THE FUNCTIONS OF PROTOPLASM AND CELLS 43 



though somewhat improperly, such substances are called colloids. The 

 existence of invisible particles may be detected and they may be counted 

 with the ultramicroscope against a dark background. Some of them 

 may be photographed by means of the electron microscope. Even the 

 fine particles are mostly larger than molecules and so may be composed 

 of more than one substance. Their composition cannot be precisely 

 known, but they must be relatively insoluble in water in order to main- 

 tain themselves as particles. There are indications that the particles 

 are surrounded by a lipoid film, which may have something to do with 

 their insolubility in water. 



This whole structure is, of course, permeated with water, and there 

 are always salts, and usually sugars, in solution. The particles of these 

 dissolved substances, being either ions or single molecules, are much 

 smaller than the dispersed emulsoid particles and confer very different 

 properties on the protoplasm. 



Diffusion and Osmosis. — The molecules and ions of a substance in 

 solution engage in continual spontaneous movement. So do the mole- 

 cules of the water or other liquid in which the substance is dissolved. 

 The particles bombard one another and the walls of the containing 

 vessel if there is one. The direction of movement of individual particles 

 is entirely impredictable. Yet if a substance is more concentrated in 

 one part of a solution than in another, the particles spread more 

 from the place of high to the place of low concentration than in the 

 opposite direction. The spontaneous random movement of the particles 

 in a solution is known as diffusion, and it tends to equalize the concen- 

 tration in all parts. Protoplasm is the scene of constant shifts of this 

 kind. The elimination of the waste product carbon dioxide is effected 

 by diffusion from a place of high concentration in a cell or tissue to a 

 place of low concentration in the surrounding air or water. The entrance 

 of oxygen into the cell is dependent on the same principle. Rapid 

 entrance of water into single-celled animals, requiring its elimination by 

 pulsating vacuoles, is practically simple diffusion. There are many 

 situations where an important physiological process is merely diffusion. 



There are places, however, in which the diffusion of different sub- 

 stances is quite unequal. The membrane of a cell — not the dead wall 

 or the secreted pellicle, but the outer film of protoplasm itself — exercises 

 a selective influence on the passage of substances through it. Some 

 substances pass through it readily, others slowly, still others practically 

 not at all. The membrane is said to be semipermeable. The exchange 

 of particles between two solutions on opposite sides of a semipermeable 

 membrane is known as os77iosis. In general, the gases (carbon dioxide 

 and oxygen) and water pass through a cell membrane rapidly. Simple 

 sugars (glucose), the amino acids (components of proteins), and glycerol 



