6o ELEMENTS OF BIOLOGY 



forms an important part of protoplasm has been mentioned several 

 times. In respiration water also plays an important role, for all 

 cells, even the brain cells of Man, are aquatic and receive their 

 oxygen and discharge their waste products, including carbon di- 

 oxide, by means of an aqueous medium. 



For the amoeba, as for all other animals, including Man, the food 

 materials taken in contain three general types of available sub- 

 stances: These are the carbohydrates, the fats, and the proteins. In 

 animal cells the carbohydrate is the most easily and most com- 

 pletely oxidized and when so oxidized such substances yield exactly 

 the same quantity of energy as when the same substance is burned 

 in a calorimeter. Exactly the same number of CO2 molecules are 

 given off as there are O2 molecules taken up. 



CoHioQe + 6O2 = 6CO2 + 6H2O. 

 In other words, in the utilization of a carbohydrate, for ex- 



CO2 



ample, grape sugar, the ratio -:— is always i. This value is 



called the respiratory quotient. Proteins are less completely oxi- 

 dized than are carbohydrates, hence their respiratory quotient is less 

 than I. Still lower ratios hold for fats, which, in general, are less 

 completely oxidized than are the proteins. The determination of the 

 respiratory quotient in an animal, or in a process, enables one to 

 understand approximately what sort of material is providing the 

 energy. For example, if an athlete while running is known to have 

 a ratio of carbon dioxide given oflf to oxygen taken in of i, or 

 nearly so, it may be concluded that the metabolic process involved 

 in running is the oxidation of a carbohydrate. No such determina- 

 tion has been made on amceba but data on the respiratory quotient 

 of many animal activities are available. 



Excretion. When a carbohydrate is oxidized in the amceba, 

 or in any cell, the products are: energy, utilized by the cell; water, 

 also used in the cell or else diffusing out; and carbon dioxide, a 

 gas dissolved in the cell liquids. When fats and proteins are oxidized 



