Photosystem II (PSII)
* Function: Captures light energy and uses it to split water molecules, releasing electrons, protons (H+), and oxygen.
* Process:
* Light energy is absorbed by chlorophyll molecules in the reaction center of PSII.
* This energy excites an electron, which is then passed along an electron transport chain.
* The electron's energy is used to pump protons from the stroma into the thylakoid lumen, creating a proton gradient.
* Water molecules are split by an enzyme called oxygen-evolving complex, releasing oxygen as a byproduct.
Photosystem I (PSI)
* Function: Uses light energy to further energize electrons and produce NADPH, a reducing agent used in the Calvin cycle.
* Process:
* Light energy is absorbed by chlorophyll molecules in the reaction center of PSI.
* This energy excites an electron, which is then passed along another electron transport chain.
* The electron's energy is used to reduce NADP+ to NADPH.
Key Differences between PSI and PSII:
| Feature | Photosystem II (PSII) | Photosystem I (PSI) |
|---|---|---|
| Electron donor | Water | Plastocyanin (PC) |
| Electron acceptor | Plastoquinone (PQ) | Ferredoxin (Fd) |
| Final product | Oxygen (O2) | NADPH |
| Wavelength of light absorbed | 680 nm | 700 nm |
Relationship between PSI and PSII:
* The electrons released from PSII are passed along an electron transport chain to PSI.
* The proton gradient created by PSII drives ATP synthesis through ATP synthase, using the energy stored in the gradient.
* The NADPH produced by PSI is used in the Calvin cycle to fix carbon dioxide into sugars.
In summary:
Photosystem II captures light energy to split water, releasing electrons, protons, and oxygen. These electrons are then passed to photosystem I, which uses light energy to further energize them and produce NADPH. The proton gradient created by PSII powers ATP synthesis. Both ATP and NADPH are essential for the Calvin cycle, which converts carbon dioxide into sugars.
