82 NUTRITION AND METABOLISM. 



pounds from the inorganic or mineral matter. Chlorophyl is the green 

 coloring substance of plants, and only green plants can use the energy 

 of sunlight for their growth. The growth of green plants is a storing of 

 the energy of light in the form of organic matter; their metabolism is 

 largely synthetic, i.e., building up. Plants without chlorophyl, like mush- 

 rooms, molds, yeasts and bacteria, have to provide for their energy by 

 some other means. 



Animals feed mainly on 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 different 

 from the plant protein and fat. Consequently, complex chemical 

 changes, which require 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 for the energy. Part of the food is used for 

 building the tissue of the body; it becomes part of the animal itself. 

 Animal metabolism is largely analytic, i.e., destructive. More organic 

 matter is decomposed than is formed. Often the same substance 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. The distinction between 

 food for energy and food for growth will be of value in the interpretation 

 of microbial metabolism. 



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 characters 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 almost entirely, since inorganic food furnishes energy only in 

 exceptional cases. In this respect they resemble the animals very much. 



The metabolism of protozoa is furnished by Todd (p. 7 1) as follows: 

 'The ingestion of food is accomplished in some protozoa by pseudopodia; 

 the protozoon simply flows around and so encloses a food particle. In the 



