1. Photosystem II (PSII):
* Light Absorption: Light energy is absorbed by chlorophyll molecules within PSII.
* Excitation: The absorbed energy excites an electron to a higher energy level.
* Electron Transfer: The excited electron is passed to an electron acceptor molecule within PSII.
2. Electron Transport Chain:
* Movement Downhill: The electron travels through a series of electron carrier molecules (like plastoquinone, cytochrome b6f complex, and plastocyanin). These carriers are arranged in order of decreasing energy levels, so the electron "falls" down the chain.
* Energy Release: As the electron moves, it releases energy. This energy is used to:
* Pump Protons: Move protons (H+) from the stroma into the thylakoid lumen, creating a proton gradient across the thylakoid membrane.
* Generate ATP: The proton gradient drives ATP synthase, which produces ATP (the energy currency of the cell).
3. Photosystem I (PSI):
* Light Absorption: PSII is excited again by light energy.
* Electron Transfer: The excited electron is passed to another electron acceptor molecule.
* NADPH Production: The electron travels through a short chain of electron carriers, ultimately reducing NADP+ to NADPH. NADPH is a reducing agent (electron donor) that will be used in the Calvin cycle.
4. Water Splitting:
* Replacing Electrons: To replenish the electrons lost from PSII, water molecules are split. This releases electrons, protons (H+), and oxygen gas.
Summary:
* The path of electrons begins at PSII, where they are excited by light and move through a series of carriers, releasing energy to pump protons and generate ATP.
* The electrons then reach PSI, where they are excited again and used to reduce NADP+ to NADPH.
* The electrons lost from PSII are replaced by electrons from the splitting of water.
Overall, the light-dependent reactions involve:
* Light energy absorption and conversion to chemical energy (ATP and NADPH).
* The release of oxygen as a byproduct.
* The creation of a proton gradient used to generate ATP.
This energy stored in ATP and NADPH will be used in the Calvin cycle (light-independent reactions) to fix carbon dioxide and produce sugars.
