16 



PHOTOCHEMICAL PRINCIPLES 



absorption spectrum of a suspension of Eiiglena in water with tlie 

 absorption spectrum of an alcoliol extract of the pigments from the 

 same quantity of cells. These measurements of Shibata, Benson, and 

 Calvin (1954) are particularly useful for our purposes because, 



20 



S'5 



05 



Eugleaa 



MO 1,00 500 600 



Xin mil 



Fig. 1. The absorption spectra of a suspension of Euglena (solid line) 

 and of an alcoholic extract from the same volume of cells (dots) (Shibata, 

 Benson, and Calvin, 1954). 



throughout the visible part of the spectrum, all the colored material in 

 the cells is soluble in alcohol. The two curves therefore tiive a direct 

 comparison of absorption of the pigments themselves in solution and 

 in their natural state. The two curves are very different, for reasons 

 that we will consider in some detail. 



The dotted line, representing the absorption by the alcohol extract, 

 is higher at some wavelengths and lower at other wavelengths than 

 the solid line representing the absorption by the cell suspension. 

 Furthermore, the positions of the peaks of the two curves are at 

 different wavelengths, a fact that is most clearly seen at the red 

 absorption peak of chlorophyll a. The red peak is at about 662 m/^ in 

 alcohol and at about 675 m/j^ in the cells. This shift of wavelength 

 position is probably due to real chemical differences of the pigment 

 in the two cases. In solution the pigments are actually combined with 

 the solvent, as is evident from the fact that the peak position depends 

 on the type of solvent. In the cells many pigments, and presumably 

 also chlorophyll, are combined with proteins. In fact, much of the 

 evidence for the chemical nature of pigments in cells comes from 

 absorption spectroscopy. There are, however, other groups of pig- 



