBP |
“PEP catches CO₂ → OAA makes C₄ acid → C₄ acid carries CO₂ → Bundle Sheath releases CO₂ → RuBisCO fixes CO₂ → PPDK regenerates PEP.”
- First stable product: 4-carbon Oxaloacetate (OAA).
- Initial CO₂ acceptor: Phosphoenolpyruvate (PEP).
- Primary initial enzyme: PEP carboxylase (PEPC).
- Calvin cycle is still present (in bundle sheath).
- Major crops: Maize, Sugarcane, Sorghum, Pearl & Foxtail Millets.
- PEPC uses HCO₃⁻ (bicarbonate), not dissolved free CO₂.
- PEPC lacks oxygenase activity; cannot trigger photorespiration.
- Requires ~2 extra ATP equivalents per CO₂ due to PPDK conversion.
- C₄ evolution is convergent (evolved independently ~60+ times).
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