Final answer:
The missing electron in photosystem II's reaction center is replenished by splitting water molecules, which provides new low-energy electrons to the reaction center.
Step-by-step explanation:
When an electron is removed from the reaction center of photosystem II (PSII), it is replenished by electrons obtained from the splitting of water. The reaction center of PSII, known as P680, loses its high-energy electron to the primary electron acceptor, which then passes through the electron transport chain. This missing electron in P680 is replaced by extracting a low-energy electron from a water molecule. During this process, water is split into oxygen, hydrogen ions, and electrons. These electrons are then used to re-reduce PSII, and the oxygen is released as a byproduct into the atmosphere.