166 Microbes and You 



ing process. Nature strives for equality in the biological as well 

 as the physical world. Equal opportunity for all was a law of 

 nature long before man applied the principle in democratic forms 

 of government. 



Solutions having equal osmotic pressure are called isotonic, 

 while solutions of high concentrations are called hypertonic, in 

 contrast to hypotonic solutions which have a low concentration. 

 Cells thrive best under isotonic conditions; hypertonic solutions 

 cause cells to shrink, while swelling occurs when cells are placed 

 in hypotonic solutions. These changes are readily demonstrable 

 with red blood cells which have more elastic cell walls than are 

 commonly found with bacteria. Observing bacteria with the micro- 

 scope under these conditions of high and low salt concentrations of 

 solutions, very little, if any, visible change in cell size can be 

 observed. Bacterial cell walls are more rigid than the walls of 

 blood cells, and this lack of elasticity prevents marked changes in 

 cell size. 



Claude Zobell has shown that marine bacteria adapted to the 

 salinity of sea water (approximately 3.5%) are quite sensitive to 

 lower salt concentrations; in fact, many of them fail to grow below 

 a salt level of 2%. Conversely, a few organisms whose natural 

 habitat is fresh water, will be inhibited by salt concentrations above 

 1%. The water of the Great Salt Lake in Utah is made up of 

 27% salt, yet many bacteria can be isolated from these waters, and 

 these organisms cannot be cultivated in the presence of less than 

 13% salt. Such salt-loving organisms are called osmophilic. 



A point oftentimes not fully appreciated is that the solute as 

 well as the solvent tend to diffuse from the region of high con- 

 centration to one of less concentration. The speed of this particular 

 reaction and the ultimate result are influenced by (1) the dissocia- 

 tion of the solute, ( 2 ) the rates of diffusion of various substances in 

 solution, ( 3 ) the molecular weight of the solute, and ( 4 ) the nature 

 of the membrane through which the diffusion is taking place. 



Sugar in a sealed parchment bag surrounded bv water will build 

 up pressure within the container. The greater the number of 

 sugar molecules, the greater will be the difference between the 



