2o8 PHYSICAL INFLUENCES 



The pressure increases in direct ratio wiih. the number of molecules 

 in -solution. Therefore, a compound with large molecules (cane sugar) 

 will produce a lower osmotic pressure than one with small molecular 

 weight (glycerin) if we compare solutions of equal concentration. 

 The osmotic pressure of protein, starch and peptone solutions can be 

 measured only with the finest instruments, while the pressure of a 30 

 per cent dextrose solution is 22 atmospheres.* 



Plasmolysis. — If a cell is brought into a strong solution of a sub- 

 stance which cannot pass the plasma-membrane, this substance will 

 cause an osmotic pressure and the concentration in the cell being lower 

 than in the medium, the water will pass out from the cell until the pres- 

 sure inside and outside is the same. This causes a shrinking of the 

 protoplasm, while the rigid cell wall keeps its shape. Such plasmolyzed 

 organisms are illustrated in Fig. 67, page 87. 



While plasmolysis is easily demonstrated with the cells of higher 

 plants, microorganisms do not show it so readily. In fact, many bac- 

 teria, like B. subtilis, Bad. anthracis, cannot be plasmolyzed by any 

 concentration of salt in solution. Others, as B. coli, B. fluorescens, 

 react promptly. But even though many are killed, the rest recover 

 from plasmolysis after a few hours, and appear normal. This indicates 

 that the salt passes slowly through the plasma-membrane and thus 

 increases the pressure inside the cell until finally the inside and outside 

 pressure are the same again. 



The fact that many microorganisms show no plasmolysis whatever 

 is explained in the same way. These organisms probably have plasma- 

 rnembranes so constructed that the salts diffuse through nearly as fast as 

 the water. An absolute exclusion of all soluble substances by the mem- 

 brane is impossible since the food can get into the cell only by diffusion 

 through the membrane. 



The resistance of various microorganisms against concentrated 

 solutions depends upon the organism as well as upon the dissolved sub- 

 stance. The sodium and potassium salts of the common mineral acids 

 act upon a culture nearly in proportion to their osmotic pressure, but 

 the potassium salts always retard growth a Httle less than the sodium 

 salts. The effect of salts upon microorganisms is therefore not due to 

 the osmotic pressure only; the chemical constitution of the salts also 

 plays an important rdle. 



* One atmosphere equals the pressure of i kg. per square centimeter or about is pounds 

 per square inch. 



