344 



NATURE 



[March i6, 1922 



Photosynthesis. 



By Prof. E. C. C. Baly, F.R.S. 



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 im- 

 possible of realisation in vitro. Recent work, however, 

 on the energy changes involved in chemical reaction has 

 shown that there is no inherent mystery in photosyn- 

 thesis, and that all reactions, including those of photo- 

 chemistry and catalysis, are completely analogous and 

 obey the same laws. Every complete reaction consists 

 of three separate stages, with each of which is associ- 

 ated 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 re- 

 action 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. 



The second stage of the reaction is the atomic re- 

 arrangement whereby new molecules are produced, and 

 it is this stage, and this stage alone, which is represented 

 by the equation of the reaction. 



The third and final stage is the change in phase of 

 the newly synthesised molecules, whereby they pass 

 into their normal and non-reactive phases. These 

 last two stages are both accompanied by an escape of 

 energy, and in each of them the amount of energy lost 

 per molecule is exactly one or more molecular quanta 

 characteristic of the new molecules. If the sum of the 

 amounts of energy evolved in the second and third 

 stages is greater than that absorbed in the first stage, 

 the reaction is exothermic ; whilst an endothermic re- 

 action is one in which the energy necessary for the first 

 stage is greater than the total amount evolved in the 

 second and third stages. 



There are three methods by which the energy 

 necessary for the first stage may be suppHed. It may 

 be supplied by a material catalyst, or as radiant energy 

 in the form of heat or light. The action of a catalyst 

 does not arise here, and need only be mentioned in 

 order to guard against any misconception. Many re- 

 actions take place in solution without the apparent 

 intervention of the first stage, but in such cases the 

 molecules have been activated by the solvent which 

 functions as a catalyst. 



In general, it is a matter of little consequence 

 whether a molecule is activated by heat or light — 

 that is, by infra-red or ultra-violet rays — in view of 

 the known integral relationships that exist between 

 the frequencies at which a molecule can absorb energy. 

 It is a matter of cardinal importance, however, in the 



NO. 2733, VOL. 109] 



case of highly endothermic reactions, in which the 

 increment of energy required for the initial phase 

 change is obviously a large number of molecular quanta. 

 When a molecule absorbs energy at its principal fre- 

 quency in the infra-red, it absorbs it in terms of its 

 molecular quantum ; but if it absorbs ultra-violet light 

 the unit of energy absorbed is a quantum which is an 

 integral multiple of the molecular quantum, the multiple 

 depending on the phase in which the molecule exists. 

 One single quantum of energy absorbed at the char- 

 acteristic frequency in the ultra-violet is always suffi- 

 cient to activate a single molecule for any reaction, 

 however endothermic this may be. 



An endothermic reaction, in the first stage of which 

 each molecule requires a large number of molecular 

 quanta to activate it, will obviously be very much easier 

 to carry out by exposing the molecules to energy of their 

 characteristic frequency in the ultra-violet, when the 

 absorption of one quantum per molecule is suflScient, 

 than by exposing them to infra-red radiation, when 

 the reaction will not proceed until a specific number 

 of quanta have been absorbed by each molecule. 

 When, as is frequently the case, this specific number 

 is ten or more, it is not surprising that the realisation 

 of the reaction by means of heat becomes impossible 

 from the practical point of view. Such a reaction, 

 however, is readily brought about by the absorption of . 

 a single quantum by each molecule at its characteristic 

 frequency in the ultra-violet. 



This may be understood more clearly from a specific 

 instance, namely, the decomposition of hydrogen 

 chloride into hydrogen and chlorine. . The molecular 

 quantum of HCl is about 5-7 xio"^^ erg, whilst the 

 quantum absorbed at the ultra-violet frequency is 

 about 9-7 X 10 "12 gj-g^ which is seventeen times as large. 

 The activation of an HCl molecule so that it may 

 decompose requires seventeen molecular quanta, and 

 this may readily be brought about by exposing the 

 gas to radiant energy of the wave-length 203 /x/x, when 

 the absorption of a single quantum per molecule is 

 sufficient. In order to bring about this reaction by 

 heat, it will be necessary for each molecule consecutively 

 to absorb seventeen molecular quanta, without losing 

 any by radiation during the process, before it can 

 decompose. The preparation of hydrogen and chlorine 

 from hydrogen chloride is therefore very difficult to 

 carry out by the aid of heat, but is readily induced by 

 light. Another highly endothermic reaction which 

 may be realised photochemically is the synthesis of 

 formaldehyde from carbon dioxide and water, for it has 

 been shown that under the influence of light of wave- 

 length 200 /t/x the reaction takes place according to 

 the equation COg + HgO = CHgO + Og. These two ex- 

 amples are sufficient to show that whilst there is no 

 essential difference between any two chemical reactions, 

 those that are highly endothermic can be realised in 

 practice only by photochemical stimulation. 



The photosynthesis of formaldehyde from carbonic 

 acid is of great importance, because it undoubtedly 

 forms the first step in the formation of the many com- 

 plex substances produced in the living plant. Some 

 recent work in Liverpool on this reaction has thrown 



