Final answer:
In photosystems, electrons in the reaction center are excited by sunlight and passed to an acceptor molecule. Photosystem II replaces these electrons by splitting water, while Photosystem I receives them from the electron transport chain.
Step-by-step explanation:
Regarding the concept of photosystems and their operation within the process of photosynthesis, it is true that electrons in the reaction center molecule are excited by absorbed solar energy and are subsequently passed along to an acceptor molecule. The energy for this excitation comes from light absorbed by various pigments, including chlorophyll a and chlorophyll b, within the light-harvesting complexes of the photosystems. These pigments pass the absorbed light energy to two special chlorophyll a molecules in the reaction center.
Within photosystem II (PS II), the excited electron that is passed to the primary electron acceptor must then be replaced. This is achieved through the splitting of water molecules, which also releases oxygen as a byproduct. However, in photosystem I (PS I), the replaced electron is provided by the chloroplast electron transport chain. The energy is then utilized in the electron transport chain to create a proton gradient across the thylakoid membrane, which ultimately helps in the production of ATP and powers the Calvin cycle to produce glucose.