CHAPTER XI 

 RESPIRATION 



THE great importance of gaseous exchange lies in the 

 liberation of energy by oxidation. All organic com- 

 pounds can take up oxygen, and are finally converted 

 into carbon dioxide and water. During the process of oxidation 

 in living organisms a certain amount of energy is rendered 

 available for biological processes. The maximum amount of 

 energy is shown by the amount of heat liberated when the 

 substance is completely oxidised, but only a certain proportion 

 of this is available energy, the remainder being lost. It is 

 usual to designate the factor energyused for externaj work 



total energy 



as the efficiency of the process. As mentioned in the preceding 

 chapter the carbohydrates and fats yield the same amount 

 of energy on oxidation in the body as when oxidised outside 

 of it, but the proteins yield less owing to a certain amount 

 of the energy being carried away in incompletely oxidised 

 substances such as urea. 



Unicellular organisms can obtain a supply of oxygen and 

 get rid of carbon dioxide by diffusion between themselves 

 and their surroundings, but multicellular organisms require 

 some means of transport whereby the gases can be transferred 

 to and from the cells. 



In most animals the gases are transferred in solution by 

 some form of circulatory mechanism, the simplest of which 

 consists of an exchange of water either by rhythmical move- 

 ments or by ciliary action. When a closed circulatory 

 mechanism has been developed the gases are exchanged in 

 one organ (gill or lung), they are transferred by the circulatory 

 mechanism and used by the cells. Thus there are three 

 processes to consider, external respiration, transport of gases 

 and internal respiration corresponding respectively to the 

 division mentioned above. 



EXTERNAL RESPIRATION 



In plants the gas exchanges take place through minute 

 openings. On the under surface of the leaves are stomata 



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