Photosynthesis in Plants

Introduction

• Revisit photosynthesis focusing on carbon fixation pathways in angiosperms.
• Carbon fixation: Conversion of CO2 into organic compounds in organisms.

C3 Photosynthesis

• Most common pathway.
• Initial fixation by rubisco adding CO2 to RuBP producing 3-PGA (three-carbon compound).
• Common in plants like soybeans, oats, wheat, and rice.
• Drawback in Dry Environments:
  • Stomata close to reduce water loss, causing O2 buildup.
  • Leads to photorespiration: O2 added to RuBP, producing no sugar or ATP.
  • Inefficient in arid environments.

C4 Photosynthesis

• Adaptation for hot, dry environments in plants like corn and sugarcane.
• Precedes Calvin Cycle by fixing CO2 into a four-carbon compound (OAA) using PEP carboxylase.
• Carbon fixation occurs in mesophyll cells; Calvin cycle occurs in bundle-sheath cells.
• Continues making sugars despite stomata closure, avoiding photorespiration.
• Convergent Evolution:
  • Evolved independently at least 20 times in the grass family.
  • About 3% of land plants use C4 fixation, mostly monocots.

CAM Photosynthesis

• Adaptation for extremely arid environments, found in plants like pineapples and aloe.
• Stomata open at night for CO2 admission; CO2 fixed into a four-carbon compound stored in vacuoles.
• During the day, stomata close, and stored acids release CO2 for the Calvin cycle.
• Convergent Evolution:
  • Evolved in different plant clades.
  • Common in epiphytes (e.g., orchids, bromeliads) and succulents (e.g., cacti).

Summary

• C3 Photosynthesis: Common but not efficient in dry areas.
• C4 Photosynthesis: Carbon fixation and Calvin cycle in different cells.
• CAM Photosynthesis: Processes occur in the same cells at different times.
• Both C4 and CAM pathways balance photosynthesis and water conservation efficiently.