274 



Microbes and You 



atmosphere contains only three parts of carbon dioxide (CO2) in 

 ten thousand (0.03%), it is difficult to conceive of plants and 

 animals existing without this material. 



The equilibrium created by plants and animals with respect to 

 carbon economy is one of those orderly processes we see so fre- 

 quently when nature takes a hand in things. Higher plants re- 



Fig. 50. The carbon cycle. 



quire carbon dioxide in order that they might synthesize sugar 

 from this gas with the aid of sunlight. This building up of energy 

 substances from water and carbon dioxide is catalyzed by the green 

 pigment, chlorophyll, found in higher plants, and the chemical 

 process is called photosynthesis. While it is generally stated that 

 plants take in carbon dioxide and give off oxygen in their metabo- 

 lism, and that animals utilize oxygen and liberate carbon dioxide 

 through respiration, plants also return some carbon dioxide to the 

 atmosphere during their life processes. 



Combustion of fuel— coal, oil, 2;asoline, kerosene, etc.— releases 



