28 A PRIMER OF BIOLOGY 



plants and animals, as products of their activity 

 when alive, or of their decomposition when dead. 

 We thus appear to have landed ourselves in a " vicious 

 circle " : protoplasm- the essential basis of the 

 living organism can be supported only by organic 

 compounds and yet organic compounds are formed 

 only as a result of the activity of protoplasm. The 

 problem before us, therefore, is, How are the necessary 

 organic compounds originally formed ? How are 

 relatively simple inorganic materials synthesised 

 into food acceptable to protoplasm ? An attempt 

 will be made to answer this question in Chapter VI., 

 meanwhile, let us endeavour to find how " food," 

 properly so-called, is a source of energy or of power 

 to do work. 



A study of dietetics teaches us that an average 

 man doing average work requires, during the twenty- 

 four hours, in round numbers, 140 gr., or about 5 

 oz. of nitrogenous compounds or proteids ; 100 gr., 

 or about 3^ oz. of fat, and 420 gr., or about 15 oz. of 

 such compounds as starch, sugar, &c., which are 

 known to chemists as carbohydrates, because they 

 contain (in addition to carbon) hydrogen and oxygen 

 Daily in the same relative proportions as they occur in 

 [let ' water. Now. it must be at once apparent that, 

 so long as it is alive, an organism, of whatever rank, 

 is constantly doing work whether it be external 

 and visible or internal and invisible. But to 

 do work, energy must be expended and this natur- 

 ally involves a source of energy. How does the 

 organism obtain the necessary energy, and in what 

 form ? 



Energy, so the physicists inform us, occurs in 



Energy, two states or conditions, potential and kinetic. A 



weight resting on a shelf possesses potential! energy, 



