EARLY HISTORY OF LIFE 



53 



movement of water molecules through a 

 semi-permeable membrane is called osmo- 

 sis. 



Osmosis can be easily demonstrated by 

 placing a strong sugar solution in a bag of 

 thin skin, such as frog skin, and tying it to 

 a small-bore tube (Fig. 2-24). The water 

 will pass into the bag, or toward the sugar, 

 thus building up a pressure within the bag 

 which will be registered by the rise of fluid 

 in the small tube. If properly constructed, 

 such an apparatus will demonstate a rise of 



significance of osmosis can be demonstrated 

 with red blood cells. These tiny disks have 

 a limiting semi-permeable membrane which 

 encompasses a large variety of particles that 

 constitute the internal protoplasm of the 

 cell. Normally when these corpuscles float 

 in the fluid of the blood, water passes in 

 and out of the cell with equal speed, so the 

 membrane is uninfluenced by the move- 

 ment. Other particles pass in and out, but in 

 so doing there is always an even distribu- 

 tion on both sides of the membrane, that 



• ^ 



:±A 



^^i 



T^ 



"T^ 



water 



sugar 



froi^ sk.in 



ar solution 



water 



Fig. 2-24. An osmometer is made from the skin of a frog's foot into which a strong sugar 

 solution is poured and a small-bore tube attached. A diagrammatic view of why the 

 membrane is permeable to water and not sugar is shown on the left. 



the fluid column to many feet, the height 

 being determined primarily by the effective 

 semi-permeability of the bag. Usually such 

 a preparation is not absolutely semi-perme- 

 able; some sugar goes the other way, that 

 is, after a time some sugar molecules will 

 have wedged their way through the mem- 

 brane to mingle with the uniforai water 

 molecules outside. 



Both diffusion and osmosis are important 

 physical processes in the movement of par- 

 ticles, not only through the membranes but 

 within the cells themselves. The practical 



is, for every particle going inside another 

 comes out, so that the numbers, not neces- 

 sarily the kinds, are approximately the same 

 all of the time. Such a surrounding fluid is 

 said to be isotonic to the corpuscles ( Fig. 2- 

 25). If the corpuscles are now separated 

 from the fluid portion of the blood and 

 placed in distilled water, a very rapid and 

 sudden change occurs. The water moves 

 into the cell because the dispersed particles 

 are greater (less water molecules) inside 

 than out ( where there are more water mole- 

 cules), so the water flows in, causing the 



