Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
The Z-scheme depicts photosynthetic electron transport as an energy diagram.

Fig. 23-12 illustrates not only The pathway of electrons passing from H₂O to NADP⁺, but also their energetic relationships. The vertical axis of this diagram represents the energy scale. Processes in which electrons are transferred "uphill" (red arrows) require an input of light energy, whereas processes in which electrons move "downhill" (black arrows) are accompanied by a decrease in Free energy. Upon the absorption of a light quantum by Photosystem I, one of its electrons is promoted from a relatively low-energy state to a high-energy state; As a result, the photochemical reaction center of photosystem I—now in an excited state—becomes a very powerful reductant, driving electrons "downhill" to NADP⁺ and reducing it to NADPH. The high-energy electrons moving from Photosystem II to photosystem I have also gained energy from light quanta, specifically those absorbed by photosystem II. The electron "holes" in photosystem II, which has now become a very powerful oxidant (electron acceptor), are filled by electrons moving "downhill" from H₂O. The Z-scheme thus depicts the pathway by which electrons travel from H₂O to NADP⁺, reducing it to NADPH—that is, moving from a large positive standard potential (+0.82 V) to a large negative one (-0.32 V). The free energy required to transfer a single electron from H₂O to NADP⁺ is supplied by two absorbed light quanta: one absorbed by photosystem I and the other by photosystem II.