C4 Pathway and Photorespiration

🌱 Calvin Cycle Recap

To make one glucose molecule via the Calvin cycle:

  • 🌿 Requires 6 turns of the cycle.
  • 🔋 Uses 18 ATP and 12 NADPH.
  • 💨 Takes in six CO2 molecules.
  • 🔄 Outputs: Glucose, 18 ADP, and 12 NADP.

Cyclic phosphorylation helps balance ATP/NADPH needs!

🌞 What Makes C4 Plants Special?

C4 plants (like maize/sorghum) grow in dry tropical regions and differ from C3 plants because they:

  • ☀️ Tolerate higher temperatures.
  • 💡 Handle high light intensities better.
  • 🌬️ Don’t do photorespiration (saves energy!).
  • 📈 Produce more biomass (higher yields!).

🍃 Kranz Anatomy (The “Wreath” Structure)

C4 leaves have unique Kranz anatomy:

  • 🌾 Bundle sheath cells wrap around leaf veins like a wreath.
  • 🔍 These cells have:
    • Lots of chloroplasts 🌿.
    • Thick walls (block gas exchange).
    • No gaps between cells.
  • 🔬 Tip: Cut a maize leaf slice to see this under a microscope!

🚀 The C4 Pathway (Hatch & Slack Pathway)

A 2-step CO2 fixation process:

  1. In Mesophyll Cells:
    • CO2 + PEP (3-carbon) → OAA (4-carbon acid).
    • Enzyme: PEP carboxylase (no Rubisco here!).
    • OAA becomes malic/aspartic acid → sent to bundle sheath.
  2. In Bundle Sheath Cells:
    • C4 acids break down → release CO2 + 3-carbon molecule.
    • CO2 enters Calvin cycle (using Rubisco here!).
    • 3-carbon molecule returns to mesophyll → becomes PEP again.
Key Equation: \(\text{PEP} + \text{CO}_2 \xrightarrow{\text{PEP carboxylase}} \text{OAA}\)

💨 Photorespiration (The “Oops” Reaction)

Only in C3 plants! Rubisco binds O2 instead of CO2 when O2 is high:

  • ⚠️ RuBP + O2 → Phosphoglycolate (2C) + PGA.
  • 🔥 Wastes energy: Releases CO2 + uses ATP (no sugar made!).
  • ❓ Why it happens? Rubisco likes CO2 more, but O2 competes!

C4 plants avoid this by pumping CO2 into bundle sheath cells, making Rubisco use CO2 only! 🛡️

📊 C3 vs. C4 Plants: Quick Compare

FeatureC3 PlantsC4 Plants
CO2 FixationDirect Calvin cycle2-step (PEP → Calvin)
PhotorespirationYes (loses energy)No (energy efficient)
Leaf AnatomyNo KranzKranz anatomy
EnzymesRubisco everywherePEPcase (mesophyll)
Rubisco (bundle sheath)

🎯 NEET Important Concepts

These ideas are frequently tested!

  1. Kranz Anatomy 🌿: Bundle sheath cells + features (thick walls, no intercellular spaces).
  2. Photorespiration 💨: Why it happens in C3 plants (Rubisco binds O2), and why C4 plants avoid it.
  3. Hatch-Slack Pathway 🔄: Steps, key enzymes (PEPcase in mesophyll, Rubisco in bundle sheath), and compounds (PEP, OAA).
  4. ATP/NADPH in Calvin Cycle 🔋: 18 ATP + 12 NADPH needed for 1 glucose molecule.
  5. C4 Advantages 🏆: No photorespiration, high temperature tolerance, better productivity.

Keep exploring plants around you — try spotting Kranz anatomy in your garden! 🌻