344 RADIATION BIOLOGY 



obtained from plants. The process of photosynthesis and the function 

 of the pigments must still be studied in the living material. 



There are three types of spectroscopic data that are used to determine 

 the identity, quantity, and function of photochemically active pigments. 

 Absorption spectra of live cells can be used to detect the presence of 

 certain pigments, but only very crudely to determine the quantity pres- 

 ent. They also give information as to the fraction of the incident light 

 absorbed by cells. Quantitative measurements of spectral absorption 

 are most useful in work on the nature and concentration of pigments in 

 solutions outside the cell. Fluorescence spectra both of solutions and of 

 live cells provide an equally powerful method of identification and pro- 

 vide, furthermore, the only direct measure of the energy received by the 

 various individual pigments when a complex mixture such as exists in 

 biological material is illuminated. Action, or effectiveness, spectra show 

 what regions of the spectrum cause a photochemical or photobiological 

 reaction to take place. Comparisons of action spectra with the absorp- 

 tion spectra of known compounds are used to identify the light-absorbing 

 pigment for the reaction in ciuestion. 



It is the purpose of this chapter to present on uniform scales the various 

 types of spectroscopic measurements that are often needed by workers in 

 the field, to summarize the main findings from spectroscopic measure- 

 ments of photosynthetic plants and of their isolated pigments, to show 

 the type of information that can be obtained by biological spectroscopy, 

 and to point out some of the more obvious gaps in the knowledge of the 

 spectroscopy of the photosynthetic pigments in live cells. The present 

 discussion has been restricted to the consideration of complete spectral 

 curves and intentionally omits all data reported only as wave length of 

 absorption, fluorescence, or effectiveness maxima. 



The spectroscopy of photosynthetic pigments has very recently been 

 given a comprehensive and critical treatment by Rabinowitch (1951), 

 where many of the subjects are covered in more detail than can be done 

 here. Algal pigments have been reviewed by Cook (1945). 



By measurements of action spectra it has been found that light absorp- 

 tion by at least 10 pigments can induce photosynthesis — namely, chloro- 

 phyll a, chlorophyll b, bacteriochlorophyll, bacterioviridin, fucoxanthin, 

 some bacterial carotenoids, C-phycocyanin, R-phycocyanin, C-phyco- 

 erythrin, and R-phycoerythrin. Of these pigments, chlorophyll a is the 

 only one that has been found in all photosynthesizing plants, with the 

 exception of bacteria that have one of two pigments, bacteriochlorophyll 

 or bacterioviridin, closely related to chlorophyll a. The universal occur- 

 rence of chlorophyll in organisms capable of photosynthesis has led 

 observers to wonder whether it is the only pigment that is essential for 

 the process. If so, perhaps the other pigments act merely by transferring 

 their absorbed energy to chlorophyll. The answer to this question has 



