INTRODUCTION 143 



destructive although a limited amount of energy is required for the 

 chemical changes and molecular rearrangements which are essential 

 to animal tissue formation — a synthetic process. Accordingly more 

 organic matter is decomposed than is formed. Often the same sub- 

 stance can serve both purposes; the meat eaten by a dog furnishes, to 

 it energy as well as material for growth. In other cases, certain food 

 compounds execute only one function and not the other. This dis- 

 tinction between food for energy and food for growth must also enter 

 into the interpretation of microbial metabolism. 



It might appear from this discussion that energy is needed only by 

 growing cells, as the full-grown cells do not increase in size or weight 

 or number. They also need energy, for in all living cells, there is 

 noticed a continuous breaking down {katabolism) and rebuilding 

 (anaboUsm) of the cell constituents. This process is commonly called 

 metabolism. The katabolic processes (the breaking down) in a cell 

 will continue even if the cell receives no food. The cell loses in weight, 

 and the starvation which follows will ultimately result in the death of 

 the tell. All living cells require food for the maintenance of life. 



In the first part of this book, microorganisms have been divided 

 into plants and animals, but attention has been called in various places 

 to the fact that it is often hard to determine whether the plant char- 

 acters or the animal characters prevail. This holds true not only 

 with the morphology, but also with the physiology of microorganisms. 

 Since none of the plants discussed in this text-book possesses chlorophyl, 

 none of them can use light as a source of energy, therefore they depend 

 entirely upon chemical energy obtained by the digestion of food. This 

 means that they require organic food ^Imost entirely, since inorganic 

 food furnishes energy only in exceptional cases. In this respect they 

 resemble the animals very much. 



The metabolism of protozoa which in some respects calls for dif- 

 ferential and special treatment is furnished by Todd and Tyzzer as 



follows: 



"The ingestion of food is accomplished . in some protozoa by 

 pseudopodia; the protozoon simply flows around and so encloses a food 

 particle (Fig. 99). In the same way, these protozoa flow away from 

 waste particles which are to be eUminated. Other protozoa have defi- 

 nite mouth areas for the ingestion of food, and definite anal areas for 

 the discharge of residual material. Those protozoa which ingest soUd 



