278 Nutrients 



Green Plants + O2 



/ / 



H2O + CO2 + Nutrients + Energy / 



'\Animals 



Colorless plants 



Modes of Nutrition 



The various modes of nutrition in the living world may be classified 

 according to the accompanying scheme. 



Autotrophic 



Holophytic (Phototrophic) 



Chemotrophic 

 Heterotrophic 



Holozoic 



Saprophytic 



Parasitic 

 Mixotrophic 



Autotrophic, or "self-nourishing," organisms can synthesize all es- 

 sential organic components entirely from inorganic substances; they 

 are therefore not directly dependent upon other organisms for food. 

 This group composed principally of holophytic forms which use light 

 as a primary source of energy through photosynthesis is represented 

 most prominently by the green plants, although the purple bacteria are 

 also included. The chemotrophic organisms, lacking chlorophyll, are 

 not able to make direct use of solar energy but instead derive their 

 energy from the oxidation of certain inorganic substances. This spe- 

 cialized mode of nutrition is limited to relatively few kinds of organ- 

 isms. Among those encountered frequently in natural environments 

 are the sulphur bacteria, which oxidize hydrogen sulphide first into 

 free sulphur and then into sulphate compounds, and the iron bacteria, 

 which oxidize ferrous salts to ferric salts. 



Heterotrophic organisms require already formed organic com- 

 pounds as a basis for their nutrition and hence are dependent upon 

 autotrophic organisms for food directly or indirectly. The most 

 prominent subdivision of this group is composed of animals that dis- 

 play the holozoic mode of nutrition. These free-living animals char- 

 acteristically ingest solid food and digest it internally. Saprophytic 

 organisms commonly lack a digestive cavity and absorb organic food 

 directly from the environment, usually employing external digestion. 

 Included in this category are most bacteria, many fungi, some flagel- 

 lates, and a very few higher plants. The parasitic mode of nutrition 



