Learning Objectives

• CO2 Fixation in Calvin Cycle: 3 molecules of CO2 are fixed during each turn.
• Stages of Photosynthesis:
  • Light Dependent Reactions:
  • Inputs: H2O & photons (light energy)
  • Products: O2 (released), ATP, NADPH
  • Location: Thylakoid membrane
  • Light Independent Reactions (Calvin Cycle):
  • Inputs: CO2, ATP, NADPH
  • Products: PGAL (Glyceraldehyde 3-phosphate)
  • Location: Stroma
• Photorespiration Conditions: Occurs in high light intensity, high O2, low CO2 conditions.
• Carbon Fixation Definition: The process of converting inorganic CO2 into organic compounds.
• Carbon Fixation in Plants:
  • C3 Plants: First fixed compound is PGA (3-Phosphoglycerate); enzyme: Rubisco.
  • C4 Plants: First fixed compound is Oxaloacetate; enzyme: PEP carboxylase.
  • CAM Plants: First fixed compound is Oxaloacetate; enzyme: PEP carboxylase, but processes are time-separated.
• Comparative Analysis of Photosynthesis:
  • Photosynthesis Efficiency: C4 is more efficient than C3 due to better CO2 fixation under hot conditions.
• Stages of C4 Photosynthesis:
  • Carbon Fixation Steps:
  1. CO2 combines with phosphoenolpyruvate (PEP) to form Oxaloacetate, catalyzed by PEP carboxylase.
  2. Oxaloacetate is reduced to Malate (or Aspartate).
  3. Malate is then processed in bundle sheath cells.
  4. CO2 is released from Malate for the Calvin Cycle.
  • Photosynthesis Location:
  • Mesophyll cells fix CO2, while the Calvin cycle occurs in bundle sheath cells.
  • Special Leaf Anatomy: Bundle sheath cells surrounded by mesophyll prevent O2 from entering during the Calvin cycle.
• C3 vs. C4 Plants:
  • C3 Plants:
  • Prefer cooler environments, optima range at 65° – 75°F; examples: wheat, rye.
  • Inefficient under high temperatures due to photorespiration.
  • C4 Plants:
  • Optimal at 90° - 95°F; examples: corn, Indiangrass.
  • Require less water and are more efficient in sunny conditions.
• CAM Plants Adaptation:
  • Stomatal Behavior: Stomata open at night to minimize water loss, CO2 is fixed then.
  • Daytime Process: Light reactions and the Calvin cycle occur using stored malate.
• Comparison Summary:
  • C3 Plants: No separation, operate generally during the day in cool wet environments.
  • C4 Plants: Spatial separation between mesophyll and bundle sheath, optimal for hot sunny environments.
  • CAM Plants: Temporal separation between night and day, best for very hot dry environments.