C4 Photosynthesis vs C3 Photosynthesis

Rucete ✏ Biology In a Nutshell

1. C3 Photosynthesis (Typical Pathway)

• Occurs in: Mesophyll cells
• Carbon fixation enzyme: Rubisco
• First product: 3-carbon compound (3-phosphoglycerate)
• Problem: When CO₂ levels are low, Rubisco binds O₂ instead of CO₂, leading to photorespiration, which wastes energy and reduces efficiency.

2. C4 Photosynthesis (Adapted Pathway)

• Occurs in two types of cells:
  • Mesophyll cells: CO₂ is initially fixed
  • Bundle sheath cells: Calvin cycle takes place
• Carbon fixation enzyme: PEP Carboxylase
  • Has higher affinity for CO₂ and does not bind O₂, minimizing photorespiration.
• First product: 4-carbon compound (Oxaloacetate)
  • Transported to bundle sheath cells → releases CO₂ → enters Calvin cycle
• Adaptation Purpose:
  • Reduces photorespiration
  • Conserves water
  • More efficient in hot, dry environments

3. Key Differences Between C3 and C4 Plants

Feature	C3 Plants	C4 Plants
Location of Calvin Cycle	Mesophyll cells	Bundle sheath cells
CO₂ Fixation Enzyme	Rubisco	PEP Carboxylase (initial)
Photorespiration	High	Minimal
Efficiency in Heat/Drought	Low	High
Common Plants	Most plants, rice, wheat	Corn, sugarcane, sorghum
First CO₂ Fixation Product	3-carbon compound	4-carbon compound

In a nutshell

C3 is Common, C4 Conserves!

• C3: Common pathway, less efficient in heat
• C4: Conserves water and reduces photorespiration