Step 4: 🌞 Absorption of Light by PS-I

• Now, light hits Photosystem-I (PS-I).
• Light energy is passed to its central chlorophyll molecule called P700 (absorbs light best at 700 nm).
• P700 gets excited and releases its electrons to its own primary electron acceptor.
• The electrons that came from PS-II now fill the electron hole in P700.

Step 5: 🔄 Electron Flow from PS-I to NADP⁺

• The excited electrons from PS-I are passed down a second electron transport chain.
• This chain includes a molecule called ferredoxin (FD).
• An enzyme called NADP⁺ reductase transfers the electrons from ferredoxin to NADP⁺.
• With the addition of a hydrogen ion (H⁺), NADP⁺ becomes NADPH, a molecule full of energy.

Summary of Non-Cyclic Photophosphorylation:

• Uses both PS-II and PS-I
• Produces:
  • ATP
  • NADPH
  • Oxygen gas (O₂)
• Electrons move in one direction — they do not return to the starting point.

🌀 This whole journey of electrons makes a zigzag path, called the Z-scheme.

🔁 b) Cyclic Photophosphorylation (The Backup Method)

Sometimes, the plant only needs ATP but not NADPH. In such cases, it uses a shortcut called cyclic photophosphorylation.

🌿 What Happens Here?

• Only PS-I is used, PS-II is not involved.
• Electrons from P700 of PS-I get excited and go to the primary acceptor.
• Instead of going to NADP⁺, they go back to the electron transport chain.
• This cycle repeats, so the same electrons keep moving in a circle.

✨ Key Features:

• Only ATP is produced
• No NADPH is made
• No water is split, so no oxygen is released
• This is a cyclic pathway — electrons return to their source

🕒 When Does This Happen?

Cyclic photophosphorylation usually happens when:

• The Calvin cycle slows down (due to lack of CO₂ or some other reason)
• NADPH accumulates because it is not being used
• The plant still needs ATP to carry out some internal processes

So the plant chooses to make more ATP using the cyclic method.

🌞 Summary of Light-Dependent Reactions:

Process	PS Used	ATP	NADPH	O₂
Non-cyclic photophosphorylation	PS-II and PS-I	✅	✅	✅
Cyclic photophosphorylation	Only PS-I	✅	❌	❌

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🧠 Final Explanation in Simple Words:

Light-dependent reactions are like a solar power station inside the leaf. Light hits the leaf, and special pigment molecules use it to excite electrons. These electrons move through special chains and help to create ATP and NADPH, which are like batteries full of energy. This energy will be used later to make food (glucose). In this process, oxygen is released as a byproduct — the oxygen we breathe!