Biochemistry, Vol. 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
ADP phosphorylation is coupled with photosynthetic electron transport

So, we now know how photosynthetic electron transport from H₂O to NADP⁺ generates one of the two high-energy products of the light reactions, namely NADPH. But what about the second high-energy product—namely, ATP?

In 1954, Daniel Arnon and his colleagues at the University of California, Berkeley, discovered that illumination of spinach METABOLISM/14.html">Chloroplasts results in the synthesis of ATP from ADP and phosphate during photosynthetic electron transport. Simultaneously and independently, Albert Frenkel at the University of Minnesota observed ATP Synthesis in illuminated chromatophores (membrane-bound pigment-containing structures) isolated from photosynthetic Bacteria. Both research groups concluded that a portion of the light energy captured by the Photosynthetic Systems of these organisms is transformed into the energy of the ATP phosphate bond. This process came to be known as photosynthetic phosphorylation, or Photophosphorylation, to distinguish it from Oxidative Phosphorylation taking place in respiring Cell/35.html">Mitochondria.

Recall (Section 17.13) that the phosphorylation of ADP to ATP in mitochondria is driven by the Free energy released when high-energy electrons travel "down" an Electron Transport Chain from a substrate to oxygen. Similarly, the photophosphorylation of ADP to ATP is coupled to electron transport; in this case, energy is released as high-energy electrons move "down" the photosynthetic electron transport chain from excited Photosystem II to the electron holes of Photosystem I.

Most evidence indicates that for every pair of electrons transported along the chain connecting Photosystems II and I, one molecule of ATP is produced. Some researchers, however, suggest that two, rather than one, ATP molecules are formed per electron pair.