Where does Photo System I get the electrons from?
The plastocyanin protein in the electron transport chain after Photosystem II.
Photosystem I is really the second photosystem. When light photons excite the pigments in the light-harvesting complexes of the photosystem, their electrons get excited. This energy is passed along from pigment molecule to pigment molecule until it reaches a special pair of chlorophyll molecules which instead of transferring their energy, transfer their electrons to the "final electron acceptor."
These two special chlorophyll molecules, called P700 for Photosystem I, are then electron deficient. The electrons asked about in the questions are the ones that now come to replenish the P700 pigments to the cycle can be repeated.
These electrons come from the process the directly proceeds Photosystem I, which is the electron transport chain.
Recall the "road map" for the light reactions:
Visualizing the structures (here, we cant to focus on the electron transport chain after Photosystem II, which goes through the proteins plastoquinone, the cytochrome complex, and plastocyanin):
We see that the general process goes:
- Light excitation of Photosystem II
- Electron transfer from P680 to electron acceptor
- Travel of electrons down electron transport chain (creating ATP)
- Simultaneously, the excitation of pigments in Photosystem I
- Transfer of these electrons to P700
The specific protein that is part of the electron transport chain that comes directly before the passing-on of electrons to Photosystem I is the small protein plastocyanin.