Photosynthesis in Plants: Metabolic Pathways

Introduction

• Focus on the diversity of plants and their photosynthetic pathways.
• Revisiting photosynthesis, particularly the differences between three metabolic pathways in angiosperms.
• Carbon Fixation: Conversion of CO2 into organic compounds.

C3 Photosynthesis

• Process:
  • Rubisco enzyme adds CO2 to RuBP.
  • Produces 3-carbon compound 3PGA.
• Commonality: Most frequent method of photosynthesis.
  • Found in plants like soybeans, oats, wheat, and rice.
• Issues in Arid Environments:
  • O2 builds up when stomata close, leading to photorespiration.
  • Photorespiration yields no sugar or ATP and consumes previously fixed carbon.

C4 Photosynthesis

• Plants: Corn, sugarcane.
• Process:
  • Precedes Calvin cycle with CO2 fixed into a 4-carbon compound.
  • Uses enzyme PEP carboxylase to form oxaloacetic acid (OAA).
  • High CO2 concentrations maintained in bundle sheath cells, continuing sugar production.
• Adaptation: Better suited for hot, dry environments as it avoids photorespiration.
• Convergent Evolution: Independently evolved multiple times in various plant families.

CAM Photosynthesis

• Plants: Pineapples, aloe, succulents.
• Process:
  • Opens stomata at night to fix CO2 into 4-carbon compounds stored in vacuoles.
  • During the day, the stomata close and stored CO2 is used in the Calvin cycle.
• Suitability: Extremely arid environments.
• Convergent Evolution: Evolved in different plant clades, more common in epiphytes and succulents.

Summary

• C3 Photosynthesis: Most common but inefficient in dry climates.
• C4 Photosynthesis: Spatial separation of carbon fixation and the Calvin cycle.
• CAM Photosynthesis: Temporal separation of these processes.
• Both C4 and CAM pathways offer solutions to balance photosynthesis with water conservation.
• Understanding these processes provides insight into plant adaptations in various environments.