Читать книгу Molecular Mechanisms of Photosynthesis - Robert E. Blankenship - Страница 48

The cornerstone of our current understanding of photosynthesis is that it is a light‐induced reduction–oxidation (redox) chemical process. This principle was first clearly set forth in the 1930s by the Dutch microbiologist Cornelis van Niel (1897–1985), working at Stanford University. Van Niel carried out a series of experiments on the metabolic characteristics of non‐oxygen‐evolving (anoxygenic) photosynthetic bacteria (van Niel, 1941). These organisms contain bacteriochlorophylls, pigments related to, but distinct from, the chlorophylls contained in cyanobacteria, algae, and plants. They assimilate CO₂ into organic matter, but do not produce molecular oxygen. In order for these bacteria to assimilate CO₂, they must be supplied with a reducing compound. Many different compounds will suffice, most notably H₂S, which is first oxidized to elemental sulfur and then further oxidized. In place of H₂S, a variety of organic compounds can also be utilized, or even molecular hydrogen. Van Niel's seminal contribution was the recognition that these compounds could all be represented by the general formula H₂A, and that the overall equation of photosynthesis could be reformulated in a more general way as follows:

(3.4)

The oxygen‐evolving form of photosynthesis can then be seen as a special case of this more general formulation, in which H₂O is H₂A and O₂ is 2A. When presented in this manner, the redox nature of photosynthesis is much more obvious. In fact, it is a simple further step to separate the oxidation and reduction into two chemical equations, one for the oxidation and the other for the reduction:

(3.5a)

(3.5b)

This separation into oxidation and reduction reactions leads to a number of important predictions. First, it suggests that the two processes might possibly be physically or temporally separated. Indeed, this was found to be the case by Hill in his classic experiments on the use of artificial electron acceptors, which will be discussed in the next paragraph. A second prediction is that the oxygen produced by oxygen‐evolving photosynthesis comes from H₂O and not from CO₂. This is indeed the case, although it took many years for this fact to be established unequivocally, because of the rapid exchange of oxygen between CO₂ and H₂O via carbonic acid, H₂CO₃, which is constantly forming and breaking down. Van Niel correctly imagined that the oxidation and reduction reactions of Eqs. (3.5a) and (3.5b) were not the primary processes carried out by light, but were, rather, the result of a primary oxidant and reductant generated in the light, which went on to react with the substrates to form the products. However, some of van Niel's detailed ideas about the nature of the primary oxidant and reductant were incorrect, and he was overly rigid in insisting that all assimilated carbon had first to be oxidized all the way to CO₂ before it could be subsequently reduced. Nevertheless, the truth of the basic principle of photosynthesis as a light‐induced redox process was unequivocally established by van Niel's work, along with that of other scientists of the time, notably Hans Gaffron (1902–1979).