20 INTRODUCTION TO GENERAL PHYSIOLOGY 



(0.3 m.) as 10 per cent, cane sugar should preserve their normal 

 volume. Such a solution has a concentration of 5.4 per cent., since 

 the molecular weight of glucose is 180, and 180x0.3 = 54 gm. in 

 the litre. If we try the effect of such a solution we shall find it to 

 be equivalent to 10 per cent, cane sugar. There are many organic 

 substances which can equally replace cane sugar in the same 

 molecular concentration. We may say, then, that the molecular 

 concentration of the red blood corpuscles, so far as concerns those 

 substances to which their membrane is semi-permeable, is 0.3 molar. 



But there are others, a solution of urea, for example, which 

 behave apparently just as water does. Is this because the 

 membrane is permeable to urea, as it is to water ? Let us consider 

 what would happen in such a case. For a moment, the number of 

 molecules in equal areas on both sides of the membrane is not the 

 same, but in a very short time urea molecules pass through the 

 membrane, and rapidly become equal in number on both sides, so 

 that there is no longer any difference, as far as urea molecules go, 

 and there is nothing to oppose the inflow of water caused by those 

 molecules which cannot pass through, 



We see how we can utilise the changes in volume of cells to 

 find out whether or not their membranes are permeable to various 

 solutes, remembering, of course, that these solutes must not cause 

 injury to the cell membrane. It may also be pointed out that 

 there is no satisfactory explanation of this behaviour of cells to the 

 molecular concentration of solutions, and not to other properties, 

 other than that they have a membrane around them semi-permeable 

 as regards the particular solute in question. 



As mentioned before, other cells may be used, and a method 

 with plant cells, known as that of " plasmolysis? has played a large 

 part in the investigation of the phenomena. In this method, 

 plant cells containing in a large vacuole inside the protoplasm 

 a coloured solution, "cell sap," are subjected to the action of 

 different solutions. The protoplasm forming a coating inside the 

 cell wall has a membrane of similar semi-permeable nature to that 

 of the blood corpuscles. If a solution of a higher molecular 

 concentration than that of the cell sap be applied, water will escape 

 through the protoplasm, and a space will be formed between it and 

 the cell wall, visible owing to the coloured fluid in the protoplasmic 

 bag. The experiment may be tried with the staminal hairs of 

 Tradescantia (E., p. 171). By testing various strengths of cane 

 sugar solutions, one will be found which is only just sufficient to 

 cause perceptible plasmolysis. The solution of equimolecular 

 concentration to that of the cell is, therefore, a little below this. 



