510 



COLLEGE ZOOLOGY 



Burning 



Bacterial 

 respiration 



CARBON DIOXIDE (CO2) 



Light energy 



Animal 

 respiration 



Green plant 

 respiration 



Animal 

 eats and 



assimilates 

 green plant 



•Green plant 

 builds 



carbohydrates, 

 proteins, fats 



Bacteria 

 and fungi 

 of decay 



Fuel 



\ 



Exci'etion 



\ 



Animal and plant residue 



/ 



Oil 



Death 



Figure 376. The carbon cycle in nature. 



ically to the hemoglobin in our own blood, 

 and which in the presence of sunlight brings 

 about a combination of carbon dioxide and 

 water to form a carbohydrate (glucose). It 

 has been estimated that the amount of 

 energy fixed annually in plants by photosyn- 

 thesis is equivalent to that obtainable from 

 300 billion tons of coal. The carbon dioxide 

 is obtained principally from the air, and 

 water is absorbed from the soil by the roots 

 of the plant. The chemical equation for this 

 may be expressed as follows: 



Carbon 

 Water dioxide 



6H2O + 6CO2 + Light energy + 

 Sugar 

 (glucose) Oxygen 

 Chlorophyl -> CeHiaOa + 6O2 



The sun furnishes the energy required for 

 these chemical combinations. The oxygen 

 that is freed escapes from the tissues of the 

 plant by diffusion. In most plants the glu- 

 cose is converted almost immediately into 

 starch, the form in which food is usually 

 stored in plants. Starch can be made syn- 

 thetically in the laboratory from glucose. 



The carbon in living animals is returned 

 to the environment as a result of fermenta- 

 tion and respiration, and of decay when the 

 animals die, as illustrated in Fig. 376. Dead 

 animals are disintegrated by bacteria with 

 the aid of enymes; carbohydrates become 

 carbon dioxide and water, and proteins be- 

 come carbon dioxide, water, and ammonia 

 or free nitrogen. 



Nitrogen-fixing bacteria 



Plants are of value in another way: with 

 the aid of certain bacteria, they recover 

 nitrogen from the soil and restore it to or- 

 ganisms. Nitrogen-fixing bacteria live in the 

 soil, or in root nodules, which they have 

 stimulated certain leguminous plants, such 

 as clover, and peas, to grow. The num- 

 ber of different kinds of bacteria known 

 to be capable of nitrogen fixation has greatly 

 increased in recent years. Also certain blue- 

 green algae living in the soil of rice fields in 

 India are very active nitrogen fixers. With 

 energy supplied by the glucose resulting 

 from photosynthesis, the bacteria are able to 

 combine atmospheric nitrogen (the air con- 



