The two-step electron transport scheme in photosynthesis is called the Z-scheme. It describes how electrons are energized twice—once in photosystem II and again in photosystem I—while moving from water toward NADP+.
In linear electron flow, photosystem II extracts electrons from water, releasing oxygen and protons. The electrons then pass through carriers including plastoquinone, cytochrome b6f, and plastocyanin before reaching photosystem I. Light re-energizes them there, allowing ferredoxin and ferredoxin-NADP+ reductase to produce NADPH.
The scheme is named for the characteristic shape of its redox-energy diagram: electron energy rises at each photosystem and falls as electrons pass through intermediate carriers. Robert Emerson’s 1957 “enhancement effect,” in which two wavelengths of light worked together more effectively than either alone, helped establish the existence of two cooperating light reactions. Robin Hill and Fay Bendall presented the influential Z-scheme model in 1960.
The Z-scheme should not be confused with the Q cycle, which is a specific mechanism within cytochrome b6f, or with cyclic electron flow around photosystem I.