PHYSICOCHEMICAL STATE OF CHLOROPHYLL 311 



regarding the condition of the chlorophyll in the chloroplasts. If we are ever 

 to know how the chlorophyll of living cells operates it will be necessary to 

 discover how it is related to the other constituents of the chloroplast. At 

 least three different views have been expressed regarding the physicochemical 

 state of the chlorophylls as they occur in the chloroplasts: (i) that they are 

 coUoidally dispersed in the stroma of the chloroplast, (2) that they are dis- 

 solved in a lipoidal solvent which itself may be colloidally dispersed through 

 the stroma, and (3) that they are physically or chemically bound up with 

 proteins or lipids or with both. 



The first view has been advanced by Willstiitter and Stoll (1913)- They 

 adduce two main lines of evidence in its support. In the first place when 

 the absorption spectrum of a solution of chlorophyll is compared with the 

 absorption spectrum of a colloidal sol of chlorophyll dispersed in water it is 

 found that the absorption bands of the colloidal sols are farther toward the 

 red end of the spectrum than those of true solutions of chlorophyll. Since the 

 absorption bands of living green leaves also show a shift towards the red as 

 compared with chlorophyll solutions this is considered to be evidence that the 

 chlorophyll in the living leaf is in the colloidal state. In the second place, 

 perfectly water-free organic solvents will not remove chlorophyll from dried 

 green leaves but in the presence of a small amount of water the extraction 

 readily occurs. Willstatter and Stoll have suggested that the water dissolves 

 small amounts of electrolytes from the leaf which "precipitate" the chlorophyll 

 from its colloidal condition and that the "precipitated" chlorophyll dissolves 

 readily in organic solvents. Chlorophyll can not be removed from the hydro- 

 sol condition by the use of the usual solvents unless small amounts of electro- 

 lytes are present. 



Colloidal sols of chlorophyll dispersed in water do not exhibit fluorescence, 

 differing in this respect from true solutions of chlorophyll. Stern (1920), 

 Lloyd (1924) and others have shown that the chlorophyll in living cells 

 exhibits fluorescence. This is considered to be evidence that the chlorophyll 

 in the chloroplasts is not in a colloidal condition. The results of these in- 

 vestigations have led to the suggestion that the chlorophyll is dissolved in some 

 lipoidal substance which may in turn be colloidally dispersed in the stroma of 

 the plastid. 



Noaclc (1927) considers the chlorophylls to be adsorbed on the protein 

 constituents of the chloroplasts. Since adsorbed chlorophyll fluoresces this is 

 in accord with the observation that the green pigment of living cells exhibits 

 fluorescent properties. It is also possible that the chlorophylls may be present 

 in the chloroplasts in chemical combination with proteins and some evidence 

 in support of this possibility has been obtained (Smith, 1938). In this con- 



