754 General and Applied Biology 



from the rather rigid, resistant cell wall. Such shrinking of protoplasm 

 from the cell wall or membrane during the loss of water is called plas- 

 molysis (Gr. plasma, form; lysis^ loose). A solution with less concentra- 

 tion (more water) than the protoplasm, and which places water into the 

 protoplasm of the cell, is known as a hypotonic solution. In this case 

 the addition of water to the protoplasm causes a condition known as 

 turgor (L. turgeo, to swell). If carried to extreme, the cell may be 

 destroyed. A solution which has the same concentration as the proto- 

 plasm and which neither withdraws nor adds water to the cell is known 

 as an isotonic solution. In this case pressures are equal on both sides of 

 the cell membrane and there is no shrinking or swelling. There can be 

 no passage of materials to or from a cell under such conditions. It is 

 quite clear that hypertonic and hypotonic solutions around a cell deter- 

 mine the passage of materials out of the cell and into the cell. The 

 securing of foods and the elimination of wastes probably are accom- 

 plished in this way. 



Diffusion and Conduction 



Diffusion may be defined as the movement of two kinds of molecules 

 in a solution, gas, or solid whereby the molecules of each kind tend to 

 be uniformly distributed in all parts of the substance. Molecules always 

 pass from a region of high concentration to a region of lower concen- 

 tration. The molecules of a gas may diffuse through another gas or 

 through a liquid. One liquid may diffuse through another. A solid 

 may dissolve and then diffuse through a liquid. A crystal of copper 

 sulfate in water will go into solution and then diffuse through the water 

 until there is a uniform distribution of copper sulfate molecules. In this 

 manner, by diffusion, the molecules of gases, liquids and solids taken in 

 through the cell membranes are made available to the protoplasm of 

 the entire cell. 



The firmness or solidity of matter is determined by the distance 

 which the molecules can travel without colliding with another molecule. 

 In liquids the molecules are attracted and usually cannot escape from 

 each other because of cohesive force. The application of heat over- 

 comes this cohesion and the molecules escape in the vapor. Some liquids 

 are very volatile and vaporize easily on contact with air. Gas molecules 

 have no cohesion and can move freely throughout another gas. 



Materials may be conducted from one cell to its neighbors, or they 

 may be conducted great distances. In the latter case they may be 

 rather quickly transported by the transporting system of the organism 



