94 THE HUMAN MECHANISM 



within the cell into simpler substances. Hence there is a 

 constant cellular loss of protein, which in the animal body 

 can be made good only from protein in the food. Plants, on 

 the other hand, have the power of manufacturing proteins 

 from sugars and certain mineral salts, the latter supplying 

 the needed nitrogen and sulphur. The plant kingdom is, 

 therefore, in the long run the sole source of protein food for 

 animals ; for while some animals (carnivores) get their pro- 

 tein entirely by eating the flesh of other animals, the latter 

 (herbivorous animals) in turn have obtained their protein 

 from plants. 



Unlike fats and carbohydrates, protein is an absolute 

 essential of animal diet; that is to say, protein food per- 

 forms certain functions in the animal body which cannot be 

 performed by fats or carbohydrates, while the two latter 

 nutrients perform no functions which cannot also, when 

 necessary, be met by proteins. Some proteins, however, are 

 incapable of meeting all the protein requirements of the 

 organism, although they may meet some of them. Of these 

 the most important in use as food is the fibrous connective 

 tissue (pp. 7, 8), whose fibers in the uncooked state consist 

 of the insoluble protein substance collagen, which by heating 

 in the presence of water is converted into the closely related 

 but soluble gelatin. Collagen and gelatin belong to the 

 albuminoids, one of the subclasses of proteins. The chief 

 protein of Indian corn is similarly incapable of meeting all 

 the protein requirements of the organism. 



6. The group of carbohydrates ; the plant cell as a food 

 factory. The carbohydrates constitute a very large chemical 

 group, although comparatively few members of it (starch 

 and sugars) are of importance as food. They are all com- 

 pounds of the elements carbon, hydrogen, and oxygen, and 

 contain no nitrogen or sulphur; those used as food are 

 manufactured in the cells of green plants. This production 

 of carbohydrates by the plant cell is another example of the 



