Final answer:
Electrons needed by the water-splitting photosystem in photosynthesis originate from water.
Step-by-step explanation:
The electrons needed by the water-splitting photosystem in photosynthesis originate from water. When sunlight is absorbed by chlorophyll molecules in the photosystems, the energy is transferred to the primary electron acceptor. This energy is then used to excite electrons from the splitting of water, which releases oxygen as a byproduct. The excited electrons are subsequently passed through the chloroplast electron transport chain.