94 Microbes and You 



cells have their own chemical configuration. Protoplasm has been 

 defined as a system of chemical compounds held together in a 

 colloidal suspension and containing carbon, hydrogen, oxygen and 

 nitrogen, among other elements, in the form of proteins, carbo- 

 hydrates and fats. The English physicist, Thomas Graham, intro- 

 duced the word colloid in 1861 to describe substances that will not 

 diffuse through a membrane. Those substances which can diffuse 

 are called crystalloids. The minute colloid particles are suspended 

 in a liquid, and there is a constant interplay and exchange of 

 various atoms, molecules, electrical forces, physical and chemical 

 stresses and attractions— all in a delicately balanced state of equi- 

 librium. Any abnormal alteration of any of these forces will 

 affect the functioning of the cell and may result in its death. 

 Microbiologists employ this knowledge in their attempts to create 

 as favorable conditions for growth as possible, and also in their 

 attempts to destroy microbes by chemical and physical forces when 

 destruction is desirable. 



The properties of protoplasm are those which distinguish living 

 from dead substance: movement, irritability, reproduction, metab- 

 olism, and death. The latter term, death, is the final distinguish- 

 ing characteristic of protoplasm, in a given stage, but to define 

 death is not easy. Just to say that it is cessation of life is hardly 

 satisfactory, because a definition of life is equally difficult. We do 

 know that the colloidal structure of protoplasm is markedly altered 

 when death steps in, but other than that, our knowledge is 

 fragmentary. 



INDIVIDUAL STRUCTURES OF RACTERIA 



CELL WALL AND CELL MEMBRANE 

 The cell walls of plants are more rigid than the walls normally 

 found on animal cells. This membrane varies in its permeability 

 between organisms, and the permeability within a given species 

 can be materially altered by application of chemical or physical 

 forces. Most evidence points to the conclusion that the wall 

 functions principally as a protective device for the underlying 

 structures and gives the cell its shape. If it were not fairly rigid. 



