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 

 energ)^, in the food-making process (see Fig. 17, and Fig. 23, 

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



Carbon dioxid 



Oxygen 



• H • H • H • 

 H C H • • C H 



• • • (^* H • • 

 H • H • • H C 

 C H • H C • • 



HH HH HH HH HH HH 



Carbo- 

 hy- 

 drate 



Chloro-jv.A 

 phyl ^'■■-' 



CH« 



eg. 



0h Chloro- 

 '■£/ phyl 



^.Hc^ 



CarbO' 

 hy- 

 drate 



fi 



Water 



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 ligJit that is used in the process, and not the Jieat. 



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 



