56 



MOVEMENTS OF MATERIALS INTO AND OUT OF THE CELL 



the cells in large quantities, they cause the cell membrane to disintegrate 

 with resultant death of the cell. 



Osmosis 



Osmosis is a term used to describe a special type of diffusion of 

 water through a differentially permeable membrane. An illustration 

 will help to explain this process. Let us place a 5 per cent solution of 



r\ 



<& 



W 



3 



-Thistle tube 



-Rising sugar 

 solution 



Sugar solution 

 Water 

 Membrane 



■Movement of 

 water 



Fig. 4.3. Osmosis. This apparatus demonstrates the movement of water through a 

 membrane by means of osmosis when there is a difference in concentration of dis- 

 solved substances on the two sides of a differentially permeable membrane. 



sucrose (table sugar) in water into a membrane which is permeable to 

 water, but not readily permeable to the sucrose molecules. Such a 

 membrane can be prepared by spreading a thin film of collodion in a 

 beaker and allowing it to dry, or we may use some natural membrane 

 such as the bladder from a pig or the skin from the leg of a frog. 

 The membrane containing the sugar solution is then suspended in a 

 jar of plain water as shown in Fig. 4.3. Within a few minutes it can 

 be seen that the volume of the solution within the membrane is in- 

 creasing, for the mixture will begin to rise in the open tube. Water 



