Primary Quantum Conversion in Photosynthesis

Abstract

One of the outstanding features, possibly a unique feature, of the process of photosynthesis as it occurs in nature today is the ability of the organism, either green plant or bacterium, to utilize a quantum of energy of the order of 38,000 calories for green plants and 40,000 for the bacteria to accomplish an ordered chemical transformation at room temperature with a relatively high degree of efficiency. The apparatus which accomplishes this, we must remember, is of labile organic construction, and the thermal reactions which can be performed by such a system rarely, if ever, involve energy changes higher than 10,000 or 15,000 calories; thus, manipulation of a package of energy two or three times that size without damage to the apparatus and in a highly directed and specific way is an impressive accom-.plishment indeed.The ultimate products of this energy transformation have long been known to us, in the form primarily of carbohydrate and oxygen but of course including all of the plant substances. In fact, it is currently possible to describe some more immediate products of this energy conversion process in terms of more transient, specific energy-storing materials. We have every reason to believe that two such energy-storing intermediates which can be used to produce the final, or more long-term, 23 NOVEMBER 1962 storage materials are reduced pyridine nucleotide and adenosine triphosphate. It may turn out that other transient energy-bearing chemical intermediates may be still closer to the energy transformation step itself. At this point it is perhaps worth while to define, as nearly as we can, the properties of the energy-bearing intermediate or intermediates which we consider to be the earliest form of chemical energy into which the electromagnetic quantum may be converted. Such a material would be the first chemically definable compound in thermal equilibrium with its environment, but, quite clearly, not in chemical equilibrium with it since, under such a definition, oxygen itself could not be evolved.We will consider the primary quantum conversion act, then, as that act, or sequence of events, that follows the absorption of the electromagnetic quantum and terminates with the appearance of the thermally relaxed, chemically defined individual which may then proceed, by direct thermal work-performing reactions, to produce the next transient and, finally, the ultimate products of photosynthesis.Under such a definition, an electronically excited state of a molecule, or array of molecules, such as might result from the primary absorption of a quantum of light would not be considered a chemical entity distinct from the parent material before light absorption. Only after the energy stored in this electronically excited state had been transformed into new chemical species which could then proceed to react, or interact, with their environment in accordance with thermodynamic principles would we consider the quantum conversion process accomplished. All succeeding reactions from these initial chemical species would...

The oxidation-reduction potential of the reaction-centre chlorophyll (P700) in Photosystem I. Evidence for multiple components in electron-paramagnetic-resonance signal 1 at low temperature

“…This is different from the situation at room temperature, where there is very good evidence that only one free radical induced by illumination or chemical oxidation is observed, which is due to the reaction centre (Warden & Bolton, 1973;Calvin & Androes, 1964). This difference is presumably due to differences in the saturation characteristics of the signals.…”

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Biological Components and Bioelectronic Interfaces of Water Splitting Photoelectrodes for Solar Hydrogen Production

“…[4] In a very recent paper, artificial photosynthesis was identified as a route to meet the objectives of the Strategic Energy Technology-Plan for the European Union, but the need for scientific breakthroughs was emphasized. [5] In pioneering papers of the late 50s and early 60s, Melvin Calvin [6][7][8] demonstrated how photosynthetic components from shredded plants such as spinach can be sublimated over interdigital electrodes and provide a bioelectronic interface where the functionalities of proteins can be directly measured with electric signals upon light exposure ( Figure 1). Calvin was one of the first who rationalized the photosynthetic apparatus as a molecular machine, or even as a molecular industrial complex.…”

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