THE PHYSICAL BASIS OF LIFE 23 



Thus proteins, carbohydrates, and fats represent large classes 

 of substances which are distinctly characteristic of living matter, 

 not being found in nature except as the result of protoplasmic ac- 

 tivity ; although biochemists now can artificially construct certain 

 fats and carbohydrates as well as the amino acid constituents of 

 some proteins. Proteins undoubtedly play the most important 

 part in the organization of protoplasm, while the carbohydrates 

 and fats contribute largely to the supply of available energy. 

 However, it is impossible to draw a hard and fast distinction in 

 regard to their respective contributions because, for example, as 

 we shall see later, carbohydrates form the foundation upon which 

 proteins are built by green plants. 



Proteins, carbohydrates, and fats are frequently referred to as 

 the foodstuffs, but it will be recognized that while, in a way, they 

 constitute the chief groups, all the constituents of protoplasm must 

 be available. Accordingly, inorganic salts, water, and free oxygen 

 are really foodstuffs. Furthermore, recent investigation has dis- 

 closed another class of organic substances which are absolutely 

 necessary and are known as vitamins. These accessory food sub- 

 stances are referred to as vitamin A, B, C, etc., though now the 

 chemical constitution of some of them is known. Scurvy, for in- 

 stance, is a disease induced by the lack of vitamin C which has 

 proved to be a hexuronic acid. Finally must be mentioned a great 

 group of organic catalyzers, called enzymes. These are not food 

 substances but- special proteins formed in organisms where they 

 play a major part in chemical processes. However, when all is said, 

 our knowledge of the chemical complexities of protoplasm affords 

 no adequate conception of how they are related to life. 



3. Metabolism 



We have emphasized that living matter is continually changing, 

 and this fundamental fact is reflected in nearly all attempts to 

 define life. Aristotle described life as "the assemblage of opera- 

 tions of nutrition, growth, and destruction"; deBlainville, as a 

 "twofold internal movement of composition and decomposition"; 

 and Spencer, as "the continuous adjustment of internal relations 

 to external relations." 



This interaction consists of chemical and physical processes in 

 which combustion, or oxidation, plays the chief role. Over a century 

 ago it was shown that animal heat results from a slow burning of 



