Summary of The Light Reactions of Photosynthesis

Photosynthesis Overview

Consists of two main stages: light reactions and Calvin cycle reactions. Light reactions occur in the thylakoid membranes of chloroplasts.

Photosystems:
- Two photosystems involved: Photosystem I (PSI) and Photosystem II (PSII).
- Named based on the order of discovery, not their order of operation.
Photon Absorption:
- PSII absorbs photons, energizing electrons in the chlorophyll reaction center.
Electron Transport Chain:
- Energized electrons from PSII move to an electron acceptor and down the Electron Transport Chain.
- Water is split to replace lost electrons, releasing oxygen, hydrogen ions, and electrons.
Hydrogen Ion Gradient:
- Energy released from electrons moving through the chain is used to pump hydrogen ions into the thylakoid space, creating a gradient.
ATP Production:
- The gradient drives hydrogen ions through ATP synthase, converting ADP and inorganic phosphate into ATP.
NADPH Production:
- Electrons from PSI, after being energized by light, move down a second Electron Transport Chain.
- These electrons, along with hydrogen, reduce NADP to NADPH.

Oxygen gas is released as a byproduct of water splitting.

Both are essential for the light reactions to occur; without them, Photosynthesis halts, leading to plant death.