I 2 8 PHYSIOLOGY OF NUTRITION 



much greater for these small openings than would be the case if it were proportional to 

 the areas of the cross sections of the openings. 



4. Diffusion of Dissolved Substances. — Solute particles diffuse outward in the 

 solvent, much as do gas particles in space, and tend to become equally distributed 

 throughout its volume. Solute diffusion does not extend beyond the spatial limits of 

 the solvent. Mass streaming or convection accelerates or retards the apparent rate 

 of diffusion, just as is true for gases. Solute and solvent particles attract each other. 

 If pure water is separated from an aqueous solution by a septum permeable to both 

 solvent and solute, diffusion of both substances occurs through the septum and a 

 uniform solution on both sides finally results. If the septum is permeable only to the 

 solvent (water), then diffusion takes place only in the direction from solvent to solu- 

 tion, and osmotic pressure is developed in the latter. This is like gas pressure in 

 many respects, being proportional to the outward-diffusing tendency of the solute 

 particles. Salts dissociate or ionize to some extent in solution, and the osmotic 

 pressure that can be developed by a given solution (its osmotic value) is nearly 

 proportional to the total number of particles contained in a unit of volume; more 

 precisely, it is proportional to the quotient of the number of solute particles present 

 divided by the total number of particles (solvent and solute). 



When a septum — such as the outer surface of the protoplasm of a cell — separates, 

 two different aqueous solutions, each containing many kinds of solutes as well as 

 water, the septum may retard the diffusion of water or that of any of the solutes, but 

 its presence does not render diffusion any more rapid than it would be if the septum 

 were not present. Retardation may be greater for some substances than for others. 



Plasmolysis is the tearing of the protoplasmic lining away from the cell wall, 

 frequently due to the presence of more non-permeating solute particles, per unit of 

 volume, on the outside of the protoplasmic periphery than on the inside. Turgor 

 results largely from the reverse condition, being generally due to osmotic pressure 

 developed within the cell, by solutes to which the protoplasm is impermeable. This 

 results in the protoplasm being pushed outward against the cell wall, which becomes 

 stretched. 



5. Absorption of Dissolved Substances. — Most dissolved substances diffuse 

 through cell walls rather rapidly, but the protoplasm is frequently impermeable to 

 many solutes that are present and it retards the inward (or outward) diffusion of 

 others. The permeability of the protoplasm of a cell to the various solutes within and 

 without, alters from time to time, according to conditions in the surroundings and 

 within the cell. Carbon dioxide and oxygen (and other gases in the air) pass into 

 solution in the water, etc., of cell walls and then diffuse as other dissolved substances. 

 These materials, and also salts, etc., dissolved in the soil solution, diffuse through the 

 cell walls and protoplasm of roots. (It appears that they may also be carried in by 

 mass streaming when the transpiration rate is high; if this occurs, the peripheral cells 

 of the roots may act somewhat as filters, allowing the soil water and some of its solutes 

 to enter with the stream but causing other solutes to remain outside or to enter more 

 slowly than do water and the solutes that penetrate the membranes readily.) 



A solute may accumulate in the interior of a living cell until its concentration there 

 is higher than that in the solution from which it diffuses. This phenomenon is some- 

 times to be explained on the ground that the accumulating solute is chemically altered 

 upon passing into the cell, in which case it only apparently surpasses the concentration 

 of the solution from which it comes. In other cases the physical explanation is still 

 uncertain. The osmotic value of cell sap is generally between two and six atmos- 



