SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 33 



comparison with the directly measured curve. On the lower right is 

 the absorbance curve measured with the recording Beckman spectro- 

 photometer for a culture of Chlorella. The lower right shows the 

 derivative spectrum of the same Chlorella sample. The notch and the 

 extra peak in the neighborhood of 650 m/x is due to chlorophyll b, 

 which shows only as a small shoulder on the absorbance curve. The 

 most interesting part of the Chlorella absorption curve is, however, the 

 shoulder of 675 to 680 mix. This shoulder means that the chlorophyll 

 a band in Chlorella is made up of two components having absorption 

 peaks at slightly different wavelengths. The shape of this shoulder is 

 dependent on temperature. At 5°C a separate peak, rather than a 

 shoulder, shows on the derivative curve. 



With the derivative spectrophotometer we can easily study the 

 shape of the chlorophyll c band in brown algae. In Ulva the chloro- 

 phyll b band was found to consist of two components. 



This machine is showing fascinating and previously undetectable 

 details in absorption spectra of common objects. Work with it is much 

 like applying a microscope for the first time to the study of objects 

 well known macroscopically. The major question of immediate con- 

 cern is the shape of the red chlorophyll a band in green plants. Does 

 its variation in shape mean that there are many different kinds of in 

 vivo chlorophyll al Are the observed differences due to only two 

 forms of chlorophyll a occurring in various proportions? Is the varia- 

 tion merely a secondary result of the flattening effect being dependent 

 on size of particles? Are the different spectroscopic forms of chlo- 

 rophyll a correlated with differences in photosynthetic function? Many 

 of these questions have arisen from the work of the Krasnovskii group 

 (translations by Rabinowitch and by Milner). To answer them com- 

 pletely we may have to wait for the development of methods for 

 measuring action spectra with resolving power equal to that of absorp- 

 tion spectroscopy. 



The absorption of pigments in trace amounts in living cells can be 

 measured with great accuracy by another means known as difference 

 spectrophotometry. Difference, or differential spectrophotometry, as 

 it is also known, is extremely useful in comparing two samples which 

 differ only by the presence or the reduction state of the significant 

 pigments. A modification of this procedure is to compare, for a single 



