CONVERSION OF CHEMICAL ENERGY 77 



The first step in the discussion is to examine the way in 

 which cells are prevented from mixing with their surroundings. 



DEMARCATION OF CELLS FROM THEIR SURROUNDINGS 

 There are two physico-chemical conceptions which may 

 represent the conditions which prevent cells from mixing 

 with the liquids in contact with them. They are : 



(1) That the cell consists of a surface membrane in which 

 the cell contents are contained in the same way as the contents 

 of an osmometer are held inside its semi-permeable membrane. 



(2) That the cell consists of a fluid which is immiscible 

 with its surroundings just as a solution of water in phenol 

 does not mix with a solution of phenol in water. 



Apart from these two conceptions the only other possibility 

 is that the cells are expending energy in a continual struggle 

 to maintain their integrity, an idea which is acceptable only 

 after the two physico-chemical schema have been found 

 untenable. 



The first supposition implies that the exchanges between 

 cells and their surroundings depend on the nature of the 

 substances that can pass through the ^membrane : that is 

 that the texture of the membrane is the main factor in regu- 

 lating the income and output of the cell. 



The second supposition implies that the exchange of 

 material between cells and their surroundings depends on the 

 nature of the cell contents and the relative solution pressure 

 of a substance in the cells and their surrounding solutions. 

 Any substance which is more soluble in the cell contents or is 

 removed in some way from solution in the cell contents will 

 accumulate inside the cells ; that is, the main factor in regu- 

 lating cell processes is the nature and behaviour of the cell 

 contents. 



This as we have stated above is the explanation offered for 

 the accumulation of organic compounds in cells. 



The second step is to examine the evidence as to the nature 

 of the exchanges between the cells and their surroundings. 



EXCHANGES BETWEEN CELLS AND EXTRA CELLULAR 



SOLUTIONS 



Increase in the active concentration of a substance inside a 

 cell will cause a rise in osmotic pressure with an attraction of 

 water into the cell and a tendency for the substance under 

 consideration to escape through the surface of the cell. If 

 this substance cannot escape the rise in osmotic pressure will 

 persist, but if it can escape the osmotic pressure will fall as the 



