Light-Dependent Reaction

Rucete ✏ Biology In a Nutshell

1. What is the Light-Dependent Reaction?

• The first stage of photosynthesis, occurring in the thylakoid membrane of chloroplasts.
• Uses light energy to generate ATP and NADPH, which are required for the Calvin cycle (light-independent reactions).
• Oxygen (O₂) is released as a byproduct.

2. Noncyclic Photophosphorylation (Main Light-Dependent Pathway)

• Involves both Photosystem I (PSI) and Photosystem II (PSII).
• Produces ATP, NADPH, and O₂.

Process Steps:

1. Photosystem II (PSII Activation)
   • Light energy excites electrons in PSII, causing them to leave the chlorophyll molecule.
2. Primary Electron Acceptor
   • High-energy electrons are transferred to a primary electron acceptor.
3. Electron Transport Chain (ETC)
   • Electrons move through the ETC, releasing energy to pump H⁺ ions into the thylakoid lumen.
4. ATP Formation (Photophosphorylation)
   • The H⁺ gradient powers ATP synthase, converting ADP + Pi → ATP.
5. Photosystem I (PSI Activation)
   • Light excites electrons in PSI, which are replaced by electrons from PSII.
6. NADPH Formation
   • Electrons combine with NADP⁺ + H⁺ to form NADPH.
7. Water Splitting (Photolysis)
   • H₂O is split to replace lost electrons in PSII, producing O₂, electrons, and H⁺ ions.

Overall Reaction:

$$H_2O + ADP + P_i + NADP^+ + light → ATP + NADPH + O_2 + H^+$$

3. Cyclic Photophosphorylation (Alternative Pathway)

• Involves only Photosystem I (PSI).
• Produces ATP only (no NADPH or O₂).
• Electrons cycle back to PSI instead of reducing NADP⁺.
• Used when more ATP is needed without producing extra NADPH.

4. Differences Between Noncyclic & Cyclic Photophosphorylation

Feature	Noncyclic Photophosphorylation	Cyclic Photophosphorylation
Photosystem Involved	PSI & PSII	Only PSI
ATP Production	Yes	Yes
NADPH Production	Yes	No
O₂ Production	Yes	No
Electron Pathway	Electrons end in NADPH	Electrons return to PSI

Light-Dependent Reactions In a nutshell

• PSII = Splits Water, releases O₂.
• ETC = Pumps H⁺, powers ATP Synthase.
• PSI = Generates NADPH.