SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 25 



no particular problem in action spectra measurements, since all parts 

 of the sample are supposed to be equally illuminated. 



Each photobiological process requires special consideration of the 

 means of measuring the response used to determine an action spec- 

 trum. Such things as a chemical change, a mechanical motion, or a 

 color change may be the final result. As a general principle, however, 

 it is far better to work with the intensity of light of various wave- 

 lengths needed to give the same amount of response than to use con- 

 stant intensity and measure the variable response. In this way non- 

 linearity of response causes no difficulty. 



In the 1880's Englemann observed action spectra for photosynthesis 

 in filamentous algae by projecting a microspectrum directly on a single 

 filament. The liquid surrounding the filament was a suspension of 

 motile bacteria. These bacteria congregated where oxygen was evolved. 

 The bacteria showed activity at the red end of the spectrum near the 

 chlorophyll absorption band and also near the blue absorption band of 

 chlorophyll. More recent work on the action spectra of photosynthesis 

 in various organisms has confirmed Englemann's qualitative conclu- 

 sions. Emerson and Lewis (1943) measured the action spectrum for 

 photosynthesis of Chlorella and compared the action with the absorp- 

 tion by the whole cells. The approximate agreement of these curves 

 shows that most of the pigments present were active in photosynthesis. 

 The yield, however, was somewhat lower, around 480 m/x, where 

 carotenoid absorption is very strong. 



A very different shuation was found by Haxo and Blinks (1950). 

 Their data for a red alga are given in Fig. 5 as plotted by French and 

 Young to show the spectrum of the inactive as well as of the active 

 components. The absorption from 475 to 625 m/x is due to phycobilin 

 pigments, and the activity in this region is high. The striking fact about 

 this set of measurements is the relative inactivity of the chlorophyll 

 which is evident both in red and blue. If red algae were more widely 

 distributed in nature, we would probably think of chlorophyll as a 

 second-rate photosynthetic pigment. 



One of the most spectacular uses of action spectrum measurements 

 was the determination of the chemical nature of the cellular respira- 

 tory enzyme which combines directly with oxygen. Warburg's respira- 

 tory enzyme, which the British prefer to call cytochrome oxidase, 



