26 



PHOTOCHEMICAL PRINCIPLES 



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TOTAL ABSORPTION 



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Fig. 5. The absorption and photosynthesis action spectra of a red alga 

 replotted from Haxo and BHnks (1950) to show also the spectrum of the 

 inactive absorption (French and Young, 1956). 



combines with carbon monoxide as well as with oxygen. The carbon 

 monoxide complex is inactive, but the carbon monoxide may be dis- 

 sociated by exposing the cells to light. Therefore the inhibition caused 

 by adding carbon monoxide to a suspension of cells is lessened when 

 the cells are illuminated. Warburg and Negelein (1928) measured the 

 action spectrum for the decrease of carbon monoxide inhibition of the 

 respiratory activity in several microorganisms. These spectra led to the 

 identification of the active group of the respiratory enzyme as a 

 porphyrin, an achievement for which Warburg was awarded the Nobel 

 Prize in 1931. This investigation was a most remarkable accomplish- 

 ment because the spectrum of the active material was entirely ob- 

 scured by other colored components of the cells, and the enzyme could 

 not be isolated. Many other action spectra have led to equally 

 definitive results, but the use of this technique has not always proved 

 equally rewarding. 



Let us consider the action spectrum for chlorophyll formation. No 

 chlorophyll is formed in leaves grown in the dark. A colored pre- 

 cursor, protochlorophyll, does, however, accumulate in trace amounts. 

 Upon illumination of dark-grown leaves protochlorophyll is trans- 

 formed immediately into chlorophyll a. This transformation, reviewed 

 by Smith and Young (1956), may be measured by direct spectro- 



