Chapter 15 



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THE PIGMENT SYSTEM 

 A. General Composition * 



When pigments are encountered in animal or plant tissues, their 

 presence never fails to attract attention, even though their color may be 

 irrelevant from the point of view of their biochemical function. The 

 deep red color is a striking attribute of blood, which makes it so much 

 more exciting than the colorless lymph — although the capacity of 

 hemoglobin to absorb visible light has no direct bearing on its biological 

 activity, the transportation of oxygen. In photosynthesizing cells, the 

 presence of a colored substance acquires special significance. Knowing 

 that, in these cells, light is converted into chemical energy, we ask: 

 Does this pigment participate in conversion, or is its color, in analogy 

 with the red color of blood, only a coincidence? 



The pigment system of photosynthesizing plants is a complex mixture 

 whose analysis presents many difficulties. Extraction disrupts the 

 chemical units, containing the plastid pigments in the natural state, 

 dilutes them with the pigments (which have no relation to photosynthesis) 

 from the vacuoles and cell walls, and allows them to come into contact 

 with cell components which may affect them chemically (e. g., acids and 

 enzymes). The separation of the extracted mixture into its constituents 

 can easily lead to further " denaturation " by contact with air, solvent, 

 or adsorber. Complete separation is made difficult by the fact that the 

 pigment mixture contains isomers or other components which differ only 

 slightly in solubility and chemical properties. 



We cannot enter here into details of the methods which have been 

 perfected to overcome these difficulties. The two main procedures are 

 distribution between immiscible solvents and fractional adsorption on 

 columns of talcum, sugar, urea, or other adsorbing powders ("chromato- 

 graphic analysis")- The first method was used by Stokes in 1864 in the 

 historic investigation which proved that the pigment of green leaves 

 consists of four major constituents. It has been further developed by 

 Willstatter and Stoll (1913, 1918) and more recently by Schertz (1928), 

 Meyer (1939), and Hanson (1939). Chromatographic analysis, which is 



Bibliography, page 432. 



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