42 PRINCIPLES OF ANIMAL BIOLOGY 



to last for months in a bottle; but if a pinch of manganese cUoxide (Mn02) 

 is added, the extra oxygen of the peroxide comes away so rapidly as to 

 produce a froth. The manganese dioxide acts as a catalyst, which is the 

 name applied to inorganic accelerating agents. Now, many living tissues 

 are constantly producing hydrogen peroxide, but it is promptly decom- 

 posed. Something in the cells does what manganese dioxide does in the 

 bottle. That something is called catalase. It is one of many organic 

 accelerators called enzymes. 



For the first time in 1926 an enzyme was isolated, and now some 30 

 of them have been purified. All of these are apparently proteins or 

 protein compounds. Some of them work in the cells; others, as the 

 digestive enzymes, are extruded from the cells and do their work outside. 

 They work best at temperatures of 30 to 40°C., are inhibited by tempera- 

 tures around 50°, and destroyed by prolonged exposure to this tempera- 

 ture. Each enzyme accelerates some particular reaction, and all cells 

 possess a wido variety of these agents. Theoretically an enzyme may 

 accelerate a reversible reaction in either direction, and the direction is 

 dependent on other conditions. Actually, however, the other conditions 

 in living things are usually such that the enzyme works only one way. 

 Som.e enzymes ordinarily break down substances (for example, the 

 digestive enzymes); others build up materials into more complex sub- 

 stances. The destructive type may be extracted and work in about the 

 same way under artificial conditions. Those of the constructive class, 

 however, seldom work outside of cells. Perhaps protoplasm could be 

 manufactured in the lal^oratory if constructive enzymes worked as Avell 

 in test tubes as the analytical or destructive ones do. 



Physical Structure of Protoplasm. — No matter how smooth and 

 structureless protoplasm may look to be in a microscope, it is far from 



homogeneous. In general, it consists 

 of particles of various sizes, mostly 

 very minute, distributed through a 

 supporting liquid substance. In the 

 terms of physical chemistry, proto- 

 plasm is a ''system" consisting of two 

 "phases," of which the particles are 

 '^gQS:C);#BlQ%MlC the "dispersed" phase and the sup- 

 ,, ^ ^^. , , porting liquid is the "continuous" 



i'lG. 27. — Diagram of an emulsion, i i- 



illustrating the physical structure of a phasc. In SO far as thc dispersed 

 very common kind of protofjiasm. particles are liciuid and large enough 



to be visible in a microscope, such 

 a mixture is an emulsion (Fig. 27). If the particles are submicro- 

 scopic in size and liquid, as they usually are, the mixture is an emulsoid. 

 Material in such a finely divided state is also said to be colloidal, or, 



