METABOLISM. 



INTEGRATION OF THE CHEMICAL PROCESSES. 

 CONSTITUENTS OF A CELL. 



A living cell consists of a mixture of organic and inorganic sub- 

 stances dissolved or suspended in water ; water makes up about three- 

 quarters of the total mass of the cell, the other constituents about one- 

 quarter. 



The twelve elements : carbon, hydrogen, nitrogen, oxygen, sulphur, 

 phosphorus, chlorine (iodine), sodium, potassium, calcium, iron and 

 magnesium enter into the composition of the various compounds which 

 compose the material of all living organisms ; of these, only oxygen 

 and nitrogen exist in the free state dissolved in the liquid. 



The inorganic compounds are water and salts, chiefly the chlorides, 

 sulphates and phosphates of sodium, potassium, calcium, magnesium. 



The organic compounds are very numerous in their variety. 

 Proteins, carbohydrates, fats and nucleins are the essential substances, 

 also the most complex, t Alcohol, lactic acid, amino acids, urea and 

 purines represent stages in the processes of their building up and 

 breaking down, i.e. in their synthesis and decomposition, or anabolism 

 and catabolism. 



LIFE-CYCLE OF THE ELEMENTS. 



During the life processes of animals and plants an interchange of 

 inorganic into organic and of organic into inorganic compounds occurs. 

 The change of inorganic into organic is effected by the plant from the 

 carbon dioxide in the atmosphere and the ammonia and salts in the soil. 

 The change of organic into inorganic is effected by the animal. The 

 organic compounds synthesised by the plant enter the animal as food 

 and are converted into carbon dioxide which leaves the body by the 

 lungs, salts, urea and other simple organic compounds which leave 

 the body in the urine. The faeces contains matter which has not been 

 utilised by the animal. The urea and the other organic compounds 

 in the excreta are finally broken down into carbon dioxide, ammonia, 

 etc., by bacteria. There is thus a complete life-cycle of the elements 

 and a balance-sheet can be made. 



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