484 



Absorption Spectrophotometry /26 : 2 



standard spectrophotometers which increase their utility for biological 

 research. 



It is quite possible to use a spectrophotometer without any idea of the 

 physical basis for the observed spectra. However, spectrophotometry is 

 so important in biological studies that it is hoped the reader will want to 

 know more about the spectra in order to appreciate both the limitations 

 and the unexplored possibilities of spectrophotometry. The physical 

 basis for the characteristic absorption spectra of biological molecules is 

 described in the following chapter. 



2. Units and Symbols of Absorption 



Almost all biophysics, biochemistry, and physiology laboratories use 

 some form of spectrophotometry. The most frequently used types are 

 absorption spectra of compounds in liquid suspension or within the 



660 



A (muJ 



Figure I. Absorption spectrum of ethyl chlorophyllide b. 

 This spectrum is essentially identical to that for chlorophyll b. 

 See Chapter 20 for a discussion of chlorophylls. After A. S. 

 Holt and E. E. Jacobs, Am. J. Botany 41 : 710 (1954). 



living cell. These absorption spectra are used both to identify com- 

 pounds and also to quantify the amount of a given compound in solution 

 (or in the cell). Typical absorption spectra are shown in Figures 1 

 through 4. 



