54 



ELEMENTARY BIOLOGY 



The machines or instruments directly involved are different 

 from the machines with which we are familiar. Instead of 

 having wheels or levers or other moving parts, these machines 

 are chemical engines, each consisting of a lump of protein 

 with some of the chlorophyl that gives the familiar plants their 

 distinctive color. This chlorophyl is the tool, or transformer of 

 energy, in the food-making process (see Fig. 17, and Fig. 23, 

 p. 70). The chlorophyl-bearing particle is called a chloroplast. 



Carbon dioxid 



Oxygen 



Light 



from 



Oxygen 

 FIG. 17. Starch-making by chlorophyl 



We may think of photosynthesis as taking place in two stages : in the first the raw 

 materials, water and carbon dioxid, are broken up into their constituents carbon, 

 hydrogen, and oxygen ; in the second these elements are recombined into carbohy- 

 drates, and the surplus oxygen is set free. The energy for this chemical process is 

 sunlight ; the transformations are brought about through the action of chlorophyl 



The energy for doing this work is the light from the sun. 

 Although the work cannot go on at too low a temperature, it 

 is the light that is used in the process, and not the heat. 



83. Oxygen a by-product. The starch that is formed from 

 water and carbon dioxid by the action of sunlight through 

 chlorophyl contains the elements found in the raw materials 

 namely, carbon, hydrogen, and oxygen. In starch, as in 

 most carbohydrates, hydrogen and oxygen occur in the same 

 proportions as they do in water. The raw materials taken in 

 by the plant therefore contain an excess of oxygen. This 



