Introduction of Photorespiration

Photorespiration is a metabolic process that occurs in plants, particularly in C₃ plants. It creates an important difference between C₃ and C₄ plants. This process involves the enzyme RuBisCO binding with oxygen instead of carbon dioxide, leading to wasteful reactions that decrease photosynthetic efficiency. Understanding photorespiration is crucial for comparing C₃ and C₄ plant physiology.

1. RuBisCO Enzyme and Its Dual Activity

1.1 Role of RuBisCO in Calvin Cycle

• First CO₂ fixation step: RuBisCO catalyzes the reaction where RuBP combines with CO₂ to form 2 molecules of 3-PGA (3-phosphoglycerate).
• Reaction: RuBP + CO₂ → 2 × 3-PGA (catalyzed by RuBisCO)
• Most abundant enzyme: RuBisCO is the most abundant enzyme in the world.

1.2 Unique Characteristic of RuBisCO

• Dual binding capacity: The active site of RuBisCO can bind to both CO₂ and O₂.
• Name origin: The enzyme name RuBisCO reflects its ability to act on both carbon dioxide and oxygen.
• Competitive binding: The binding of CO₂ and O₂ to RuBisCO is competitive in nature.
• Affinity for CO₂: RuBisCO has much greater affinity for CO₂ when the CO₂:O₂ ratio is nearly equal.
• Determining factor: The relative concentration of O₂ and CO₂ determines which gas will bind to the enzyme.

2. Photorespiration in C₃ Plants

2.1 Mechanism of Photorespiration

• O₂ binding: In C₃ plants, some O₂ binds to RuBisCO instead of CO₂.
• Decreased CO₂ fixation: Oxygen binding reduces the rate of carbon dioxide fixation.
• Alternative pathway: RuBP binds with O₂ instead of being converted to 2 molecules of PGA.
• Products formed: One molecule of phosphoglycerate (3-carbon) and one molecule of phosphoglycolate (2-carbon) are formed.
• Pathway name: This alternative reaction pathway is called photorespiration.

2.2 Characteristics of Photorespiratory Pathway

• No sugar synthesis: The photorespiratory pathway does not result in synthesis of sugars.
• No ATP synthesis: There is no synthesis of ATP in this pathway.
• No NADPH synthesis: NADPH is not synthesized during photorespiration.
• CO₂ release: Photorespiration results in the release of CO₂.
• ATP utilization: The pathway utilizes ATP instead of producing it.
• Unknown function: The biological function of photorespiration is not yet known.

3. Photorespiration in C₄ Plants

3.1 Absence of Photorespiration

• No photorespiration: Photorespiration does not occur in C₄ plants.
• CO₂ concentration mechanism: C₄ plants have a mechanism that increases CO₂ concentration at the enzyme site.

3.2 Mechanism to Prevent Photorespiration

• C₄ acid breakdown: C₄ acid from mesophyll cells is broken down in bundle sheath cells.
• CO₂ release: The breakdown releases CO₂ in bundle sheath cells.
• Increased intracellular CO₂: This results in increasing the intracellular concentration of CO₂.
• RuBisCO as carboxylase: High CO₂ concentration ensures RuBisCO functions primarily as a carboxylase.
• Minimized oxygenase activity: The oxygenase activity of RuBisCO is minimized in C₄ plants.

4. Comparative Advantages of C₄ Plants

4.1 Productivity and Efficiency

• Better productivity: C₄ plants show better productivity due to lack of photorespiration.
• Higher yields: Yields are better in C₄ plants compared to C₃ plants.
• Temperature tolerance: C₄ plants show tolerance to higher temperatures.
• High light efficiency: CO₂ fixation rate is high under high light conditions in C₄ plants.

5. Key Differences Between C₃ and C₄ Plants Related to Photorespiration

5.1 Calvin Cycle Location

• C₃ plants: Calvin cycle takes place in mesophyll cells.
• C₄ plants: Calvin cycle takes place in bundle sheath cells.

5.2 Initial Carboxylation

• C₃ plants: Initial carboxylation occurs in mesophyll cells.
• C₄ plants: Initial carboxylation occurs in mesophyll cells (but forms C₄ acid).

5.3 Cell Types Fixing CO₂

• C₃ plants: One cell type (mesophyll) fixes CO₂.
• C₄ plants: Two cell types (bundle sheath and mesophyll) are involved in CO₂ fixation.

5.4 Primary CO₂ Acceptor

• C₃ plants: RuBP (Ribulose-1,5-bisphosphate) is the primary CO₂ acceptor.
• C₄ plants: PEP (Phosphoenolpyruvate) is the primary CO₂ acceptor.

5.5 Carbons in Primary CO₂ Acceptor

• C₃ plants: 5 carbons in RuBP.
• C₄ plants: 3 carbons in PEP.

5.6 Primary CO₂ Fixation Product

• C₃ plants: 3-PGA (3-phosphoglycerate) is the primary CO₂ fixation product.
• C₄ plants: OAA (Oxaloacetic acid) is the primary CO₂ fixation product.

5.7 Carbons in Primary CO₂ Fixation Product

• C₃ plants: 3 carbons in 3-PGA.
• C₄ plants: 4 carbons in OAA.

5.8 Presence of RuBisCO

• C₃ plants: RuBisCO is present in mesophyll cells.
• C₄ plants: RuBisCO is present in bundle sheath cells.

5.9 Presence of PEP Carboxylase (PEPCase)

• C₃ plants: PEPCase is not present or not significant.
• C₄ plants: PEPCase is present in mesophyll cells.

5.10 CO₂ Fixation Rate Under High Light

• C₃ plants: Low to medium CO₂ fixation rate under high light conditions.
• C₄ plants: High CO₂ fixation rate under high light conditions.

5.11 Photorespiration at Different Light Intensities

• C₃ plants at low light: Photorespiration is present but at lower levels.
• C₃ plants at high light: Photorespiration is high.
• C₄ plants: Photorespiration is negligible at both low and high light intensities.

5.12 Photorespiration at Different CO₂ Concentrations

• C₃ plants at low CO₂: Photorespiration is high.
• C₃ plants at high CO₂: Photorespiration is negligible or low.
• C₄ plants: Photorespiration is negligible at both low and high CO₂ concentrations.

5.13 Temperature Optimum

• C₃ plants: Temperature optimum is 20-25°C.
• C₄ plants: Temperature optimum is 30-40°C.

5.14 Anatomical Difference (Kranz Anatomy)

• C₃ plants: Do not have Kranz anatomy (specialized bundle sheath arrangement).
• C₄ plants: Have Kranz anatomy with well-developed bundle sheath cells around vascular bundles.

Photorespiration represents a major difference in efficiency between C₃ and C₄ plants. While photorespiration wastes energy and releases CO₂ in C₃ plants, C₄ plants have evolved a specialized mechanism to concentrate CO₂ and prevent this wasteful process. This gives C₄ plants advantages in productivity, especially under high light intensity, high temperature, and low CO₂ conditions.