THE CELL 33 



non-living semi-permeable membranes it is possible to demonstrate 

 the fact that other qualities of the penetrating substance are impor- 

 tant. Thus water will not pass through a rubber membrane, while 

 alcohol, consisting of much larger molecules, will pass. This is 

 regarded as due to the fact that alcohol will dissolve in a membrane 

 of rubber while water will not. The boundary of the cell, being 

 composed of watery protoplasm and also containing fatty sub- 

 stances, has solubility properties that in part determine its per- 

 meability. So the passage of a substance through a semi-permeable 

 membrane into and out of a cell is determined by at least two con- 

 ditions, namely, the size of its molecules and its solubility in the 

 membrane. Any material entering or leaving the cell must be in 

 such physical and chemical state that it may pass the cell boundary. 

 Colloidal particles and large molecules like the protein molecules 

 may be expected to be restrained by the cell boundary. 



As cells differ in other respects so do they differ as regards the 

 property of boundary permeability. The cells that make up the 

 muscle of the human arm, for example, have properties of per- 

 meability that are somewhat different from those of the cells that 

 compose the human brain. Thus the plasma membrane of the 

 individual cell is a most important organ in regulating the life of 

 the protoplasm. 



Many important membranes in the animal body are composed 

 of layers of cells and the permeability of such membranes is the sum 

 of the permeabilities of the cells of which they are composed. The 

 lining of the human intestine is a layer of cells that has the property 

 of allowing certain substances to pass; similarly, the lining of the 

 human lung allows certain gases to pass. The former cannot be 

 substituted for the latter because of these differences in permeability. 



Diffusion. If a crystal of some water-soluble substance, such as 

 copper sulphate, be placed in water and allowed to dissolve undis- 

 turbed, the molecules will diffuse out through the water and in 

 time the concentration, that is to say, the number of molecules of 



