Calvin Cycle and Challenges

Introduction to Calvin Cycle

• Involves an enzyme known as RuBisCo (Ribulose bisphosphate carboxylase oxygenase)
• RuBisCo facilitates the bonding and reaction of molecules in the Calvin Cycle
• Standard reaction involves:
  • Carbon dioxide
  • Ribulose-1,5-bisphosphate (RuBP)
  • Produces 3-phosphoglycerate (3-PGA) which converts to phosphoglyceraldehydes (PGALs)

Production Cycle

• For every 6 PGALs produced:
  • 5 are recycled back to form RuBP
  • 1 PGAL is used to create carbohydrates

Problem: Photorespiration

• RuBisCo can mistakenly fix oxygen instead of carbon dioxide
• This leads to the production of phosphoglycolates instead of 3-PGA
• Results in inefficiency, using ATP and NADH without producing carbohydrates

Solutions to Photorespiration

C-4 Photosynthesis

• Evolved in some plants to bypass photorespiration
• Involves a different initial carbon fixation process
• Carbon dioxide reacts with phosphoenolpyruvate (PEP) in mesophyll cells
  • Facilitated by PEP carboxylase (only fixes carbon dioxide, not oxygen)
  • Produces a 4-carbon molecule (oxaloacetate)
• Oxaloacetate converts to malate or aspartate
• Malate is transported to bundle-sheath cells (deeper in the leaf)

C-4 Photosynthesis Process

1. Mesophyll Cell

   • Air containing CO2 and O2 enters
   • CO2 reacts with PEP via PEP carboxylase
   • Produces oxaloacetate, then malate

2. Bundle-Sheath Cells

   • Malate transported via plasmodesmata
   • Malate converts back to CO2 (isolated from O2)
   • CO2 fixed in the Calvin Cycle to produce sugars

Benefits of C-4 Photosynthesis

• Calvin Cycle occurs in an environment with only CO2
• Avoids the wasteful process of photorespiration
• More efficient in producing carbohydrates in high oxygen environments

Summary

• C-4 Photosynthesis optimizes carbon fixation by separating initial CO2 fixation and Calvin Cycle spatially
• Ensures high efficiency through selective fixation of CO2