MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 57 



is coming through the membrane and causes this increase in volume. 

 Such movement of water through a membrane toward an area of 

 greater concentration of a dissolved substance is known as osmosis. 

 Now let us try to determine the cause of this unequal movement of 

 water molecules through the membrane. 



We must remember that water molecules are moving just the same 

 as the molecules of a dissolved substance. We have a membrane sepa- 

 rating solutions of different concentrations, but there can be little dif- 

 fusion of the sucrose molecules through the membrane because they 

 are too large. Water molecules, on the other hand, can pass through 

 freely because of their smaller size. On one side of the membrane we 

 have 100 per cent water (water with no dissolved substances in it), but 

 on the other side of the membrane we have only 95 per cent water, for 

 5 per cent of the solution is sugar. Hence, according to the principles 

 of diffusion which we have already learned, the water molecules will 

 diffuse through the membrane from the area where they are more con- 

 centrated to the area where they are less concentrated. Of course, mole- 

 cules of water can pass through in both directions, but more water 

 molecules will go through the membrane from one side for the simple 

 reason that there are more water molecules on that side to bombard 

 the membrane. This will cause an increase of volume in the sugar 

 solution, and a force will be generated that will cause a rise in the tube 

 against the force of gravity. With this new concept we can say that 

 osmosis is the diffusion of water through a differentially permeable 

 membrane from an area of greater concentration of water to an area 

 of lesser concentration of water. (It is possible to have osmosis of 

 other liquids, but since water is the primary solvent in living material 

 we use water in our definition.) 



The water will continue to rise in the tube until the pressure of 

 gravity overcomes the osmotic pressure. When the force of gravity 

 becomes great enough it will force the water molecules back through 

 the membrane as fast as they diffuse through by osmosis, and an equi- 

 librium will be established. 



Diffusion and Osmosis in the Cell 



The protoplasm of a cell consists of a series of dispersion systems 

 as we learned in Chapter 3. The inorganic minerals and the sugars 

 exist in true solution, but the more complex carbohydrates, proteins, 

 and some of the fatty compounds exist as colloidal particles. Also, 

 there will be some of the fats in the emulsion state and aggregations 

 of particles of various kinds in suspension. Since the plasma mem- 



