COMPARISON OF ANIMAL AND PLANT BIOLOGY 125 



All the above applies to our human food-supply, for our 

 dependence upon plants as food-producers is exactly the same 

 as that of all animals. We may eat food from plants, or we 

 may eat meat from animals which acquired their store of 

 foods from plants. So, directly or indirectly, all our food 

 comes from plants and depends upon the action of sunlight 

 in the photosynthetic processes of green plants. 



113. Oxygen Required by both the Animal and the Plant. 



Oxygen is necessary in all living cells, because oxidation 

 of cell-substances, chiefly absorbed foods, goes on con- 

 tinuously as long as the animal or the plant lives. In other 

 words, oxidation in cells is inseparably connected with life- 

 activities. In both animals and plants there are special 

 breathing mechanisms for getting the required oxygen from 

 the air and for distributing it throughout their bodies. (De- 

 scribe these mechanisms in frog and bean plant.) The 

 nitrogen of the air seems to play the part of a diluting sub- 

 stance, for oxidation goes on too rapidly when an animal or 

 a plant is placed in a jar of pure oxygen. Animals require 

 more oxygen in proportion to weight than do plants, because 

 in animals life-activities (especially movement), are more 

 intense, and oxidation is more rapid. 



114. Metabolism Occurs in Both Animal and Plant Cells. 



In each case foods which reach the cells may be oxidized 

 to furnish energy, or may be used in forming new particles 

 of protoplasm (repair and growth), or may be stored until 

 needed. Plant cells require less food than do animal cells 

 for supplying energy and for repairing protoplasm; and 

 hence more is available for growth and for storage. This 

 enables plants to grow rapidly and to store up great quantities 

 of foods (sugar, starch, proteins, oils). Animals take ad- 

 vantage of this food stored by plants. The great difference 

 between animal and plant metabolism is that plant proto- 

 plasm can make such foods as sugar, starch, oils and proteins 

 from simple materials like carbon dioxide, water, and nitrates. 



