CHAPTER X 



MEMBRANES (PLASMAHAUT) 



THE HOUSE OF 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. 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. 



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

 be regarded as a watery solution, containing all three classes of 

 solutes (i.e. colloids, dissociated and non-dissociated crystalloids) 

 and also substances in suspension. 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. 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. Nageli, Pfeffer, De Vries and others have 

 demonstrated the existence of such a cell wall. If plant cells are 

 immersed 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 will pass from 

 the cell, causing the cell substance to shrink. But the cell wall 

 does not shrink and a space is left between the contents and the 

 container, so rendering the latter apparent. This process is 

 termed plasmolysis. 



The cell wall differs considerably from the cell contents in 

 chemical composition as well as in physical state. It is a secretion 



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