165. Energy Stored In Plant Substance

Back of it all, and this is what interests us, is the animal's food. As a result of years of patient investigation, it has become known that through the combustion of the carbon compounds of vegetable and animal origin, which serve as nutrients, chemical energy may be transformed into those other forms that are manifested in the activities of living beings. When we ask from whence comes the energy given up by the plant compounds, we arrive at our last stage of inquiry. Here we enter the domain of plant life, and it is a notable triumph of the human intellect that we are able to declare with certainty that the ceaseless and multiple activities of life on this planet are sustained by an energy which comes to the plant in the sun's rays through almost limitless space.

166. Energy Unit

It is obvious that if the internal and external work performed by man is sustained by the food, it is desirable to measure the energy available in different foods, provided, of course, that they differ in this respect, as we know they do. In order to measure anything, we must have a standard or unit of measurement. In this case, it cannot be a unit of space or of mass; that is, we cannot declare that wheat flour contains so many cubic feet or pounds of available energy. Energy has neither dimensions nor weight. If we measure it at all, it must be by units of temperature or of work performed. Units of this kind are applied to the measurement of food energy. The one most commonly in use is the calorie, this being the energy which, in terms of heat, is sufficient to raise the temperature of one pound of water 4° F. Expressed in terms of work, the calorie is very nearly 1.53 foot tons, or, in other words, it is equivalent to the work involved in lifting one ton 1.53 feet.

167. Energy Units In Food Compounds

The total energy or heat units developed in the combustion of human foods is determined in an apparatus called a calorimeter. The latest form of this device is one in which food material is burned under pressure in the presence of pure oxygen, and the heat evolved is all used in warming a known weight of water. Data are thus obtained from which it is possible to calculate the calories in the particular material burned. The energy value of single compounds, such as albumin, starch, and sugar, may also be found in the same way, as has been done in a large number of instances. These data show that the heat resulting from the combustion of the compounds of the same class is not the same in all cases. The value in calories of one gram (about one-twenty-eighth of an ounce) of the several nutrients is shown in the following table:

Table XXIII Energy Values Of Food Compounds