38 PHOTOSYNTHESIS 



Subsequent to extraction from the Cell both the absorption 

 and the fluorescence bands shift towards shorter wave- 

 lengths, e.g. the chlorophyll red absorption band maximum 

 changes from about 680 rcifi to 660 m/n, whilst in bacterio- 

 chlorophyll it is shifted, possibly by 100 m/i, from the far to 

 the near infra-red (there still remaining considerable uncer- 

 tainty as to the correlation between the bands of the pigment 

 in extract and those of cell suspensions). The absorption 

 maxima of the carotenoids in extract are shifted towards 

 shorter wavelengths by some 10-30 mju. All these figures 

 are rough estimates derived from an attempt to analyse the 

 absorption curve of a living cell suspension into a number of 

 component curves. There remains considerable uncertainty 

 in any such analysis since, in addition to the shift in the ab- 

 sorption maxima, the bands of the pigments in the cell also 

 appear broader owing, in part, to the effect of scattering (see 



Fig. 3.6). 



Green aqueous juices may be extracted from green leaves 

 either by pressing or by grinding at low temperature in dis- 

 tilled water. The absorption spectrum of the juice is practi- 

 cally identical with that of the leaf and the chlorophyll is 

 stable towards oxygen and light in the same way as that in 

 the plastids. Such preparations show only a weak fluores- 

 cence. Similarly bacteria may be ground or broken, using 

 ultrasonic waves and a clear colloidal solution obtained with 

 an absorption spectrum similar to that of bacterial cell sus- 

 pensions. These preparations are irreversibly altered by 

 heat, suggesting that the properties shown depend on the 

 presence of native protein. Thus there is good circumstan- 

 tial evidence that the chlorophyll and probably the caro- 

 tenoids are present in the cell in the form of a protein com- 

 plex. How this structure is related to the lipoids of the 

 plastids is not yet known. 



Fluorescence of the pigments in vivo; sensitized fluorescence 



In the plant the 'position' of the fluorescence band of 

 chlorophyll is further tow^ards the red and at the same time 

 the intensity of fluorescence is reduced to between one- 

 tenth and one-hundredth compared with that in organic 



