LIGHT ABSORPTION BY PIGMENTS ITl VIVO 



1861 



which a pre-ilkiminated Chlorella suspension was conducted through the 

 cuvette of a spectrophotometer. Only the narrow range 540-570 m/x 

 was examined — a range in which the absorption spectrum of Chlorella, 

 pretreated with ascorbic acid, shows the a-bands of the reduced cyto- 

 chromes b, c, and /. (The amounts of c and / are similar, the concentration 

 of c being much higher in Chlorella than in the leaves of the higher plants.) 

 Upon illumination, the 556 m^ band of reduced cytochrome / is found to 

 be weakened, indicating oxidation, while those of the cytochrome b and c 

 are unchanged; that of b may be even slightly enhanced. Duysens 



T 1 1 \ 



400 



350 



Fig. 37C.40. Changes in the ultraviolet absorption spectrum of illu- 

 minated Chlorella and Porphyridium (after Duysens 1954''). 



fig. 37C.38 did not show any effects above 550 m/x, but his subsequent 

 experiments (1954^) revealed a small dip at 555 mju, in agreement with 

 Lundeg&rdh's findings. 



Duysens (1954'') applied his stationary crossed-beam method also to 

 a suspension of the red algae Porphyridium cruentum. The difference 

 spectrum is shown in fig. 37C.39. Bands at 420 and 555 m^, character- 

 istic of the oxidation of cytochrome / (or of another very similar pigment) 

 appear more clearly on this picture, because the relatively stronger spectral 

 effects, exhibited by Chlorella between these two bands are absent in the 

 red alga. 



Duysens (1954^) also noted a reversible increase, during the illumina- 

 tion of both Chlorella and Porphyridium, of the absorption in the near 

 ultraviolet, with the indication of a peak at about 350 m/z (fig. 37C.40). 

 This effect was tentatively interpreted as evidence of the reduction in 

 light of a pyridine nucleotide; if this interpretation is correct, about one 

 molecule of pyridine nucleotide must be reduced, in the photostationary 

 state, for about 100 chlorophyll molecules present in the cell. 



