20 THE CELL AS A PHYSICO-CHEMICAL UNIT 



places where they can interfere with these processes of 

 degradation and rebuilding of the constituent molecules 

 of the cell. It is probable that permanent interference 

 with any one stage in these processes will eventually 

 cause the death of the cell. 



Defence Mechanisms 



Many of the defence mechanisms of the cell are physico- 

 chemical in nature. They may in some measure be di- 

 vided into mechanisms of short term importance and 

 of long term importance. Among the short term mecha- 

 nisms available to the cell for dealing with foreign bodies 

 are concentration in vacuoles or granules, and detoxi- 

 cation processes. A foreign substance which becomes 

 introduced into the cytoplasm of the cell may be prac- 

 tically removed from most of the cell and concentrated 

 either in vacuoles, as is often the case with neutral red, 

 or in granules as is often the case with trypan blue. As 

 a result of these processes, most of the rest of the cell 

 can function unaffected by the foreign substance. Al- 

 though these processes are only obvious in the case of 

 coloured substances, there is, of course, no particular 

 importance in a substance being coloured. A colourless 

 substance is concentrated in the same ways as are col- 

 oured substances. Then, possibly after a substance has 

 been concentrated in the interior of a cell in this way, it 

 may be subjected to detoxication processes. Often these 



