FOOD DIGESTION AND RESPIRATION 65 



an alcohol). Glycerol has three alcohol groups united together by 

 loss of hydrogen from the middle one : 



CH 2 OH 



CHOH 



CH 2 OH 



The acid in an ester may be either organic or inorganic, and the 

 combination takes place with the elimination of the elements of 

 water. Ethyl alcohol and acetic acid unite thus : 

 C 2 H 5 (aHT ~H)COOCH 3 



forming what is often called ethyl acetate ; but since it is not a salt, 

 its correct name is acetic ethyl-ester. The acidic properties of the 

 acid have disappeared, although not by combination with a base. 



Ordinary fats and oils are esters of glycerol with fatty acids of 

 a large number of carbon atoms. Thus, olive oil or olein is the 

 tri-glyceride of oleic acid, having three oleic acid residues united to 

 the alcohol. Oleic acid contains eighteen carbon atoms, while the 

 stearin of mutton fat is the corresponding ester of an acid also 

 containing eighteen carbon atoms. The well-known difference in 

 their properties is due to the smaller number of hydrogen atoms in 

 oleic acid, some of the carbon atoms being unsaturated. Reference 

 will be made to this again later. 



There is an interesting group of complex fatty substances called 

 " lipines," which form important constituents of cells, although their 

 function is not altogether clear. One of their uses is doubtless to 

 take part in the production of the cell membrane, but there must 

 also be some more distinctively chemical part to be played by 

 them. Probably they take some share in the oxidative mechanisms. 

 Some of these contain phosphorus and nitrogen, such as lecithin, 

 which may be regarded as phosphoric acid combined with a fat 

 and an organic nitrogen base. Others are devoid of phosphorus, 

 but contain fatty acid, a nitrogen base and a sugar, and are found 

 chiefly in the brain. 



The relative proportion of the three kinds of constituents of a 

 diet may be varied to a large extent without injury to the healthy 

 individual, so long as the energy value is not decreased. The 

 Royal Society Food Committee recommends the following as the 

 basis of calculation for the food supply of a nation : 



Protein - 70 gm. = 280 calories 



Fat - 90 = 8 10 



Carbohydrate - 550 =2,200 



Total =3,290 calories 



per day per 7o-kilo man, doing moderate work. 



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