82 BIOLOGICAL CHEMISTRY 



directly negative the impermeability of the membrane. If 

 the red blood corpuscles are impermeable to potassium how is 

 it that potassium has accumulated inside the cell so that its 

 concentration in the corpuscles is many times that of its 

 concentration in the surrounding plasma ? * One can assume 

 that the potassium entered the cell during its growth and that 

 the cell membrane only became impermeable at a later stage, 

 but Hamburger and Bubanovic have shown that red blood 

 corpuscles are permeable to potassium and to sodium. f 



Changes in cell permeability are said to occur ; thus the 

 escape of liquid from the pulvinar cells when a leaf petiole 

 droops have been ascribed to an increase in permeability. On 

 the other hand, the escape of liquid has been ascribed to 

 changes in the osmotic concentration of the cell contents as the 

 solution that escapes is too dilute to be some of the cell con- 

 tents which have filtered through a more permeable wall, i 



The absence of a true membrane round some cells is proved 

 by the process of amoeboid movement which cannot take place 

 if there is a fixed structure such as a membrane. Also the 

 fact that it is possible to pass a glass capillary into the cells 

 and to inject substances into the cells without a leak from the 

 cell surface shows that the integrity of the cell does not 

 depend on an intact membrane. 



The second view that the cells consist of a fluid immiscible 

 with the surrounding fluid leads to certain definite comparisons 

 between cells and the behaviour of two immiscible liquids. 



At the junction of the two liquids there is a surf ace tension, 

 and if the specific gravity of the two liquids is nearly the same 

 the suspended liquid assumes that shape which has the least 

 surface for the given volume, namely, it becomes a sphere, 

 and this is the resting shape of a naked isolated cell such as an 

 amoeba. 



Owing to the pull of the surface tension there will be a slight 



T T 



pressure inside the sphere according to the formula P = -f 



r\ ?z 



where P is the pressure, T = surface tension, and r l and r 2 are 

 the radii of curvature of two sectors at right angles to each 

 other (p. 48). 



* B. Moore in Recent Advances in Physiology, edited by L. Hill. 

 Edward Arnold, 1906, p. 147. 



f H. J. Hamburger and F. Bubanovic, Arch. Internal, d. PhysioL, 

 iqio, vol. 10, p. j. 



J V. H. Blackman and S. G. Paine, Ann. of Bot., 1918, vol. 33, 

 p. 69. 



G. L. Kite, Amer. Journ. PhysioL, 1913, vol. 32, p. 146. 



