544 INTERRELATIONSHIPS OF ORGANISMS 



upon what they can secure directly or indirectly from the green plants. 

 Many elaborate "food chains" have been developed, in some of which 

 the last and most powerful animal, or the last and most remote parasite, 

 obtains its food at fourth, fifth, sixth, or seventh hand after its original 

 synthesis by the plant. The synthesized molecules may circulate so long 

 as they never are broken below the level of simple sugars or amino acids; 

 but in each organism and at each transference some of them must be 

 broken below this level to provide animal and plant energy. 



Moreover, not all protoplasm is eaten. Many plants and animals die 

 and fall to the ground, where their dead substance would accumulate 

 and remain locked up were it not for the processes of decay carried on by 

 soil bacteria. In these processes the organic substances are broken down, 

 with loss of heat to the soil and atmosphere, and the original elements and 

 inorganic compounds are returned to the air and soil, where they again 

 become available for the carbon and nitrogen cycles. A similar return 

 is also constantly made by t>he respiration and the nitrogen catabolism of 

 all living organisms. 



The soil thus plays a very important part in the energy cycle. It not 

 only maintains a huge population of bacteria, fungi, and other organisms 

 that slowly but constantly oxidize the compounds that have escaped the 

 catabolism of higher organisms, but it also serves as a storehouse for the 

 end products of this decay until they are again taken up by the green 

 plants to enter the synthesis part of the cycle. In fact, a very considerable 

 part of the soil is formed by the decay-producing organisms themselves 

 and the end products of their oxidative processes. 



Some quantitative considerations. The preceding account of the 

 energy cycle has been given largely in qualitative terms. Actually, the 

 whole process is capable of being treated in a quantitative way. The 

 utilization of the radiant energy from the sun to propel all life processes 

 is governed by the same quantitative energy laws that hold for any engine 

 or any physical utilization of power, and the same total energy is freed 

 whether foods are burned under a boiler or oxidized in the animal or 

 plant body. As in the engine, only a part of the energy released by oxida- 

 tion within the organism is convertible into useful work, and the rest 

 is lost as heat. The efficiency of the conversion in human muscle, some 

 20 to 33 per cent of the total energy available for work, compares favor- 

 ably with that of the best existing steam engines. 



Some Quantitative Values 

 1 gram of sugar by oxidation yields 4.1 Calories 



1 gram of fat by oxidation yields 9.4 Calories 



1 gram of protein by oxidation yields 5.6 Calories 

 1 gram of carbon by oxidation yields 8.0 Calories 

 1 gram of hydrogen by oxidation yields 34.5 Calories 



