ABSORPTION 35 



According to the molecular theory, all matter, whether solid, 

 liquid, or gas, consists of molecules in constant motion. The mole- 

 cules of solids are close together and there is very little space for 

 them to move. There is more space between the molecules of 

 liquids, and still more between those of gases. Consequently, 

 molecules of liquids move more freely than those of solids, and 

 molecules of gases move even more freely than those of liquids. 



As the molecules in gases or solutions keep bounding up 

 and down, knocking against each other and against the cell mem- 

 brane, some of them move through the small openings. If the 

 molecules are very active, as in gases or certain liquids, these 

 molecules can pass readily through the membrane. The mole- 

 cules of denser liquids move more slowly and therefore will take a 

 longer time to go through the membrane. In general, the rate of 

 osmosis of a substance depends upon the number of its molecules. 

 During the process of osmosis the molecules of any substance 

 pass through a permeable membrane more rapidly from the area 

 of greater concentration to an area of lesser concentration. Osmosis 

 never stops, although when there is a balance of materials on 

 either side of the membrane, there may be so even an exchange 

 of molecules that osmosis is not obvious. When the membrane 

 is a living one, as in the cell membrane, the cytoplasm exercises 

 a definite selective action, taking in and keeping in the materials 

 needed by the cell and eliminating those not needed. 



Demonstration. To illustrate the movement of molecules. 



Make a very dilute solution of powdered carmine or India ink in water. 

 Place a drop under the compound microscope and observe under both low and 

 high magnification. 



Describe the behavior of the particles observed in this experiment. 

 According to the molecular theory, molecules are always in a state of motion 

 somewhat similar to that of the particles observed by you. 



Robert Brown, who was the first person to observe the nucleus 

 in plant cells, first described the movement that you have just 



