Light-Dependent Reactions Quiz

Study Notes

Location: Thylakoid membranes of chloroplasts
Photolysis of Water: Water is split into oxygen, protons, and electrons.
- Equation: 2H₂O → 4H⁺ + 4e⁻ + O₂
- Provides electrons to replace those lost by chlorophyll
Photosystem II (PSII):
- Light energy excites electrons in chlorophyll.
- Excited electrons move along the electron transport chain (ETC) to Photosystem I (PSI).
Electron Transport Chain (ETC):
- Electrons pass through protein complexes in the thylakoid membrane.
- Energy released during this transfer pumps protons from the stroma into the thylakoid space.
- Creates a proton gradient
Photosystem I (PSI):
- Light energy re-excites electrons in chlorophyll.
- Excited electrons are transferred to NADP⁺, along with protons from the stroma, to form reduced NADP (NADPH).
Chemiosmosis and ATP Synthesis:
- The proton gradient drives ATP synthesis via ATP synthase.
- Protons flow back into the stroma through ATP synthase, providing energy to convert ADP + Pi into ATP
- This process is called photophosphorylation.
End Products:
- ATP and NADPH are produced and used in the light-independent reaction (Calvin cycle).
- Oxygen is released as a by-product.

Non-cyclic Photophosphorylation: Involves both PSII and PSI, producing ATP, NADPH, and oxygen.
Cyclic Photophosphorylation: Involves only PSI, producing ATP but no NADPH or oxygen.