200 NUTRITION AND METABOLISM 



Animals construct their bodies mainly of organic matter. Their 

 body substances as protein, fat, etc., are derived from the protein, 

 fat, cellulose, etc., of plants or of animals. Nevertheless, a certain 

 amount of energy is required in this assimilation process, since the 

 animal protein and fat are somewhat different from the plant protein 

 and fat. Consequently, complex chemical changes and rearrangements, 

 which require some energy, are necessary for growth. Energy is also 

 lost by radiation of heat and by locomotion. Animals, being entirely 

 unable to use the sunlight as a source of energy, obtain their energy 

 from the digestion of organic food. The larger part of this food is 

 oxidized completely; this part provides the energy. The smaller 

 part of the food is used for building the tissues of the body; it becomes 

 part of the animal itself. Animal metabolism is largely analytic, i.e., 

 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 othe r 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 

 (anabolism) 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 cell. 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 



