March i6, 1922] 



NATURE 



345 



It on the mechanism utiHsed by the living plant 

 carrying it out, and also on the general problem 

 [the photosynthesis of vegetable products. Although 

 ! synthesis of formaldehyde has been carried out photo- 

 Bmically in the laboratory with light of wave-length 

 /!A/4, this is certainly not the case in the plant, for 

 »re is present in sunlight no radiation of this wave- 

 igth. The plant must in some way carry out the 

 iction with the absorption of visible light, for it is 

 ^11 known that visible light only is necessary for the 

 )to-assimilation of carbon dioxide. It has been 

 )wn experimentally that, if a visibly coloured basic 

 stance be added to the aqueous solution of carbon 

 cide, formaldehyde is produced on exposing the 

 cture to visible light. The coloured substance, being 

 basic, forms a complex with the carbonic acid, and 

 within such a complex the components possess an 

 identical infra-red frequency ; that is to say, the 

 molecular quanta of the two are identical. The energy 

 absorbed by the coloured component is radiated at this 

 common infra-red frequency and re-absorbed by the 

 carbonate component. The necessary increment of 

 energy is thus gained by the carbonate component, 

 which is converted into a molecule of formaldehyde and 

 a molecule of oxygen. This type of reaction has been 

 named photocatalysis, the coloured substance acting 

 as photocatalyst. It has been proved that malachite- 

 green, methyl-orange, and /)-nitrosodimethylaniline 

 act as photocatalysts in this reaction, and in the 

 presence of carbon dioxide give formaldehyde on 

 exposure to visible light. 



It has been shown by Willstatter that chlorophyll 

 as it occurs in the plant combines with carbonic acid, 

 and hence there is Httle doubt that it functions as a 

 photocatalyst. The green-coloured complex absorbs 

 visible light, and the energy so absorbed is transferred 

 to the carbonic acid through the identity of infra-red 

 frequency, with the result that formaldehyde and 

 oxygen are produced. Although this gives a satis- 

 factory explanation of the mechanism by means of 

 which the living plant is able to produce formaldehyde 

 with the aid of visible light alone, the story is far from 

 complete, for there are yet to be considered the forma- 

 tion of carbohydrates from the formaldehyde, and the 

 details of the process whereby the oxygen set free in 

 the photosynthesis is transpired by the plant as gaseous 

 oxygen. 



It was shown by Moore and Webster that aqueous 

 solutions of formaldehyde on exposure to ultra-violet 

 light are polymerised to reducing sugars, but no 

 evidence was given of the nature of these sugars 

 or of the wave-length of the light required. It has 

 been shown more recently in Liverpool that the 

 necessary wave-length of the light is 290/>i/x, which 

 at once establishes the fact that the polymerisation 

 is photochemically distinct from the synthesis of 

 formaldehyde. It has also been shown indirectly 

 that the polymerisation of formaldehyde can be 

 photocatalys6d ; but this is of scientific interest only, 

 since there is no need to postulate such a mechanism 

 in the plant. On exposure to ultra-violet light the 

 formaldehyde molecules are activ^ated, and it is these 

 activated molecules which undergo polymerisation to 

 sugars, since it is well known that ordinary formalde- 

 hyde does not polymerise in this way. When the 

 formaldehyde molecules are first produced by photo- 



NO. 2733, VOL. 109] 



synthesis they are in the activated form, and may 

 therefore lose energy in one of two ways, either by 

 change of phase to produce ordinary formaldehyde or 

 by polymerisation to give sugar molecules. But it 

 has been proved that the photochemically activated 

 molecules at once polymerise to sugar, and therefore 

 the photochemically synthesised molecules do the same. 

 There is thus no need to consider the activation of 

 the formaldehyde in the plant, for it is already acti- 

 vated when produced. The absence, therefore, of free 

 formaldehyde in the growing leaf is explained by the 

 fact that the photosynthetic process from carbonic 

 acid to sugar takes place without a break. (Baly, 

 Heilbron, and Barker, Trans. Chem. Soc, vol. 119, 

 p. 1025, 1921.) 



The mechanism of the process whereby the oxygen, 

 which is produced with the formaldehyde in the 

 photosynthesis, is transpired as gaseous oxygen 

 is one of great importance in view of the energy 

 changes involved. Willstatter has shown that 

 chlorophyll is in reality a mixture of two substances, 

 chlorophyll A and chlorophyll B, and that a mole- 

 cule of chlorophyll B contains one atom of oxygen 

 more and two atoms of hydrogen less than a mole- 

 cule of chlorophyll A. Two atoms of oxygen, 

 therefore, are required to convert a molecule of 

 chlorophyll A into a molecule of chlorophyll B, and 

 since this is the exact relation required in the photo- 

 synthetic operation it is impossible to believe that 

 it is not utilised. It is in the highest degree probable 

 that a molecule of chlorophyll A combines with a 

 molecule of carbonic acid, and that this complex 

 on exposure to light gives a molecule of activated 

 formaldehyde and a molecule of chlorophyll B. 

 Willstatter hesitates to accept this view, because he 

 found that the ratio of chlorophyll B to A is not 

 altered during photosynthesis ; but since he also 

 proved that the velocity of transpiration of the 

 oxygen is equal to that of the absorption of carbon 

 dioxide, this cannot be accepted as evidence. It means 

 only that there is present in the leaf some mechanism 

 whereby the chlorophyll B is deoxidised and recon- 

 verted into chlorophyll A. Willstatter has further 

 proved that an aqueous solution of chlorophyll, satu- 

 rated with carbon dioxide, decomposes on exposure 

 to light, no measurable photo-assimilation of carbon 

 dioxide taking place. This affords an additional proof 

 that there is present in the living plant a mechanism 

 for maintaining the chlorophyll equilibrium. 



In the living photosynthetic cell there exist, along 

 with the chlorophylls, two more pigments, carotin, 

 Cj^Hgg, and xanthophyll, C^oHggOg, the relation be- 

 tween the two as regards oxygen being the same as 

 that between chlorophyll A and B. It may therefore 

 be suggested that carotin has the power of reducing 

 chlorophyll B to chlorophyll A, itself being oxidised 

 to xanthophyll. This is supported by Willstatter's 

 observation that the ratio of xanthophyll to carotin is 

 increased during the photosynthetic operation. This 

 increase, though perfectly definite, is not large enough 

 to decrease materially the amount of oxygen transpired. 

 The complete reaction, H.^O + C02 = CH20 + 02. is 

 highly endothermic, and is accompanied by the ab- 

 sorption of about 150,000 calories per gram-molecule 

 of formaldehyde produced ; and it is interesting to 

 note that one quantum of energy absorbed in the 



