🌀 a) Non-Cyclic Photophosphorylation (The Usual Way)

This is the main method used by plants to make ATP during light reactions. It is called “non-cyclic” because the electrons do not return to where they started — they go from water to NADP⁺.

This process involves both photosystems:

• Photosystem-II (PS-II)
• Photosystem-I (PS-I)
  And uses two electron transport chains.

Let’s go step by step.

Step 1: 🌞 Absorption of Light by PS-II

• When sunlight hits Photosystem-II (PS-II), its pigments absorb light energy.
• This energy is transferred among pigment molecules until it reaches a special chlorophyll molecule called P680 (named because it absorbs light best at 680 nm).
• P680 becomes excited and releases two high-energy electrons.
• These electrons are passed to the primary electron acceptor.
• This leaves an “electron hole” in P680, which means it is now missing electrons and wants them back.

Step 2: 💧 Photolysis of Water (Splitting of Water)

• To fill the “hole” in P680, water molecules are split.
• This process is called photolysis (photo = light, lysis = splitting).
• The split water produces:
  • 2 electrons (to refill P680)
  • 2 hydrogen ions (H⁺)
  • 1 oxygen atom, which combines with another to form O₂ (oxygen gas)

Example:
2H₂O → 4H⁺ + 4e⁻ + O₂
This is where plants release oxygen — it comes from water, not carbon dioxide!

Step 3: ⚡ Electron Flow from PS-II to PS-I

• The excited electrons from PS-II now travel through an electron transport chain (ETC) — like a conveyor belt.
• This chain includes:
  • Plastoquinone (PQ)
  • Cytochrome complex
  • Plastocyanin (PC)
• As the electrons move through this chain, they lose energy step by step.
• This lost energy is used to pump H⁺ ions into the thylakoid space, creating a proton gradient.
• The cell uses this gradient to make ATP, using an enzyme called ATP synthase. This whole process is called chemiosmosis.