Other Modes of Nutrition; Conservation of Food Elements - 181 



the conversion of free nitrogen (N 2 ) into the nents of their protoplasm. This kind of nu- 

 nitrate ( — NO s ) form of nitrogen, and this trition, in which metabolism is supported by 

 fixation of nitrogen is an uphill process. Un- energy derived from inorganic oxidations, is 



like most oxidation reactions, the fixation of 

 nitrogen reqtiires the absorption of consider- 

 able energy. As a source for this energy, the 

 nitrogen-fixing bacteria are dependent upon 

 glucose (or other carbohydrate) provided by 

 the root tissues of the host plant. In return, 

 the host receives the excess quantities of ni- 

 trate compounds formed by the bacteria. 



Rich cultures of the nitrogen-fixing bac- 

 teria live in the cytoplasm of the cells of 

 nodulelike swellings that can be seen on 

 the roots of the leguminous plants, such as 

 beans and peas, and upon the roots of other 

 plants, such as alfalfa and clover (Fig. 10-6). 

 Frequently such crops are "rotated" with 

 ordinary soil-depleting crops. The nitrate 

 compounds formed by the bacteria, with the 

 help of the host plant, may be returned to 

 the soil directly, if the crop is plowed under; 

 or the nitrates may be converted into pro- 

 teins that return nitrogen to the soil indi- 

 rectly, via the metabolism of animals that 

 have eaten the protein, or of saprophytes 

 that accomplish their decay. In any event 

 the nitrogen-fixing bacteria accomplish the 

 important function of reclaiming free nitro- 

 gen and restoring it to "circulation" in other 

 forms of life. 



CHEAAOTROPHIC NUTRITION 



called chemotrophic nutrition. Likewise, this 

 kind of synthesis, which may be formulated 

 as follows: 



6C0 2 + 6H 2 

 energy from inorganic oxidations 



chemosynthesis 



C 6 H 12 6 + 60 2 



glucose oxygen 



is sometimes referred to as chemosynthesis 

 (in contrast to photosynthesis). 



Inorganic substances capable of yielding 

 energy through oxidation are not very abun- 

 dant in the environment, and therefore 

 chemotrophic modes of nutrition are re- 

 stricted to a relatively small number of bac- 

 terial species. 



The nutrition of the sulfur bacteria (Fig. 

 10-7) is typically chemotrophic. These bac- 

 teria oxidize hydrogen sulfide (ITS), first 

 into free sulfur (S) that is stored intracellu- 

 lar!}' (Fig. 10-7), and then into sulfate ( — S0 4 ) 

 compounds as further energy may be needed. 

 Other chemotrophic bacteria oxidize free 

 hydrogen (to water), carbon monoxide (to 



The capacity of green plants to synthesize 

 their organic essentials entirely from inor- 

 ganic substances represents the primary 

 source of organic foods utilized by other 

 organisms. However, there are a few kinds of 

 bacteria that also can grow in the complete 

 absence of organic foods. These forms, lack- 

 ing chlorophyll, cannot utilize the sun's 

 energy to support their metabolism. Instead, 

 they resort to the oxidation of certain inor- 

 ganic substances, thus obtaining energy for 

 synthesizing glucose; and this glucose then 

 provides a source of matter and energy for 

 the synthesis of the other organic compo- droplets are free sulfur 



ACHROMATIUM 



THIOSPIRILLUM 



Fig. 10-7. Two kinds of sulfur bacteria. The spherical 



