Predict how an inhibitor of electron passage through pheophytin would affect electron flow through each of the two photosystems. Photosystem II Electron flow would not change. Electron flow would probably slow down but not stop. Electron flow would stop. Electron flow would speed up. Photosystem I Electron flow would speed up. Electron flow would probably slow down but not stop. Electron flow would stop. Electron flow would not change.

Photosystem II electron flow would stop.

Photosystem I electron flow would probably slow down but not stop.

Explanation:

In photosystem II, electron flow is exclusively dependent on pheophytin which transfers electron from the excited reaction-center P680, to cytochrome b₆f complex and then the electrons are transferred to photosystem I via plastocyanin. Any inhibitor of electron passage through pheophytin will thus, ultimately stop electron flow through Photosystem II.

However, in the case of Photosystem I, even if electron flow through pheophytin is inhibited, it has an alternate pathway for electron flow known as the cyclic electron flow. In cyclic electron flow, electrons passing through its reaction-center P700 does not continue to NADP⁺, but is recycled back through cytochrome b₆f complex to plastocyanin and back to P700. Thus, electrons can continue flowing but at a probably slower rate since no new electrons are coming from Photosystem II.