How are C₄ plants more efficient than C₃ plants?

C₄ plants are photosynthetically more efficient than C₃ plants, particularly under conditions of:

• High temperature
• High light intensity
• Water scarcity

Efficiency Comparison:

Parameter	C₃ Plants	C₄ Plants	Efficiency Gain
Water Loss per CO₂ Fixed	400-1000 H₂O/CO₂	250-400 H₂O/CO₂	2x more water-efficient
Photorespiration	Significant (25-50% loss)	Minimal (~2% loss)	~10-15% higher productivity
CO₂ Compensation Point	50-100 ppm	0-10 ppm	Can photosynthesize at lower CO₂
Photosynthetic Rate	15-35 mg CO₂/dm²/hr	40-80 mg CO₂/dm²/hr	2-3x higher
Temperature Optimum	15-25°C	30-40°C	Thrive in hot climates
Biomass Production	Lower	Higher	Greater yield

Why C₄ Plants Are More Efficient:

1. CO₂ Concentrating Mechanism

Anatomical Basis—Kranz Anatomy:

• Bundle sheath cells: Surround vascular bundles; site of Calvin cycle
• Mesophyll cells: Outer layer; site of initial CO₂ fixation
• Spatial separation: Prevents photorespiration

Biochemical Pathway:

In Mesophyll Cells:

1. CO₂ + PEP (3C) → Oxaloacetate (4C) [enzyme: PEP carboxylase]
2. Oxaloacetate → Malate or Aspartate (4C acids)
3. Transported to bundle sheath cells

In Bundle Sheath Cells: 4. 4C acids decarboxylated → CO₂ released 5. High CO₂ concentration (10-60x atmospheric) created around RuBisCO 6. CO₂ + RuBP → Calvin cycle operates efficiently 7. Sugar produced; 3C compound returns to mesophyll

Key Enzyme Difference:

• C₄ plants:PEP carboxylase (no O₂ affinity, high CO₂ affinity even at low concentrations)
• C₃ plants:RuBisCO (binds both CO₂ and O₂, causing photorespiration)

2. Suppression of Photorespiration

Photorespiration in C₃ Plants:

• RuBisCO binds O₂ instead of CO₂ at high temperatures
• Produces 2-phosphoglycolate (toxic)
• Loss of 25-50% of fixed carbon

C₄ Advantage:

• High CO₂ concentration in bundle sheath favors RuBisCO's carboxylase activity
• Oxygenase activity suppressed
• Photorespiration reduced to ~2%

3. Water Use Efficiency

Mechanism:

• C₄ plants achieve higher photosynthetic rates even with partially closed stomata
• PEP carboxylase efficiently captures CO₂ at lower concentrations
• Less water lost per unit of CO₂ fixed

Adaptive Advantage:

• Thrive in arid/semi-arid environments
• Dominate tropical grasslands, savannas

Examples of C₄ Plants:

• Agricultural: Maize (Zea mays), sugarcane (Saccharum), sorghum (Sorghum)
• Grasses: Bermuda grass, crabgrass
• Weeds: Amaranthus (pigweed)

C₃ Plant Examples (for comparison):

• Rice, wheat, soybean, most trees

Trade-off: C₄ photosynthesis requires extra ATP (2 ATP per CO₂) for the C₄ cycle, but this cost is outweighed by benefits in hot, bright, dry environments.

Evolutionary Significance: C₄ photosynthesis evolved independently ~60 times, demonstrating strong selective advantage in specific ecological niches.