142 NUTRITION AND METABOLISM 



is always obtained and always in the same amount regardless of the 

 method chosen in burning the sugar. It has been definitely determined 

 to be 674 calories for i g. molecule (180 g.) of sugar. The complete 

 equation of sugar combustion is therefore written 



CsHiaOe + 12O = ~6C02 + 6H2O + 674 Cal. 



Consequently the same amount of energy will be needed to produce 

 sijgar from carbon dioxide and water; for the law of the conservation 

 of energy requires that, if a certain process liberates a certain quantity 

 of energy, the reverse process will require the same quantity of energy. 

 Green plants get their energy from the sunlight; exactly the opposite 

 proceeds in the equation which should read from right to left; CO2 

 and H2O are absorbed by the plant resulting in the formation of sugar. 

 But it is evident from the equation that CO2 and H2O are not sufficient 

 to produce sugar since it takes 674 calories of heat in addition. The 

 radiant energy of light is transformed by the chlorophyl granules of the 

 plant leaves into chemical energy which causes the formation of organic 

 compounds from the simple inorganic or mineral matter. Chlorophyl 

 is the green coloring substance of plants, and only green plants can use 

 the energy of sunlight for their growth. 



The growth of green plants is a storing of the energy of light in the 

 form of organic matter; their metabolism is largely synthetic, i.e., 

 building up. Plants without chlorophyl, however, like mushrooms, 

 molds, yeasts and bacteria, have to provide for their energy by some 

 other means. 



Animals construct their bodies mainly of organic matter. Their 

 body substances as protein, fat, etc., are derived from the protein, 

 fat, cellulose, etc., of plants or of animals. Nevertheless, a certain 

 amount of energy is required in this assimilation process, since the 

 animal protein and fat are somewhat different from the plant protein 

 and fat. Consequently, complex chemical changes and rearrangements, 

 which require some energy, are necessary for growth. Energy is also 

 lost by radiation of heat and by locomotion. Animals, being entirely 

 unable to use the sunlight as a source of energy, obtain their energy 

 from the digestion of organic food. The larger part of this food is 

 oxidized completely; this part provides for the energy. The smaller 

 part of the food is used for building the tissues of the body; it becomes 

 part of the animal itself. Animal metabolism is largely analytic, i.e., 



