340 LIFE : OUTLINES OF GENERAL BIOLOGY 



amount of oxidisahle hydrogen. The same is true of proteins. A very 

 interesting case is the respiratory quotient of hibernating mammals 

 (marmots): within tlie body there takes place, it would seem, a 

 conversion of compounds poor in oxygen (fats) into oxygen-rich 

 compounds (carbohydrates), so that a large amount of oxygen is 

 breathed in without the immediate appearance of a corresponding 

 amount of carbon dioxide. The oxygen is, in fact, stored in the 

 form of carbohydrate; for this reason the respiratory quotient falls 

 extremely low, to about 3 : 10, in hibernating mammals. 



A few approximate figures may now be given. Energy is required 

 by the body for three chief purposes, to maintain constant tempera- 

 ture (animal heat), to vaporise water, and to do work — whether 

 external as in moving, or internal as in the beating of the heart. 

 It is not possible to reduce this output of energy, even at rest, below 

 about 2,500 calorics per day for an adult man; this is about the 

 energy yielded by two pounds (one kilogram) of bread and meat. 

 In addition, about 100 grams (quarter of a pound) of protein must 

 be allowed for daily to make good the constant breakdown of the 

 materials of the tissues themselves. 



2. The Chemical Aspect of Nutrition. — The preceding sentence 

 leads naturally on to a consideration of the chemical requirements of 

 the body, as distinct from its energy requirements. The distinction 

 between these two aspects of the question of nutrition is made 

 clearer if the case of the higher plants is considered. These are able, 

 with the help of their green pigment (chlorophyll), to utilise the 

 energy of the sunlight, with which they build up from carbon 

 dioxide and water such compounds as starch and sugars, in which 

 the energy of the sunlight may be said to be stored. But in spite 

 of this independence of a supply of energy-containing food-materials, 

 the plant has certain very definite chemical requirements — it must 

 be supplied with nitrogen-containing compounds and with various 

 inorganic salts. It is evident in the first place that plant and animal 

 alike must be supplied with the chemical elements which enter into 

 the composition of their tissues, since there is a certain amount of 

 inevitable waste, and it is impossible to transmute one element 

 into another. Apart from carbon, hydrogen, nitrogen, and oxygen, 

 many other elements are essential to the life of the body, the most 

 important being iron, sodium, potassium, and calcium among the 

 metals, and sulphur, phosphorus, chlorine and iodine among the 

 non-metals. All these, and probably several others, must be repre- 

 sented to some extent in a comprehensive diet. In some organisms, 

 both plants and animals, there is a constant presence of rarer 

 elements, such as arsenic, fluorine, zircon — a matter that requires 

 further investigation. 



The second feature of the chemical requirements of the body is 

 related to the (limited) power of synthesis which the animal body 



