CHAPTER VII 



INTER-CONVERSION OF CARBOHYDRATES, FATS 

 AND PROTEINS 



IN Chapter VI we discussed the reduction of carbon 

 dioxide and water to form carbohydrate. From carbo- 

 hydrate plants obtain sufficient energy to build up fats 

 and, with the addition of nitrogen, to form protein. We 

 know these facts because plants can be grown without a 

 supply of fat or protein, yet these substances are found in the 

 grown plant in larger amounts than were present in the seeds. 

 In the case of animals the experiments of Lawes and Gilbert 

 proved that fat can be formed from the carbohydrate of the 

 food.* The formation of protein requires a supply of amino 

 acids, so we have to deal with the problem of how ammo-acids 

 can be formed from carbohydrate. Experimental evidence 

 to be given later (Chap. X) shows that mammals cannot 

 synthesise certain amino acids, and that these must be 

 furnished ready-made in the food, but that other amino acids 

 may be formed in the body. We do not know whether all 

 animals are incapable of synthesising the same amino acids 

 which cannot be formed in mammals, but for the present we 

 need consider only the synthetical problem in its broadest 

 aspect. 



By the conversion of carbohydrate into fat a more con- 

 centrated supply of energy is produced. One gram of 

 carbohydrate furnishes 4-1 Calories, whilst one gram of 

 fat furnishes 9-3 Calories. Therefore, as a minimum, 2-27 

 grams of carbohydrate are required to furnish the energy 

 value of one gram of fat or 



2-27gr. carbohydrate + oxygen = igr. fat + carbon 



dioxide and water. 



It is not impossible that other forms of energy might be used, 

 but carbohydrate is the coin of exchange, and its value is 

 required even if the energy for fat synthesis should be shown 

 to be obtained directly from sunlight. 



* J. B. Lawes and J. H. Gilbert, British Association Reports, 1852* 

 PP- 323-353. 



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