ROLE OF LIPIDES IN CHLOROPLASTS 391 



improbable that the 30-40% of the dry material of leaves which consti- 

 tuted Neish's "chloroplast fraction" could represent free grana. To 

 justify the opposite point of view, Mommaerts (1938) and Hanson, 

 Meeuse, Mommaerts, and Baas-Becking (1938) spoke of " chloroplasts 

 tightly packed with grana," and identified the volume of the grana with 

 the total volume of the chloroplast — an assumption which defies the 

 evidence of microphotography. 



According to page 411, the chlorophyll content of a single chloroplast 

 of Mnium is of the order of 2.5 X lO-^^ gram. Assuming that this 

 chloroplast has a volume of 40 y?, and contains 2 X 10-^^ g. dry matter, 

 of which 1 X 10-11 g. is protein, the mass ratio of chlorophyll : protein 

 becomes 0.25, in approximate agreement with the figures of Granick, 

 Smith and Bot in table 14. IX. 



The outcome of this discussion is: first, that although the association 

 of chlorophyll with proteins in chloroplasts is highly probable, the 

 existence of a chlorophyll-protein complex of uniform composition is not 

 proved by experiments; and second, that the chlorophyll : protein ratio 

 in the grana is at least four times larger than in hemoglobin or cj^to- 

 chrome. 



The values for the protein : chlorophyll ratio in table 14. IX may 

 have to be further increased if one assumes (as suggested on page 361) 

 that the concentration of lipoids in the grana is higher (and that of the 

 proteins correspondingly lower) than in the stroma. In fact, there is 

 enough lipoid material in most chloroplasts to fill the grana completely, 

 thus increasing the chlorophyll : protein ratio in the grana to infinity. 



4. Role of Lipides in Chloroplasts ; the Model of Hubert 



On page 371 et seq., we discussed the occurrence of proteins and 

 lipides in the chloroplasts, and stated that the pigments may be associated 

 with either or both of these components. We then reviewed (page 382 

 et seq.) the evidence of chlorophyll-protein association. The possible 

 association between the pigments and lipides will now be considered. 

 While the pigment-protein link may be a true chemical bond and thus 

 lead to stoichiometric relations, the pigment-lipide association is more 

 likely to be of a "physical" nature, with the lipide molecules tending to 

 surround the pigment molecule and bring it into solution. 



Liebaldt (1913), who thought the chloroplasts to be microscopically 

 homogeneous, suggested that they may contain a submicroscopic emulsion 

 of a lipide in which the pigments are dissolved. She observed that, 

 when surface-active substances are introduced into the cell, small oil 

 drops appear in the chloroplasts, and assumed this to be the result of 

 the coalescence of submicroscopic lipide drops. (Zirkle, 1926, suggested, 



