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PRINCIPLES OF ANIMAL BIOLOGY 



and fatty acids (components of fats) pass through slowly. The ions of 

 inorganic salts, and the disaccharides (sucrose, etc., page 40) penetrate 

 the membrane very slowly, and the proteins, polysaccharides, and fats 

 practically not at all. For some of these substances the inability to 

 traverse the membrane is explained by the large size of their particles. 

 For the ions of salts it is probal)ly their electric charges which keep them 

 out. The cell membrane itself has a charge, usually negative, which 

 repels ions of like charge; and since the oppositely charged ions cannot 



part company, both are excluded. There are 

 probably other reasons, not yet understood, for 

 the retardation of passage of particles through 

 membranes. 



The result of osmosis is easily illustrated by 

 tying a piece of bladder tightly over a thistle 

 tube, filling the tube with sugar solution, and 

 immersing the expanded end in a dish of piu-e 

 water (Fig. 28). After a short time it is found 

 that the sugar solution in the tube has risen to a 

 higher level, but that it is not so concentrated 

 as at first. Water has obviously passed through 

 the bladder into the sugar solution. A little, 

 but not much, of the sugar has also found its 

 way through the membrane into the Avater. 

 The molecules of water are in constant motion, 

 striking the walls of the dish, the membrane, and 

 other molecules of water. Their impacts against 

 the membrane drive some of them through. 

 Now the water inside the thistle tube is also in 

 motion, and some of its molecules pass out into 

 the water of the larger vessel. But there are 

 fewer molecules in a given volume of the sugar 

 solution because the sugar molecules take some 

 of the space, and their movement is less vigorous 

 owing to hindrance by the sugar. Hence fewer 

 molecules of water get out /of the thistle tube than would do so if the 

 sugar were not there. Water is thus passing through the membi-ane 

 in both directions, but more of it goes toward the sugai- solution than 

 away from it. The sugar solution thus rises in the tul)e but becomes 

 more dilute. 



Surface Phenomena. — An important consequence of the colloidal 

 structure of protoplasm is the enormous surface exposed by the dis- 

 persed particles. Extremely finely divided ])articles present a greater 

 surface relative to their volume than do larger particles. This great 



Fig. 28. — Diagram of 

 apparatus used to illus- 

 trate osmosis. T , inverted 

 thistle tube covered with 

 animal membrane and con- 

 taining a solution of sugar 

 in water; V, vessel of 

 water. 



