THE ORIGIN OF FOOD 55 



element is given off in a free, or uncombined, state during 

 the process of starch-making. 



84. Sunlight and life. The process which we have called 

 starch-making is really a group of processes. In some green 

 plants starch is never found, and yet the transformation of 

 carbon-hydrogen-oxygen materials by the sunlight, acting through 

 chlorophyl, goes on in these plants. The common fact in all 

 these processes is that some kind of carbohydrate (usually 

 some kind of sugar) is produced. This process of carbohydrate 

 formation is called photosynthesis, from Greek words meaning 

 "light" (compare photo-grwpti) and "put together." In addi- 

 tion to forming sugar, some plants have a way of condensing 

 the sugar, shortly after it is formed, into starch grains (Fig. 17). 



Without going deeply into the chemistry of photosynthesis we 

 may note that in the making of carbohydrate the energy of the sun- 

 light has practically broken up a combination (CO 2 ) that is ordinarily 

 formed with the liberation of energy. That is to say, through the 

 action of light, carbon and oxygen have become separated so that 

 they are capable of again combining and liberating energy. Carbo- 

 hydrate may thus serve as a source of energy by becoming oxidized, 

 either in the bodies of living things or in a flame. We may thus see 

 that all the energy that plants or animals use as a result of the 

 oxidation of carbohydrates is derived from the sun's energy. There 

 is more than poetry in the statement that every human act is a 

 transformed sunbeam. 



85. Origin of fats. All organic materials appear to be derived 

 directly or indirectly from carbohydrates. It has been found 

 that fats originate in the cells of animals as well as of plants, 

 by a modification of starches or sugars. Fats are characterized 

 by containing a large proportion of carbon and a small propor- 

 tion of oxygen. The chemical process by which carbohydrates 

 are changed into fats is not understood. 



86. Origin of proteins. The foods of the third group, the 

 proteins, consist of very complex substances. They all contain 

 nitrogen, in addition to carbon, hydrogen, and oxygen. Some 



