ACCUMULATION OF ENERGY 97 



The development of colour in the skins of animals is possibly 

 due to the action of an oxidase.* 



Rhodopsin, the visual purple, is bleached by light and this 

 bleaching may be of fundamental importance in the photo- 

 chemistry of vision. 



EXTINCTION COEFFICIENT 



Absorption of light by a solution is proportional to the 

 thickness of the solution and to the strength of light falling 

 on it. As the strongest light falls on the surface the outer 

 layers absorb most of the light. The extinction coefficient 

 is the reciprocal of the thickness of solution required to reduce 

 the light to one-tenth of its incident value. 



CHLOROPHYLL 



The chlorophyll is contained in small oval bodies called 

 chloroplastids. As it occurs largely on the surface of these 

 plastids the absorption of light is more efficient than if the 

 chlorophyll were evenly distributed throughout the plastids. 



Chlorophyll can be extracted from leaves by means of 

 alcohol or other solvents. The crude extract contains two 

 forms of the green pigment, namely, a and p chlorophyll, with 

 two yellow pigments, carotin and xanthophyll. The solution 

 in alcohol has a dark green colour by transmitted light, but 

 by oblique illumination it appears red : that is, the solution is 

 fluorescent. 



The red fluorescence of chlorophyll indicates that the 

 solution absorbs shorter wave lengths near or beyond the 

 blue end of the spectrum, and gives out waves nearer the 

 red end of the visible spectrum. 



Examined by means of the spectroscope a solution of 

 chlorophyll in acetone shows an absorption band in the red 

 near the Fraunhofer line C, three absorption bands with 

 decreasing intensity towards the violet, and a second absorp- 

 tion maximum which completely absorbs the blue and violet 

 end of the spectrum. By filling the air spaces of a leaf with 

 water it becomes transparent, and the spectrum is seen to 

 correspond to a colloidal solution in one per cent, acetone. 



On extraction of chlorophyll by alcohol the enzyme chloro- 

 phylase acts upon the chlorophyll, splitting it into the 

 unsaturated alcohol phytol and a crystalline substance, 

 chlorophyllin, which is the magnesium salt of a carboxylic 

 2 cid. Borodin's crystals are the ethyl ester of chlorophyllin, 



* F. M. Durham, Proc. Roy. Soc., 1904, vol. 74, p. 310. 



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