Light Reaction in Photosynthesis

🌈 Key Pigments & Light Absorption

Photosynthesis works best with blue and red light 🌈, but other colors help too! Here’s how:

  • 💚 Chlorophyll a: Main pigment that traps light.
  • 🟢 Accessory pigments (Chlorophyll b, Xanthophylls, Carotenoids):
    • Absorb different light wavelengths
    • Transfer energy to Chlorophyll a
    • Protect Chlorophyll a from damage (photo-oxidation)

⚡ What Happens in Light Reaction?

Light reaction = “Photochemical phase” ✨. It includes:

  • ☀️ Light absorption
  • 💧 Water splitting
  • 💨 Oxygen release
  • 🔋 Making energy carriers: ATP and NADPH

🔬 Photosystems – The Light Factories

Pigments are organized into Light Harvesting Complexes (LHC) inside two structures:

  • Photosystem I (PS I) & Photosystem II (PS II)
    • Named by discovery order, not work order!
  • Each LHC has:
    • Hundreds of pigment molecules + proteins
    • An “antenna” 🌟: All pigments except one Chlorophyll a
    • A reaction centre: Single Chlorophyll a molecule

🔎 Reaction Centres – The Heart of Photosystems

  • PS I → P700 (absorbs 700 nm light)
  • PS II → P680 (absorbs 680 nm light)

🚀 Electron Transport Kick-off!

In PS II:

  1. P680 absorbs red light (680 nm) 🔴
  2. Electrons get excited and “jump”!
  3. An electron acceptor grabs these electrons
  4. Electrons move to an electron transport system

⭐ NEET Must-Knows

  1. Pigment roles: Chlorophyll a (main), accessory pigments (expand light use + protection)
  2. Light reaction outputs: ATP, NADPH, O₂ (from water splitting)
  3. Photosystems: PS I (P700) & PS II (P680) – discovery vs. function order!
  4. Reaction centres: P700 (PS I) and P680 (PS II) wavelengths
  5. Electron transport start: PS II’s P680 absorbs light → electrons excited → transported

Keep shining! You’ve got this 💪🌱