Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Electron transfer from H2O to NADP+ occurs as a result of the interaction between photosystems I and II.

The array of light-harvesting, or antenna, pigments along with its reaction center—which supplies high-energy electrons for the reduction of NADP⁺—constitutes Photosystem I, maximally excited by light at a wavelength of 700 nm. It has been found, however, that the maximum rate of oxygen evolution is achieved only when METABOLISM/14.html">Chloroplasts absorb not only 700 nm light, but also shorter-wavelength light, such as 600 nm. Conversely, illumination with 700 nm light alone (without 600 nm light) results in a sharp drop in The rate of oxygen evolution, a phenomenon known as the red drop because 700 nm marks the far edge of the red spectrum. These observations led to the Conclusion that there are Two Photosystems with distinct absorption maxima, and that they function cooperatively in the light reactions of plant Photosynthesis responsible for oxygen evolution. Fig. 23-12 presents a scheme (commonly referred to as the Z-scheme due to its zigzag shape) that illustrates The pathway of electron flow between photosystems I and II, as well as the energetic relationships between these two photosystems during the light reactions.

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Fig. 23-12- Cooperative action of photosystems I and II. This zigzag scheme (Z-scheme) illustrates the pathway of electron flow from H₂O (bottom right) to NADP⁺ (top left) in noncyclic plant photosynthesis, along with the underlying energetic relationships. For electrons originating from H₂O molecules to reach an energy level sufficient to reduce NADP to NADPH, each electron must be "boosted upward" twice (bold pink arrows) by photons absorbed by photosystem I and Photosystem II. In each photosystem, one quantum, or photon, is expended per boosted electron. Following each such boost, the high-energy electrons cascade "downward" (bold gray arrows) along the indicated pathways. Photophosphorylation of ADP to ATP is coupled to electron flow through the chain connecting photosystem II to photosystem I (see Section 23.14). The dashed black arrow between P430 and cytochrome b denotes an alternative, or bypass, pathway taken by electrons in cyclic electron flow and phosphorylation (see text and Fig. 23-14). Only photosystem I participates in cyclic electron flow; electrons return via the bypass route to photosystem I rather than reducing NADP to NADPH.

Let us first trace the pathway of electron flow. Light serves as the driving force for this process. When light quanta are absorbed by photosystem I, energy-rich electrons are ejected from the reaction center and transferred via a chain of electron carriers to NADP⁺, reducing it to NADPH. This process creates an electron vacancy, or "hole," in photosystem I. The vacancy is subsequently filled by an electron ejected from photosystem II upon its illumination, which reaches photosystem I via the carrier chain linking photosystem II to photosystem I. However, this creates a corresponding vacancy in photosystem II, which in turn is filled by an electron supplied by H₂O. Upon splitting, a Water molecule yields: (1) electrons that fill the vacancies in photosystem II, (2) H⁺ ions released into the medium, and (3) molecular oxygen evolved into the gas phase. The equation for water Cleavage is expressed as

2Н₂О → 4Н⁺ + 4е^- + О₂.

The Z-scheme thus describes the complete pathway by which electrons travel from H₂O to NADP⁺ in accordance with the equation

For every electron transferred from H₂O to NADP⁺, two quanta of light are absorbed—one by each photosystem. To generate a single O₂ molecule, four electrons must be transferred from H₂O to NADP⁺, meaning that eight quanta must be absorbed in total, with four going to each photosystem.