8 5 6 



NA TURE 



[December 23, 1922 



Photosynthesis. 



GREAT interest was taken in the joint discussion 

 on photosynthesis between the sections of 

 Chemistry and Botany during the British Association 

 meeting at Hull. The discussion was presided over 

 by Prof. H. H. Dixon, who was supported by Principal 

 J. C. Irvine. 



The discussion was opened by Dr. F. F. Blackmail 

 with a paper entitled " The biochemical problems 

 of chloroplastic photosynthesis." Dr. Blackmail said 

 that as the next two speakers were to take up the 

 special aspects of photochemistry and energetics he 

 would restrict his remarks to certain other aspects. 

 He would deal with the active system of photo- 

 synthesis and its organisation in the living cell and 

 bring together the evidence which supported the 

 thesis that here we have to do, not with a simple 

 h mica! reaction, but with a complex system 

 m which other components, that might be described 

 provisionally as protoplasmic components, play an 

 essential part. 



The first point developed was that there are 

 numerous lower plants which obtain all their carbon 

 by the reduction of ('<>.. in the dark without the 

 intervention of radiation, and synthesise all their 

 organic compounds from this source. Here there 

 is utilised the chemical energy of the oxidation of 

 nitrogen, sulphur, or their compounds. In these 

 chemosynthetic organisms there is not a gain of 

 energy, but only an exchange of oxidation potential : 

 the gain to the organism is substance for growth. 

 It may be asked whether this power is entirely 

 absent in the higher plants and what connexion 

 the chemical machinery of it has with the chemistry 

 of photoreduction of C0 2 . 



The second point was the fact, now thoroughly 

 investigated, that the seedlings of many plants at 

 a stage when they have developed chlorophyll to 

 a full green colour may be quite incapable of reducing 

 C0 2 in light, and give out as much C0 2 from respira- 

 tion in light as in darkness. Some other component 

 or property lags behind the chlorophyll in its develop- 

 ment, and the slow, steady rate of its development is 

 the same in darkness or light. 



A third point of interest is the efficiency of photo- 

 synthesis in the golden-leaved varieties of certain 

 shrubs. Here the amount of chlorophyll may be 

 as low as 4 per cent, of the normal green form and 

 yet under medium conditions the reduction of C0 2 

 may be as great as in green leaves. The fact has 

 been established that the golden leaf needs more 

 light than the green to carry out the same rate of 

 reduction of C0 2 . It looks as if with these extreme 

 variations of chlorophyll what counted was the cube 

 root of the amount of chlorophyll present — a single 

 dimension of the colloid micella? and not the total 

 mass — which may be taken as an indication of the 

 organisation of the system. 



A fourth point considered was the relation of 

 photosynthesis to temperature. It is established 

 that for a high rate of photosynthesis it is not sufficient 

 to have intense radiation and concentrated C0 2 , but 

 a high temperature is also essential. For each 

 temperature there is a specific maximum of activity 

 which cannot be exceeded unless the temperature 

 is raised. The specific maximal values increase 

 rapidly for rising temperature, having a temperature 

 coefficient of about 2 for a rise of 10° C. This 

 temperature relation is quite different from that 

 of a pure photochemical reaction, and it provides 

 a further indication that we have to deal with a 

 complex system 111 which dark reactions may play 

 a controlling part. 



NO. 2773, VOL. I IO] 



The fifth point to be raised had to do with the 

 organisation of the active system. Warburg in 

 investigating the action of the narcotic phenyl- 

 urethane upon the rate of photosynthesis finds that 

 the process undergoes great depression of rate with 

 perfect recovery on removal of the narcotic. The 

 relation of the depression to the external concentration 

 of the drug gives a typical adsorption isotherm, 

 indicating that the narcotic acts by adsorption on 

 a surface from which it displaces temporarily some 

 reactant substance of the active photosynthetic 

 system. 



Taking all these pieces of evidence together, Dr. 

 Blackmail considered that we are forced to conclude 

 that the chloroplast contains an active system of 

 several components related together in a complex 

 organisation. 



Prof. E. C. C. Baly then presented the results 

 of experimental work on photosynthesis carried out 

 at Liverpool. The conversion of a substance A into 

 substance B might, he said, be represented as the 

 sum of the three equations : 



A + E=A' 

 A' = B'+F 

 B' =B +G 



where E, F, and G are quantities of energy and 

 A' and B' represent the reactive forms of A and B. 

 The reaction is exo- or endo-thermic according as 

 F+G-E is positive or negative. In any case a 

 quantity of energy, E, must be supplied in order 

 to start the reaction, and this may be done by means 

 of (1) heat, (2) light, or (3) a material catalyst. 

 Now the energy can only be supplied in " quanta," 

 and if E is large, only the use of radiation of short 

 wave-length makes the number of "quanta" to be 

 introduced sufficiently small to be practicable. For 

 the conversion of a molecule of carbonic acid into 

 formaldehyde and oxygen 150,000 calories are 

 necessary, and this can be supplied in a single quantum 

 by radiation at wave-length 2oo^m. Carbonic acid 

 has an absorption band at this frequency and 

 formaldehyde ought therefore to be produced when 

 a solution of C0 2 in water is exposed to ultra-violet 

 light. This has now been shown to take place. 



In order to bring about the reaction by means 

 of visible light it is necessary to have present a 

 coloured substance with basic properties, and 

 Malachite Green has been found to fulfil the conditions. 

 There seems little doubt that the formation of 

 formaldehyde in the leaf takes the following course : 



i. Chlorophyll A + H,C0 3 +light = Chlorophvll B+CH,0. 

 ii. Chlorophyll B + Carotin =Chlorophyll A + Xanthopbyll. 



iii. Xanthophyll + light = Carotin + Oxygen. 



The photosynthesised formaldehyde is extra- 

 ordinarily reactive and is best represented by the 

 formula CHOH. It is polymerised rapidly to a 

 mixture of carbohydrates, in which are found 

 hexoses (20 per cent.), cellulose, and cane-sugar. 

 In the presence of nitrite it is converted into form- 

 hydroxamic acid and hence into amino-acids and 

 a mixture of cyclic bases in which pyrrole, pyrrolidine, 

 pyridine, coniine and glyoxaline have been detected. 

 The active (energised) forms of the aminoacids are 

 the immediate source of proteins. 



Mr. <;. E. Briggs described some experiments to 

 determine the relation between the radiant energy 

 absorbed and the carbon dioxide assimilated by the 

 green leaf (Phaseohts vulgaris) in different parts of 

 the visible spectrum. For three different parts of 

 the spectrum the carbon dioxide assimilated was 

 measured, and the energy absorbed by chlorophyll a 



