578 PRINCIPLES OF GENERAL PHYSIOLOGY 



There are two forms of chlorophyll in the leaf, one a product of oxidation 

 of the other. These are accompanied by two yellow pigments, carotin and 

 xanthophyll, unsaturated and autoxidisable hydrocarbons. These latter are not 

 related to cholesterol. 



The relation between chlorophyll and haemoglobin is described in the text. 



The absorption of light by chlorophyll is chiefly in the red and is in the 

 position of the maximum energy of the solar spectrum during the greater part of 

 the day. 



The maximum action of chlorophyll is in light of a wave length corresponding 

 to that of its absorption bands. 



There is reason to suppose that formaldehyde is the first product of photo- 

 synthesis. This is subsequently, perhaps also under the action of light, polymerised 

 to higher carbohydrates. 



Although a substance giving aldehyde reactions is split off from chlorophyll 

 by the action of light in the presence of oxygen, there is no evidence that this 

 substance is other than a decomposition product of the pigment itself, perhaps of 

 the phytol constituent. Its production takes place in the absence of carbon 

 dioxide. 



Along with the production of an aldehyde by light and oxygen, a peroxide of 

 some kind is formed, just as in the case of ordinary optical sensitisation by dye- 

 stuffs. It is doubtful whether this is the source of the oxygen evolved in the 

 normal course of the photo-synthetic process. 



At the present time, therefore, we have 110 evidence that chlorophyll acts 

 otherwise than as an optical sensitiser for the reactions going on in the complex 

 system of the chloroplast. We have no information as to these complex reactions, 

 except that possibly iron plays a part in them. It must be remembered, however, 

 that the chemical structure of chlorophyll is a rather remarkable one, so that 

 conclusions as to its behaviour must not be made too hastily. 



Certain electrical changes have been noticed to occur in green leaves under the 

 action of light. They appear to be connected with the photo-synthetic action, since 

 they are absent unless carbon dioxide is present. 



When the light available is deficient in the rays absorbed by chlorophyll, other 

 optical sensitisers are found to be present and these absorb the rays which actually 

 reach the cell. 



A brief discussion of the factors affecting the rate of photo-synthesis is givm 

 in the text. 



The efficiency of the photo-synthetic process is, at its optimum, a high one. 



Many of the constituents of living cells absorb ultra-violet light to a consider- 

 able degree. 



The reaction of certain small animal organisms to ultra-violet light obeys 

 definite laws. A minimum duration is necessary; there is a limit of intensity 

 below which no reaction occurs, and the effect is independent of temperature. 

 These facts point to the occurrence of a photo-chemical reaction, whose products 

 excite skin receptors 



The direct action of ultra-violet light on micro-organisms and on tissue cells is 

 a lethal or destructive one. The shorter the wave length, the more powerful the 

 effect with equal energy of the radiation, probably owing to the greater absorption 

 of the short waves by protoplasm. 



Similar action can be produced by visible light in the presence of a dye which 

 can absorb this light (photo-dynamic seiisitisation). 



Application of the facts of photo-chemical reactions to the retinal process is 

 suggested in the text. 



