34 ELEMENTS OF BIOLOGY 



copper sulphate per unit of volume of the solution, will be exactly 

 equal in all parts of the container. This is a phenomenon that 

 always occurs when substances dissolve, whether it be a solid dis- 

 solving in a liquid or one liquid dissolving in another liquid. The 

 molecules always pass from a region of higher to a region of lower 

 concentration. So, if the plasma membrane of a cell is permeable 

 to a certain substance, for instance oxygen, whether or not any 

 oxygen actually passes into the cell will depend on how great a con- 

 centration of oxygen is already present. If the concentration of 

 oxygen is less inside the cell than outside, oxygen will diffuse 

 through the plasma membrane and enter the cell. It is likewise 

 true in the case of substances formed within the protoplasm, for 

 example carbon dioxide; if the concentration of carbon dioxide is 

 greater within the plasma membrane than without and the mem- 

 brane is permeable to carbon dioxide, the substance will pass out 

 of the cell so long as this relative concentration on the two sides of 

 the membrane exists. 



Osmotic Pressure. But the plasma membrane is not freely 

 permeable to many sorts of substances. The effect of its obstruc- 

 tion to the exit or entrance of such substances is very important 

 in vital processes. To examine the nature of this effect under the 

 most simple conditions, we may construct a model (Fig. 6). If an 

 animal membrane, for example a sausage casing, is tied over the 

 mouth of a glass cylinder and the cylinder partially filled with a 

 strong solution of cane sugar, then inverted in water, the sugar 

 molecules, being too large, will not be able to pass through the mem- 

 brane from the region of higher to the region of lower sugar con- 

 centration. Water molecules will pass through the membrane into 

 the sugar solution, however, thus obeying the principle set down 

 above and moving from the region of higher to the region of lower 

 water concentration. As the migration of the water molecules pro- 

 gresses, the sugar solution is diluted and the quantity of liquid 



