Photosynthesis

Abstract

PHOTOCHEMICAL reactions, more particularly those in which highly endothermic syntheses take place, have generally been considered as something apart from the ordinary chemical reactions of the laboratory, and, indeed, have at times savoured of the mysterious for the reason that they seemed to be impossible of realisation in vitro. Recent work, however, on the energy changes involved in chemical reaction has shown that there is no inherent mystery in photosynthesis, and that all reactions, including those of photochemistry and catalysis, are completely analogous and obey the same laws. Every complete reaction consists of three separate stages, with each of which is associated its characteristic energy change. In general, molecules in the free state exist in a phase which is non-reactive, and in order to carry out any reaction it is first of all necessary to bring them into a reactive phase. This, which is the first stage of the reaction, requires that a definite amount of energy should be supplied to each molecule, the amount necessary being the difference in energy contents of the initial phase and the particular phase necessary for the reaction in question. Each phase of a given molecule differs in energy content by a fixed quantity of energy characteristic of that molecule, which is called the molecular quantum of energy. It follows, therefore, that the amount of energy necessary to activate each molecule in the first stage of the reaction is exactly one or more molecular quanta.