CHAPTER XI 



MEMBRANES (PLASMAHAUT) 



THE HOUSE OP^ THE CELL 



" The retention of an individuality by the cell must be determined by chemical 

 and physical differences between this layer and the surrounding fluid." 



Starling. 



The unit of life is the cell. It follows that all changes that affect 

 life take place in the cell and that the metabolism of a complex 

 organism is the sum of the changes of the cells that compose it. 

 It is therefore logical to study unicellular animals with a view 

 to the application of the knowledge so gained to the elucidation 

 of the more intricate problems of multicellular organisms, provided 

 that it is borne in rnind that in a coinplex animal, each cell will he 

 modified inform and somewhat in function not only by neighbouring 

 and similar units, but by comparatively remote and dissimilar cells. 



Need for cell membrane. 



We have seen that the cell consists of protoplasm, which may 

 be regarded as water dispersed through a colloidal complex con- 

 taining dissociated and non-dissociated crystalloids and substances 

 in suspension. 



(i.) Now it is clear, that as amoeba, for instance, lives in water, 

 some skin or pellicle is necessary to prevent the protoplasm from 

 suffering infinite dilution. For example, if an amoeba is deprived 

 of its coat, say by bringing the animalcule into contact with air at 

 the surface of water, the naked protoplasm lying below the surface 

 of the water after a moment or two generally suffers dispersion 

 throughout the surrounding water. 



(ii.) Further, if osmotic pressure is to be converted into 

 hydraulic pressure, a membrane is necessary, as we have seen. In 

 plants, growth is, in part, due to osmotic energy, and therefore 

 plant cells must be bounded by a cell wall which will allow the 

 passage of water, but not of, say, sugars. Nagcli, Pfeffer, De Vries 

 and others have demonstrated the existence of such a cell wall. 

 If plant cells are inmiersed in a hypertonic salt solution, i.e. in a 

 salt solution having a greater osmotic pressure than the osmotic 

 pressure of the cell contents, then exosmosis will take place. Water 



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