Electron Donor:
* Water (H₂O): The primary electron donor in the light reactions is water. When a photon of light strikes a chlorophyll molecule in Photosystem II (PSII), it excites an electron to a higher energy level. This excited electron is then passed along an electron transport chain. To replace the lost electron, water is split, releasing oxygen (O₂) as a byproduct.
Where Electrons End Up:
1. Photosystem II (PSII): Electrons from water are initially passed to PSII, replacing the excited electron that was passed to the electron transport chain.
2. Electron Transport Chain: The excited electrons travel down an electron transport chain, releasing energy along the way. This energy is used to pump protons (H⁺ ions) across the thylakoid membrane, creating a proton gradient.
3. Photosystem I (PSI): Electrons eventually reach Photosystem I (PSI). Here, they are re-energized by another photon of light and passed to a molecule called NADP⁺.
4. NADPH: The electrons combine with NADP⁺ and a proton (H⁺) to form NADPH. NADPH is a powerful reducing agent, meaning it can donate electrons to other molecules. It serves as the primary electron carrier in the Calvin cycle (light-independent reactions) where it is used to reduce carbon dioxide into sugar.
In Summary:
* Water donates electrons to replace those lost by chlorophyll in PSII.
* These electrons travel through an electron transport chain, releasing energy to pump protons.
* The electrons finally reach PSI where they are re-energized and used to reduce NADP⁺ to NADPH.
* NADPH carries these high-energy electrons to the Calvin cycle, where they are used for carbon fixation.
