14 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



(r?) Conversion into dextrin and dextrose. To some starch solution in a 

 flask add a few drops of 25-per-cent. sulphuric acid, and boil for 

 15 minutes. Take some of the liquid, which is now clear, and show 

 the presence of dextrin and dextrose. 



9. Dextrin. Add iodine solution to solution of dextrin ; a reddish-brown 

 colour is produced. The colour disappears on heating and reappears on 

 cooling. 



10. Glycogen. Solution of glycogen is given round : (a-) it is opalescent 

 like that of starch. 



(6) With iodine solution it gives a brown colour very like that given by 

 dextrin. The colour disappears on heating and reappears on cooling. 



(c) By boiling with 25-per-cent. sulphuric acid for 15 to 20 minutes it is 

 converted into grape sugar. 



The carbohydrates are found chiefly in vegetable tissues, and 

 many of them form important foods. Some carbohydrates are, 

 however, found in or formed by the animal organism. The most 

 important of these are glycogen, or animal starch ; dextrose ; and 

 lactose, or milk sugar. 



The carbohydrates may be conveniently denned as compounds of 

 carbon, hydrogen, and oxygen, the two last named elements being in 

 the proportion in which they occur in water. But this definition is 

 only a rough one, and if pushed too far would include many substances, 

 like acetic acid, lactic acid, and inosite, which are not carbohydrates. 

 Eesearch has shown that the chemical constitution of the simplest 

 carbohydrates is that of an aldehyde, or a ketone, and that the more 

 complex carbohydrates are condensation products of the simple ones. 

 In order, therefore, that we may understand the constitution of these 

 substances, it is first necessary that we should understand what is 

 meant by the terms aldehyde and ketone. 



A primary alcohol is one in which the hydroxyl (OH) is attached 

 to the last carbon atom of the chain ; its end group is CH 2 OH. Thus 

 the formula for common alcohol (primary ethyl alcohol) is 



CH 3 .CH 2 OH. 



The formula for the next alcohol of the same series (primary 

 propyl alcohol) is 



CH 3 .CH 2 .CH 2 OH. 



If a primary alcohol is oxidised, the first oxidation product is 

 called an aldehyde ; thus ethyl alcohol yields acetic aldehyde : 



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



[ethyl alcohol] [acetic aldehyde] 



The typical end-group CHO of the aldehyde is not stable, but is 

 easily oxi disable to form the group COOH, and the compound so 

 formed is called an acid ; in this way acetic aldehyde forms acetic 

 acid : 



