The model illustrating the electron transport pathway from water to NADP+ in photosynthesis is the Z-scheme. Its name comes from the zigzag-like pattern made by electron energy levels in a redox diagram.
In oxygenic photosynthesis, electrons begin with water at photosystem II. Water is split, releasing oxygen, protons, and electrons. The electrons pass through carriers including plastoquinone, cytochrome b6f, and plastocyanin before reaching photosystem I. A second photon then re-energizes them, allowing ferredoxin and ferredoxin–NADP+ reductase to reduce NADP+ to NADPH.
The Z-scheme is a model of electron flow, not a separate reaction cycle. It is often confused with the Calvin cycle, which uses ATP and NADPH to fix carbon dioxide, or with the Hill reaction, a related experimental concept involving oxygen evolution by isolated chloroplasts. Cyclic electron flow can return electrons to the transport chain and produce additional ATP, but it does not reduce NADP+ to NADPH.