Role of Light, CO₂, and Factors Affecting Photosynthesis: Study Notes

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Role of Light, CO₂, and Factors Affecting Photosynthesis

Factors and Descriptions

• Carbon Dioxide:
  • Acts as a key limiting factor, particularly when water, light, and temperature conditions are favorable but CO₂ levels are low.
• Light Energy:
  • Essential for photosynthesis; photosynthetic activity is significantly limited in low-light conditions such as dawn or dusk.
• Water:
  • Used in photosynthesis, but it is less influential as a limiting factor compared to carbon dioxide and light energy.

Compensation Point

The compensation point is the stage where the amount of CO₂ released by respiration equals the amount used in photosynthesis.

Stages and Descriptions

• Daytime Compensation Point:
  • Occurs when the CO₂ produced by respiration is fully utilized in photosynthesis. Beyond this point, photosynthesis increases as temperature and light conditions improve.
• Evening Compensation Point:
  • As dusk approaches, the rate of photosynthesis decreases, leading to a balance between CO₂ release and usage, reaching another compensation point.

Alternative Mechanism of Carbon Fixation in Hot and Arid Climates

C4 Plants

Aspects and Descriptions

• Example Plants:
  • Tropical grasses such as maize, sorghum, and sugar cane.
• Adaptation:
  • These plants avoid photorespiration by separating RuBP and rubisco from high oxygen concentrations.
• Cell Types:
  • Bundle sheath cells, which contain RuBP and rubisco, and mesophyll cells, which absorb CO₂.
• Enzyme:
  • PEP carboxylase, which catalyzes CO₂ fixation with phosphoenolpyruvate to form oxaloacetate.
• Process:
  • Oxaloacetate is converted to malate, transported to bundle sheath cells, where CO₂ is released and delivered to RuBP by rubisco. The light-independent reaction then proceeds as usual.

C3 Plants

Aspects and Descriptions

• Characteristics:
  • The first stable product of carbon fixation is glycerate 3-phosphate (PGA), containing three carbon atoms.
• Process:
  • Carbon fixation occurs directly via the Calvin cycle, involving RuBP and rubisco.

CAM Plants

Characteristics and Descriptions

• Examples:
  • Succulent plants, various cacti, pineapples, and other plants across several families.
• Mechanism:
  • Stomata open at night and close during the day.
  • At night, CO₂ is absorbed and incorporated into organic acids through crassulacean acid metabolism (CAM).
  • Organic acids are stored in vacuoles and used for carbon fixation during the day when stomata are closed.

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