178 - The Cell 



salts may be formed. Other elementary con- 

 stituents (S, P, etc.) are also finally converted 

 to inorganic form (chiefly — S0 4 , — P0 4 , 

 etc.). 



The processes of decay are very important in 

 the general economy of life. Without decay 

 including putrefaction, significant quan- 

 tities of the inorganic substances necessary 

 for the growth of green plants would not be 

 returned to the soil and air. Man also makes 

 use of a number of saprophytic bacteria: in 

 tanning leather, curing tobacco, making 

 cheese, and disposing of sewage. All in all, 

 the benefits that accrue from the saprophytic 

 bacteria far outweigh the harm done in the 

 spoiling of foods and other valuable ma- 

 terials. Many methods have been developed 

 to preserve foods from the action of bacteria; 

 refrigeration retards their digestive and 

 metabolic processes; salting plasmolyzes the 

 cells; dehydration deprives them of adequate 

 water; chemicals kill them; and above all, 

 canning prevents the bacteria from regain- 

 ing access to food after they have been killed 

 by heat sterilization. 



AUXOTROPHIC NUTRITION 



Mixotrophic organisms are those that can 

 carry on more than one mode of nutrition. 

 For example, Euglena (Fig. 10-4) and many 

 other green flagellates can live like typical 

 plants when light is available; but these or- 

 ganisms can also grow saprophytically. Eu- 

 glena resembles a one-celled animal — in 

 having a mouth and gullet, in its ability to 

 propel itself through the water, and in lack- 

 ing a rigid cellulose cell wall. But Euglena 

 also is like a plant, in that it possesses chloro- 

 plasts that enable it to carry on photosyn- 

 thesis. Likewise Euglena stores its excess 

 glucose in the form of starch and utilizes 

 nitrate salts for the synthesis of amino acids, 

 just like typically holophytic organisms. 

 Euglena can live indefinitely without light, 

 provided the surrounding medium contains 

 adequate amounts of organic substances, 

 especially carbohydrates. At the bottom of a 



pond, where there is an abundance of de- 

 composing organic material, Euglena carries 

 on a saprophytic nutrition. Some other green 

 flagellates can also grow holozoically, utiliz- 

 ing the mouth and gullet for the ingestion 

 of organic foods. But the mouth and gullet 

 of Euglena appear to be vestigial and are not 

 capable of fulfilling these functions. 



FLAGELLUM 



GULLET 

 EYE SPOT 

 RESERVOIR 



NUCLEUS 

 NUCLEOLUS 



POSTERIOR END 



Fig. 10-4. Euglena, a green flagellate, displays a 

 mixotrophic kind of nutrition. 



PARASITIC NUTRITIONS 



A parasitic organism lives on or in another 

 organism, deriving food from the living host. 

 Parasitic species are found among all kinds 

 of plants and animals, although the parasitic 

 habit tends to be more prevalent among 

 lower organisms. Moreover, there is scarcely 

 a single species of animal or plant that does 

 not serve as host for one or more parasites. 

 Some parasites, like lice, are ectoparasites, in 

 that they never invade the interior of the 

 host's body; but others, like the tapeworm, 

 are endoparasites, which penetrate into some 

 internal cavity, or into the tissues or cells of 

 the host. 



