XIII. ACTION SPECTRA AND ABSORPTION SPECTRA 419 



Molecules in solution absorb only quanta of certain size ranges, 

 determined by their internal structure. Hence a given light-absorb- 

 ing substance can only participate in a photochemical reaction when 

 exposed to quanta of the appropriate size capable of bringing about 

 excitation of its molecules. This means that only particular wave- 

 length regions can cause a given photochemical reaction, quantum 

 size being inversely proportional to wavelength (equations 1 and 2). 

 Thus, a means of characterizing any photobiological process is pro- 

 vided, because a given process is brought about only by those wave- 

 lengths absorbed in the primary reaction (1) by the specific light ab- 

 sorber. This is the basis for the use of absorption and action spectra 

 to identify the light absorber in a photobiological process, which is 

 obviously one of the basic pieces of information required in the elucida- 

 tion of any photobiological mechanism. While the elementary prin- 

 ciple is simple enough, the actual determination of the absorbing and 

 exciting wavelengths is less so, since there are various complicating 

 factors that may lead to incorrect interpretation of the measurements 

 obtained. 



TABLE I 



Number of Quanta Required in Some Photobiological Processes 



Photobiological process and source of data Quanta required" 



Photosynthesis: formation of 1 g. carbohydrate by the green plant. 

 Based on quantum yield of approximately 1/12; see Franck 

 and Gaffron, Advances in Enzymol., 1, 199-262 (1941) 10" 



Scotopic vision: minimal sensation of light in the human eye. 



Hecht, Biol. Symposia, 7, 1-21 (1942) 5 to 14 



Photodynamic action: hemolysis of the photosensitized rabbit 



erythrocyte. Blum and Gilbert (^, Ch. 7) 1 X lO'" 



Destructive action of ultraviolet radiation: vesiculation of Parame- 

 cium. Giese and Leighton, J. Gen. Physiol., 18, 557-571 

 (1935) 1 X JO'3 



Destructive action of ultraviolet radiation: inactivation of Escheri- 

 chia coli. After Hollaender and Glaus {18) 2 X 10* 



Inactivation of one molecule of an enzyme: trypsin. Verbrugge, 



J. Biol. Chem., 149, 405 (1943) 50^ 



Inactivation of one molecule of a virus: tobacco mosaic virus. 



Uber, Nature, 147, 148 (1941) 4 X 10^ 



" All these values are based on ostimat(!S of number of quanta absorbed ; most of 



these are approximate and they may vary to a certain extent with conditions. 

 * Additional values for inactivation of enzymes are summarized by A. D. McLaren, 

 in Advances in Enzymology, Vol. IX, Literscience, New York, 1949, pp. 75-170. 



