Other Modes of Nutrition; Conservation of Food Elements - 1 85 



and matter by the nitrite bacteria, which pro- 

 duce nitrites; and the nitrites are used by the 

 nitrate bacteria, which finally restore the ni- 

 trogen to the environment in its original 

 nitrate form. The small quanity of nitrogen 

 that tends to "escape from the cycle" as free 

 nitrogen (N 2 ) is reclaimed by the nitrogen- 

 fixing bacteria. In the roots of leguminous 

 plants, these symbiotic bacteria avail them- 

 selves of preformed carbohydrate for energy 

 to oxidize free nitrogen to its nitrate form. 



SOLAR ENERGY AND THE PERPETUATION 

 OF LIFE 



Each of the elements undergoes a closed 

 cycle of chemical changes as it is utilized in 

 the metabolism of various organisms. Con- 

 sequently the material requirements of liv- 

 ing things will never be exhausted so long as 

 these cycles continue. To keep going, these 

 cyclic changes do not require new matter, 

 but they do require new energy, and this en- 

 ergy comes from the sun. Therefore the 

 perpetuation of life as we know it depends 

 upon the light of the sun. 



The radiant energy absorbed by the chlo- 

 rophyll of green plants is transformed into 

 the potential energy of free oxygen and glu- 

 cose — and indirectly into the potential energy 

 of other organic compounds. All organisms 

 get energy from organic substances and, in 

 the process, eventually convert them back 

 into their original form. The energy liberated 

 by the complete oxidation of a given amount 

 of carbohydrate is exactly equal to that pre- 

 viously absorbed in the photosynthesis of 

 this carbohydrate. All energy transforma- 

 tions whether in animate or inanimate bodies, 

 obey the First Law of Energy. Energy is con- 

 served in that the total quantity of energy 

 remains the same regardless of all transfor- 

 mations. But the energy released by the oxi- 

 dation of carbohydrate does not appear in its 

 original form. Much is liberated directly as 

 heat, and only part is transformed into the 

 chemical potential energy of other complex 

 organic compounds, such as proteins and 



lipids. Eventually all the organic compounds 

 in protoplasm are decomposed. In the catab- 

 olism of various cells, these decompositions 

 give forth many kinds of energy, mechanical, 

 electrical, osmotic, etc. — but eventually all 

 energy liberated in organisms is transformed 

 into heat. Therefore the net result of all ac- 

 tivities in living organisms is the transforma- 

 tion of a certain quantity of energy from 

 light finally into heat. 



The sun is the source of practically all the 

 available light energy on our earth. The sun 

 liberates stupendous quantities of energy, 

 probably by atomic fissions and fusions that 

 occur spontaneously at high temperatures, 

 but only a small fraction of the kinetic en- 

 ergy from the sun falls upon our earth. Of 

 this small fraction the greater part strikes 

 inanimate bodies (rocks, water, soil, etc.), 

 which absorb the light and transform it im- 

 mediately into heat. Only that minute frac- 

 tion of solar energy that happens to fall 

 upon the green plants escapes immediate 

 degradation. But this fraction launches upon 

 a peculiar and roundabout series of trans- 

 formations that motivate the complex ac- 

 tivities of living organisms. Then finally this 

 energy likewise becomes converted entirely 

 into heat. 



Unlike matter, the energy utilized by liv- 

 ing things does not form a closed cycle. This 

 energy continuously runs "downhill" — from 

 light to heat — turning the wheels of life as it 

 flows. As it is formed, the heat is dissipated; 

 it warms the water, the soil, the air, etc., and 

 gradually it escapes into interstellar space. 

 What eventually may become of this dis- 

 sipated energy — whether it may be trans- 

 formed back into some useful form in some 

 unknown part of the universe — we do not 

 know. So far as our earth is concerned, all 

 processes, in living and in nonliving nature, 

 conform with the Second Law of Energy; all 

 forms of energy are continually degraded into 

 heat and dissipated. Therefore the extinction 

 of life on this planet must inevitably occur — 

 if and when the radiant energy of the sun 

 becomes too feeble to maintain a proper tern- 



