Biochemistry
C4 and CAM Plants
Photosynthetic adaptations for hot, dry environments — minimizing photorespiration
C4 and CAM are photosynthetic adaptations that overcome the inefficiency of standard C3 photosynthesis under hot or dry conditions. C4 plants (corn, sugarcane, sorghum): spatial separation — initial CO₂ fixation in mesophyll cells via PEP carboxylase (4-carbon intermediate); CO₂ released to bundle sheath cells where Calvin cycle runs. CAM plants (cactus, pineapple, agave): temporal separation — open stomata at night, store CO₂ as malic acid; release CO₂ during day. Both: prevent photorespiration. Trade-off: extra energy cost. Important for global agriculture (corn = world's largest crop) and arid ecology.
- C3 plants~85% of plants; standard photosynthesis
- C4 plants~3% of species; corn, sugarcane, sorghum, switchgrass
- CAM plants~7% of species; cactus, pineapple, agave
- C4 mechanismSpatial separation in two cell types
- CAM mechanismTemporal separation (night/day)
- Key enzymePEP carboxylase (high CO₂ affinity)
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Why C4 and CAM matter
- Agriculture. Corn, sugarcane (C4); pineapple (CAM).
- Climate adaptation. Plants for hot, dry climates.
- Evolution. Convergent evolution example.
- Global productivity. Crucial for food supply.
- Climate change. C4 may benefit; C3 may suffer.
- Synthetic biology. Engineering C4 traits into rice (C3).
- Ecology. Different niches.
Common misconceptions
- C4 always better than C3. Depends on climate.
- C4 doesn't use Calvin cycle. Uses it; just concentrates CO₂.
- CAM only in cactus. Many plants.
- CAM = no photosynthesis at night. Light reactions need light; CO₂ stored at night.
- C4 evolved once. ~60 independent times — convergent.
- C4 grows faster. Yes — in hot, sunny conditions.
Frequently asked questions
Why did C4 and CAM evolve?
To overcome photorespiration. RuBisCO sometimes binds O₂ instead of CO₂ — wastes energy. Worse at high T (less CO₂/O₂ ratio in solution). Worse in dry conditions (stomata closed → low CO₂ inside leaf). C4 and CAM concentrate CO₂ around RuBisCO — overcomes problem. Evolved independently many times (~60 times for C4) — convergent evolution.
How does C4 photosynthesis work?
Two-cell system. Mesophyll cells: PEP carboxylase fixes CO₂ to PEP → oxaloacetate (4 C — hence "C4") → malate. Malate transported to bundle sheath cells. Releases CO₂ for Calvin cycle there. Bundle sheath cells have high CO₂ concentration, low O₂ — minimizes photorespiration. Kranz anatomy: bundle sheath cells form ring around vascular bundles.
What's PEP carboxylase?
Phosphoenolpyruvate carboxylase. Fixes CO₂ to PEP → oxaloacetate. High affinity for CO₂; doesn't do oxygenase reaction. Lacks RuBisCO's photorespiration problem. Used as initial CO₂ fixer in C4 and CAM plants. Then CO₂ released near RuBisCO — concentrated, efficient.
How does CAM photosynthesis work?
Time-separated, single-cell system. Night: stomata open; CO₂ enters; PEP carboxylase fixes CO₂ → malate; stored in vacuole as malic acid. Day: stomata closed (preserve water); malate released; decarboxylated → CO₂ for Calvin cycle. Net: same chemistry as C4 but spread across day-night cycle. Stomata open at night when cool — minimal water loss.
What plants are C4?
Corn (maize), sugarcane, sorghum, millet, switchgrass, crabgrass. ~3% of plant species but ~25% of land photosynthesis. Tropical grasses dominantly C4. Best in: hot, sunny climates with seasonal rainfall. Evolved ~25-30 Mya during atmospheric CO₂ decline.
What plants are CAM?
Cactus, pineapple, succulents, jade plant, agave (tequila plant), ice plant. ~7% of species. Adapted to extreme drought. Found in: deserts, epiphytes (orchids), some marine algae. Some plants are facultative CAM — switch on/off based on water availability.
What are the trade-offs?
C4 and CAM use extra energy (ATP) for CO₂ concentration. Worth it when photorespiration would otherwise waste more. C3: efficient in cool, wet, low-light. C4: efficient in hot, sunny, with adequate water. CAM: efficient in hot, very dry. Best plant depends on environment. C3 still dominant globally — most ecosystems aren't extreme.