CHAPTER FIVE 



RESPIRATION 



1. WE ordinarily think of respiration, or breathing, Adaptation 



, f , . , , rr<i . totheat- 



as the process of taking air into the lungs. 1 he air mos phere 

 we thus breathe consists of various gases, principally 

 nitrogen and oxygen. The oxygen, approximately one 

 fifth of the whole, is the part used in respiration. Al- 

 though nitrogen is an important constituent of proto- 

 plasm, the nitrogen of the air cannot be taken up by 

 the animal body. It serves to dilute the oxygen, and 

 the body is so constructed that the particular mixture 

 forming the atmosphere near the surface of the earth is 

 best suited to its needs. This relationship is called ad- 

 aptation, and it is obviously the body which is adapted 

 to the atmosphere, not the atmosphere to the body. 



2. Respiration, however, does not necessarily re- The use of 

 quire lungs, or any visible process of breathing. It is 

 common to all living beings, cells or individuals, plants 



or animals. Life requires free oxygen, which in 

 animals is obtained from the air. Green plants are 

 able to make sugar or other carbohydrates (containing 

 carbon, hycjrogen, and oxygen) from carbon dioxid 

 (CO 2 ) and water (H 2 O), and in the process free oxygen 

 is liberated. Lower plants, yeasts, and bacteria are 

 able to bring about fermentation, in which oxygen- 

 containing molecules are broken up. Thus, in one 

 way or another, all living cells get access to oxygen, 

 though they may live in the absence of air, as do the 

 anaerobic (Greek, "living without air") bacteria. 

 The higher plants have innumerable stomata (Greek, 

 stoma, "a mouth"), little apertures in the surfaces of 

 the leaves, through which air, containing small amounts 

 of carbon dioxid, is able to enter. 



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