MEMBRANES AND CELL-PROCESSES 139 



both lipoids and proteins, which are probably intermixed or combined 

 in some characteristic manner and vary in their relative proportions in 

 different cells, according to the specific constitution of the latter. 



What are the chief peculiarities in the physical properties of these 

 membranes, on which their physiological importance depends? Two 

 properties appear especially significant. One of these is the semi- 

 permeability which the membranes preserve during life, i. e., the abil- 

 ity to transmit water freely while holding back dissolved substances. 

 The other is their ability to undergo reversible changes in their perme- 

 ability to such .substances, either in the direction of increase or de- 

 crease. These changes of permeability may in some cells be very rapid ; 

 and there is evidence that this is especially the case with irritable tis- 

 sues, and that the power of rapid response to stimuli is directly de- 

 pendent on this peculiarity. How essential the semi-permeability of 

 the plasma-membranes is to living organisms may be realized with 

 especial clearness in the case of plants. In many of these organisms 

 the rate of growth, the normal form and habit, and the characteristic 

 movements and reactions are intimately dependent on the peculiar con- 

 dition known as turgor, which is the expression of the outward pres- 

 sure of the dissolved molecules of the cell-contents against the mem- 

 branes which enclose them and which they can not pass. The diffusing 

 molecules hence press against these membranes, often with the force of 

 many atmospheres, and keep the cellulose cell-walls stretched and rigid. 

 It is on this condition that the maintenance of the normal form often 

 depends. The entrance of the water into the cell in growth is also 

 largely due to this osmotic pressure. Thus the confinement of the mole- 

 cules within the cells by membranes impermeable to their outward 

 diffusion is an indispensable condition of the continuance of normal 

 life-processes in these organisms. The same is true of animal cells, al- 

 though here the condition of turgor is usually unimportant in itself. 

 But, as we have already seen, the preservation of the normal proto- 

 plasmic composition in the case of any cell involves the prevention or 

 restriction of any free or unselected diffusive interchange of materials 

 between the cell and its surroundings. The semi-permeability found 

 during life is the expression of the all-importance of this condition. 

 We must therefore ascribe to the insulatory or semi-permeable character 

 of the plasma membrane, not only the existence of conditions like turgor 

 in plants, but even the very possibility of the existence of a stable or 

 permanent chemical organization in any cell. 



This being the case, it is not surprising to find that simple modifica- 

 tion of permeability may profoundly modify many cell-processes. To 

 take first a relatively simple instance: if the semi-permeability of the 

 plasma-membrane is a necessary condition for continued life in any cell, 

 it ought to be impossible to increase this permeability beyond a certain 

 limited degree for any length of time without inflicting permanent in- 



