During photosynthesis, which statement describes the first step of the electron flow?

• A. Electrons are excited first in PSI before PSII.
• B. Water donates electrons to PSII, initiating electron flow.
• C. NADP+ is the first electron donor.
• D. The flow begins with ATP synthase.

Answer: (B)

The flow of electrons starts when water is split at PSII, supplying electrons to PSII and kick-starting the electron transport chain. Light energy excites the chlorophyll in PSII, but without a source of electrons to replace those lost by PSII, the chain can’t continue. Water donates electrons to PSII through the oxygen-evolving complex, and those electrons travel through the chain (to plastoquinone, cytochrome b6f, plastocyanin, and then to PSI and ultimately NADP+). NADP+ later accepts electrons to form NADPH, and ATP synthase uses the proton gradient generated by this flow to make ATP. So water donating electrons to PSII is the initiating step of the electron flow.