THE PERMEABILITY OF MEMBRANES 



117 



kind, and the question to be answered is : Must the membrane be of necessity 

 impermeable to electrolytes and other crystalloids, or is it sufficient if it is 

 impermeable to colloids ? It is plain that if the latter alternative is found to be 

 satisfactory, less difficulty will be found in imagining an adequate structure. 

 It will be remembered that no artificial membrane is known as yet semi-permeable 

 as regards potassium chloride, for example, to which the cell is usually semi- 

 permeable, but important steps have been taken already in this direction as 

 mentioned above (page 115). 



The chief evidence may be grouped conveniently under three heads : (1) The 

 phenomena of changes in volume and internal pressure under the action of 

 solutions of various concentrations. (2) The difference between the cell and the 

 surrounding medium as regards presence and concentration of crystalloids. 

 (3) The resistance of living cells to the passage of electrical currents through 

 them. 



1. When cells or blood corpuscles are placed in solutions of crystalloids of 

 various concentration, it is found that in the case of most of these, provided that 

 they do not injure the cell, there is a particular 

 concentration in which no change of volume of 

 the cell occurs. With solutions of a greater 

 strength than this, a shrinking takes place, and 

 with weaker solutions, a swelling. On the theory 

 that these results are of osmotic origin, the solu- 

 tion which causes no change is called " isotoriie,*' 

 and the others "hyper- and hypo-tonic" respec- 

 tively. But the matter is not quite so simple as 

 it might appear at first. The word "isotonic" 

 implies that the solution which causes no change 

 in the volume of the cells has the same osmotic 

 pressure as the normal contents of the cell. How 

 far this is true depends on the permeability of 

 the membrane, as the following considerations 

 will show. Suppose that we have a 5 per cent, 

 solution of sugar enclosed in a bag of an elastic 

 membrane, which is permeable to water, but 

 impermeable to sugar, and that this is immersed 

 in water. Water will enter the bag, which will 

 be distended and probably ruptured, unless sup- 

 ported by an outer envelope, such as the cellulose 

 wall of plant cells. The pressure developed 

 when the cell is not allowed to increase in 



volume is the full osmotic pressure of the sugar solution. The tense condition 

 of the cell hereby produced is known as "turgor," and is the normal state of 

 the plant cell, enabling the stems of the higher plants to remain rigid and erect, 

 as long as the cell membranes retain their semi-permeable properties. That very 

 considerable pressures do exist within plant cells is obvious from consideration 

 of the growing cambium layer between the wood and the bark of a tree. Growth 

 takes place at this situation, so that the wood is continually being increased in 

 diameter ; it is clear, therefore, that the bark must have an enormous stretching 

 force being continually applied to it, and that the growing cells must be exposed 

 to great pressure, which would crush and kill them unless opposed by an equally 

 great pressure within them. The stretched state of the bark can be seen by 

 removing a ring of it, after cutting it through at one place. If it be then replaced 

 in position, it will be found that the ends cannot be made to meet, 

 represents this fact. From the tension required to stretch the bark to its 

 original length, the pressure exerted on the cambium cells can be calculated I 

 is common to find in plant cells pressures as high as 15 atmospheres. Now, 

 pressures of this order can only be maintained either by osmotic forc.es or by 

 imbibition. The construction of a plant cell, with its inner cavity of 

 surrounded by a protoplasmic membrane, suggests at once an osmotic machine, 



Fir:. 45. To SHOW THE PRESSURE 



EXERTED ON GROWING CAMBIUM 



CELLS. Cross section, slightly 

 enlarged, of an internode of a 

 Holly branch, from which the 

 bark has first been removed and 

 then replaced around the woody 

 core. A very great tension is 

 required to make the ends meet 

 again at r. 



