VITAL ANIMAL LIFE PROCESSES 65 



Hence, the typical digestive process begins with ingestion and is fol- 

 lowed by digestion, absorption, and egestion. 



Respiration 



Food in itself is of no value to an animal for energy until it can 

 be combined with oxygen. Carbon dioxide is always produced as a 

 result of this oxidation and must be eliminated from the animal body. 

 It is through the very vital process of respiration that oxygen is ob- 

 tained and carbon dioxide given off. Lungs and gills are specialized 

 organs used by higher animals for this purpose. In some animals the 

 skin is a major organ of respiration. Frogs, for instance, have lungs, 

 but they depend upon diffusion through their skin for a large part of 

 their oxygen supply. If the skin of a frog is allowed to dry, thus 

 making it impermeable to oxygen, the frog will die even though the 

 lungs are functioning to their fullest capacity. In the earthworm there 

 are no specialized organs of respiration — the oxygen absorption and 

 carbon dioxide release must all take place through the moist skin. In 

 the one-celled animals and a few of the simpler multicellular animals, 

 diffusion of oxygen into the cell and carbon dioxide out of the cell take 

 place directly from the medium in which the organism lives. 



Water animals obtain their oxygen from the air which is dissolved 

 in the water. Fish will die if they are kept in crowded conditions, for 

 they will use the oxygen in the water faster than additional oxygen 

 can dissolve from the atmosphere. Also, any aquatic animal will die 

 if it is placed in water that has been boiled and cooled without allow- 

 ing time for air to dissolve, because boiling drives air from the water. 

 Some people who raise tropical fish keep air bubbling through the 

 water in their aquariums constantly to insure a plentiful supply of 

 oxygen for the fish and to aid in the removal of carbon dioxide. 



The process of taking oxygen into the body and of giving off carbon 

 dioxide is spoken of as external respiration. The actual utilization of 

 the oxygen within the cell with the production of carbon dioxide as a 

 by-product is known as cellular respiration. We can represent this 

 type of respiration with the oxidation of glucose : 



glucose oxygen enzymes carbon dioxide water 



C 6 H 12 6 + 60 2 ► 6C0 2 + 6H 2 + energy 



This is the basic formula for most energy release within the cell. The 

 complex sugars, starches, and even proteins are broken down into the 

 simple sugar, glucose, before oxidation. Fats may be oxidized without 

 this conversion into glucose, but since fats are low in oxygen, a larger 

 quantity of oxygen is used, with the result that more heat is produced. 



