182 - The Cell 



carbon dioxide), iron salts (ferrous to ferric 

 salts), and so on. These inorganic oxidations 

 provide the energy by which all these species 

 synthesize carbohydrate from CO., and H 2 0. 

 And since they can also utilize inorganic ni- 

 trogen compounds for the synthesis of amino 

 acids and proteins, the chemotrophic bac- 

 teria, like the green plants, are not directly 

 dependent on other organisms for their es- 

 sential nutrients. In fact, both chemotrophic 

 and holophytic organisms are said to be 

 autotrophic, or literally "self-nourishing." 



The Nitrite and the Nitrate Bacteria. These 

 autotrophic (chemotrophic) bacteria are es- 

 pecially important in relation to soil fertility. 

 Much of the nitrogen from decomposing 

 remnants of plants and animals in the soil 

 is liberated by the saprophytic bacteria in 

 the form of ammonia (NH,), or of ammo- 

 nium (NH 4 — ) salts, which are not utilized 

 very efficiently by most green plants. But 

 nitrate bacteria (Fig. 10-8) are present in 

 all rich soil, and these organisms get energy 

 for growth by oxidizing ammonia (or am- 

 monium salts) into nitrite ( — NO.,) salts. 

 Nitrate bacteria (Fig. 10-8), possessing a 



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o^ 



#.o 



% 



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<? 6 

 \ " * ^ 



NITROSOMONAS 



^ 



NITROBACTER 



Fig. 10-8. Nitrite and nitrate bacteria. N/frosomonas 

 derives energy for chemosynthesis by oxidizing am- 

 monia (NH.,), forming nitrites ( — NO.,); Nifrobacfer 

 oxidizes nitrites, forming nitrates ( — N0 3 ). 



somewhat different set of enzymes, also ob- 

 tain energy by oxidizing the nitrite ( — N0 2 ) 

 salts into nitrates ( — N0 3 ). Consequently the 

 reclamation of nitrate nitrogen from its vari- 

 ous other combinations depends upon the 

 nitrite and nitrate bacteria, as well as upon 

 saprophytic organisms. In performing this 

 function, the nitrite and nitrate bacteria gain 

 energy with which to synthesize carbohy- 

 drates and the other organic components of 

 their structure. 



VARIOUS MODES OF NUTRITION: AN 

 OUTLINE AND SUMMARY 



According to their nutritional processes 

 (Chaps. 7, 9, and 10), living organisms may 

 be classified as follows: 



I. Autotrophic. Can synthesize all essen- 

 tial organic components entirely from 

 inorganic substances; therefore not di- 

 rectly dependent upon other organisms 

 for foods. 



a. Holophytic. Utilize light (by 

 photosynthesis) as a primary 

 source of energy: green plants. 



b. Chemotrophic. Obtain energy by 

 oxidizing inorganic substances: 

 sulfur, nitrite, and nitrate bac- 

 teria, etc. 



II. Heterotrophic. Require at least a min- 

 imum of preformed organic com- 

 pounds; therefore dependent upon 

 autotrophic organisms for food. 



a. Holozoic. Obtain organic foods 

 by ingestion, digestion, etc.: most 

 animals. 



b. Saprophytic. Absorb organic 

 foods directly from the environ- 

 ment with or without external 

 digestion: many fungi, including 

 most bacteria, some flagellates, 

 and a very few higher plants. 



c. Parasitic. Obtain food from the 

 bodies of other living organisms, 

 in or on which they live; some 

 species in almost every category 

 of plants and animals. 



III. Mixotrophic. Combine autotrophic 

 and heterotrophic nutritions in vari- 

 ous ways: many flagellates and a few 

 higher plants (for example, insecti- 

 vorous species; see p. 261). 



Holophytic and holozoic organisms have 

 achieved great dominance, and practically 

 all complex organisms are clearly divisible 

 into two great groups: the plants and the 

 animals. But among unicellular and simple 

 colonial forms there are manv borderline 



