272 PRACTICAL PHYSIOLOGY 



CHAPTER I. 

 CARBOHYDRATES. 



Chemical Relationships. These are compounds of carbon, hydrogen, 

 and oxygen, in which the latter two elements usually exist in the 

 same proportion as in water. Their general formula is therefore 



QA.O.. 



Carbohydrates are found chiefly in vegetable tissues, but also occur 

 in animal tissues. They form very important food stuffs, for they are 

 easily digested and assimilated, and moreover are much cheaper than 

 proteins and fats. (See Diet.) The simplest form of carbohydrate is 

 called a monosaccharide, and all other carbohydrates can be broken down 

 into two or more monosaccharide molecules by the chemical process of 

 hydrolysis. When, by this process, two monosaccharide molecules are 

 produced, the carbohydrate is called a disaccharide ; when more than 

 two are produced, the carbohydrate is called a polysaccharide. The 

 monosaccharides and disaccharides being sweet to the taste are 

 together spoken of as sugars, 



I. MONOSACCHARIDES. 



Chemically considered, monosaccharides are either aldehydes or 

 ketones ; the former are called aldoses, the latter ketoses. The aldoses 

 are classed according to the number of carbon atoms in the molecule, 

 e.g. pentose C 5 H 10 5 , hexose C 6 H 12 6 . 



Aldoses. An aldehyde is the first oxidation product of a primary 

 alcohol, and it contains the end group - CHO. 



A primary alcohol is one in which the " OH " or " hydroxyl group " is 

 attached to the last carbon atom of the molecule as in primary propyl 

 alcohol, CH S -CH 2 -CH 2 OH, 



and it contains the end group - CH 2 OH. If, on the other hand, the 

 hydroxyl group be attached to a central carbon atom as in secondary 

 propyl alcohol, ^ _ CHOH _ ^ 



the alcohol is called secondary, and contains the group - CHOH. 



Thus, if ethyl alcohol be heated with potassium bichromate and 

 sulphuric acid, it is oxidised and acetic aldehyde is formed : 



CH 3 - CH 2 OH + O = CH 3 - CHO + H 2 0. 



Ethyl alcohol. Acetic aldehyde. 



