12 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



enter and leave the cell through the cell wall ; the rate of 

 interchange of substances will depend on the difference of 

 osmotic pressure within and without the cell. 



In fact, a very delicate method of determining differences 

 of osmotic pressure consists in immersing certain plant cells 

 in different solutions and examining the cells microscopically. 



If the osmotic pressure of the solution is greater than that 

 of the cell contents, the cell protoplasm will contract and leave 

 the walls of the cell, a phenomenon known as plasmolysis. 



If the protoplasm within the cells just begins to show 

 signs of contracting, it may be taken that the osmotic pressure 

 is equal on the two sides of the cell wall. Such solutions are 

 said to be isotonic, i.e., the number of molecules present in 

 equal volumes of the solutions within and without the cell, or 

 the molecular concentration of the dissolved substances, is 

 such that they exercise the same osmotic pressure. The 

 changes taking place in the cell must consist in the breaking 

 down of colloidal substances, notably albumin, into crystal- 

 loidal substances which escape from the cell, and the building 

 up of complex matter from other cry stall oidal substances 

 which find entry to the cell. Further, it is obvious that 

 these changes must be analogous to those chemical changes 

 which require the least complexity of chemical conditions, 

 i.e., they must be of the nature of catalysis. 



It is important, however, to note that while the chemical 

 changes are such as can be produced in many cases in the 

 laboratory, if not by ordinary chemical reagents, at any rate 

 by products or enzymes extracted from the living cell, they 

 only take place in nature when the cell is alive. The precise 

 definition of what is meant by vital action cannot here be 

 attempted; it may, however, be stated that the cell can 

 be looked upon as an energy transformer, in which the energy 

 which is characteristic of living matter, and which may be 

 termed biotic energy, is transformed into chemical activity 

 and eventually into heat in the cell processes. 



