ASPECTS OF SALT ABSORPTION IN CELLS 



105 



probably arranged somewhat as in Fig 36. The comparative ease 

 with which vacuoles enclosed by the tonoplast can be separated from 

 the remainder of the cytoplasm (see p. 21) suggests that there may be 

 only a tenuous structural connection between them. It is across the 

 tonoplast and plasmalemma that active transport is usually presumed 

 to occur, but to look upon cytoplasm as a homogenous phase 

 separated from sap and medium by bounding membranes which 

 regulate its ion content is now seen to be a much too naive approach. 

 Within the surface membrane, cytoplasm consists of a complex 

 aggregation of membranes, vesicles, and granules, the ultrastructure 

 of which has as yet been incompletely elucidated (Plate I). There is 



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OOOOOOOOOO' 



(yjQQ(j::j)Qoq)y)C 



Protein 



Lipid 



Protein 



Fig. 36. Diagrammatic representation of the arrangement of lipid and 

 protein in a simple biological membrane (redrawn from Davson and 



Danielli, 1943). 



apparently a continuous membrane system, the "endoplasmic 

 reticulum", which divides the cytoplasm into two distinct phases, 

 one of which is continuous with the nucleus, and the other perhaps 

 with the external medium. The internal membranes are similar in 

 structure to the surface membranes, and may in fact originate by 

 growth and invagination of the plasmalemma (Buvat 1958). The 

 relationship between endoplasmic reticulum and the tonoplast is 

 likewise still not certain, but the impression given by electron 

 micrographs is that the tonoplast is an independent structure not 

 connected with intracellular membranes. If it is true that the 

 plasmalemma is deeply invaginated and continuous with the 

 endoplasmic reticulum, it is evident that the cytoplasm presents a 

 much greater surface area to the external medium than has been 

 thought. Ions which become bound from the medium on this 



