Final answer:
The electrons lost from the reaction center of photosystem II are replaced by electrons from water. This occurs when water is split by the energy absorbed from sunlight, releasing electrons, hydrogen ions, and oxygen.
Step-by-step explanation:
The electrons lost from the reaction center of photosystem II (PSII) are replaced by electrons from water (H₂O). Photosystem II uses energy from sunlight to extract electrons from water. When water is split, it releases two electrons, two hydrogen ions (H+), and one oxygen atom. Splitting two water molecules is necessary to form one molecule of diatomic oxygen gas (O₂), which is then released as a waste product.
The reaction center of PSII, known as P680, transfers its high-energy electrons, one at a time, to the primary electron acceptor, and through the electron transport chain to photosystem I (PSI). This process is vital for the photosynthetic electron transport chain to function and eventually leads to the production of NADPH and ATP, which are essential for the Calvin cycle to synthesize carbohydrates from carbon dioxide (CO₂).