ROLE OF LIPIDES IN CHLOROPLASTS 393 



as the absorption hand (cf. Chapter 23, Vol. II). Seybold and Egle sug- 

 gested that the chlorophyll solution in lecithin may have its absorption 

 band in a position (668 m/x) typical of true solutions but its fluorescence 

 band in a position typical of living cells {i. e., close to 680 m/i); but this 

 suggestion was based on insufficient evidence and is not very plausible. 

 Probably, the same chlorophyll molecules account for both absorption 

 and fluorescence in vivo. The "duahstic" theory of Noack and of Sey- 

 bold and Egle also fails to give a simple explanation of the effect of heat 

 on leaf fluorescence. The latter disappears upon short immersion into 

 boiling water and re-appears after several minutes (Vol. II, Chapter 24). 

 A plausible explanation of this phenomenon is that a weakly fluorescent 

 chlorophyll-protein complex is decomposed by denaturation of the 

 protein, with the chlorophyll first left in a nonfluorescent colloidal form, 

 and then slowly passing into solution in the molten lipides, and thus 

 becoming fluorescent again. Similarly, the disappearance of fluorescence 

 upon drying is most conveniently explained by an influence of drying on 

 the proteinaceous phase. (Seybold and Egle suggested that dried pro- 

 teins attract chlorophyll from the lipoid solution — an hypothesis which 

 does not appear particularly plausible.) 



It thus seems as if the weak fluorescence of chlorophyll in vivo must 

 be attributed, not to chlorophyll freely dissolved in a lipoid phase, but 

 to chlorophyll bound in a complex to a protein. 



Hubert (1936) suggested that a chlorophyU adsorbate on protein 

 may become fluorescent if the hydrophobic end of the pigment molecule 

 is protected by a lipide. Singh and Anantha Rao (1942) observed that 

 the fluorescence of chloroplasts can be destroyed by trypsin as well as by 

 lipase, thus indicating that the fluorescent state is brought about by the 

 association of chlorophyll with both proteins and lipides. 



In Hubert's chloroplast model, represented in figure 46, layers of 

 protein carry rows of adsorbed chlorophyll molecules with their porphin 

 rings facing the proteins, while the hydrophobic phytol tails of the 

 chlorophyll molecules are attached to the equally hydrophobic molecules 

 of lipides. Hubert's model accounts satisfactorily for the optical proper- 

 ties of chloroplasts (pp. 365 et seq.), but it remains highly speculative. 

 We have found above that the concentration of phospholipides in chloro- 

 plasts often is insufficient for the role ascribed to them by Hubert. 

 According to table 14. V, one must assume that fats, at least, must partici- 

 pate in the formation of the lipide layer together with the phospholipides. 

 Hubert's assumption that the carotenoids are associated only with the 

 lipoid constituents of the chloroplasts also appears to be incorrect {cf. 

 Menke 1940). 



In the Hubert model, all chlorophyll molecules are assumed to be in the same state, 

 except perhaps those situated in the outer layers of the grana, in contact with the 



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