CHAPTER 5 



Principles of Physiology 



From the discussion of cell metabolism in the preceding chapter, it 

 should be evident that all animal cells are laced with certain common 

 problems. To have survived, each animal— vertebrate or invertebrate, 

 multicellular or unicellular— must have solved, in one way or another, 

 the problems of getting foodstuffs and oxygen, of eliminating carbon 

 dioxide and wastes, of responding suitably to stimuli from the environ- 

 ment, of moving to new areas, and of reproducing its kind. A survey of 

 the animal kingdom will reveal that in the course of evolution an almost 

 bewildering variety of solutions to these problems has arisen. At this 

 point in our discussion, however, we want to emphasize what is common 

 to the physiology and morphology of animals rather than what differ- 

 ences exist. The details of the variety of animal forms will be presented 

 in Chapters 8 to 31. 



25. Types of Nutrition 



Organisms that can synthesize their own foodstuffs are said to be 

 autotrophic (self-nourishing). An autotroph needs only water, carbon 

 dioxide, inorganic salts and a source of energy to survive. Green plants 

 are autotrophs which obtain energy from sunlight for the synthesis of 

 organic molecules, a process known as photosynthesis. Certain bacteria 

 are also autotrophic, obtaining the energy for the synthesis of foods 

 either from sunlight (the so-called purple bacteria are photosynthetic) 

 or from the oxidation of certain inorganic substances— ammonia, nitrites 

 or hydrogen sulfide. No animal is autotrophic; animals obtain their 

 foodstuffs by eating autotrophs, or by eating other animals which ate 

 autotrophs. Ultimately the foodstuff molecules of all animals are syn- 

 thesized by energy obtained by these autotrophic organisms either from 

 sunlight or from the oxidation of inorganic compounds. 



The organisms which cannot synthesize their own food from inor- 

 ganic substances, and hence miist live either by eating autotrophs or 

 upon decaying matter, are called heterotrophs. All animals and fungi 

 (molds), as well as most bacteria, are heterotrophs. Three types of hetero- 

 trophic nutrition are found in the animal kingdom; holozoic, saprozoic 

 and parasitic. 



Holozoic nutrition is the type generally found in animals: food is 

 obtained as particles of some size which must be eaten and digested 



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