THE PHYSICAL ENVIRONMENT OF ORGANISMS 



543 



of these building-up processes and is, in fact, a waste or by-product of 

 photosynthesis; but it is required for the utilization of the energy stored 

 in food. All utilization of this energy is by means of oxidation, and oxygen 

 is thus the key that unlocks the stored energy for use. As a consequence, 

 all living things require a supply of oxygen and, except for some bacteria 



Fig. 32.1. Diagram of the carbon cycle in the organic world. 



and a few fungi, algae, and parasitic animals, are dependent upon un- 

 combined oxygen, which they acquire by respiration. 1 



The transference of stored energy. Once the plant has built up the 

 higher carbon and nitrogen compounds, all other forms of life crowd 

 in to share the product. Colorless plants, unable to utilize the sunlight, 

 and animals, unable to synthesize any basic food substance, must exist 



1 The role of green plants in maintaining a supply of oxygen in the atmosphere was 

 discussed on p. 171. 



