FOOD DIGESTION AND RESPIRATION 43 



however, that the pentoses are formed indirectly from the 

 hexoses. 



The simplest " carbohydrate " is naturally that with one carbon 

 atom ; what is this ? If we try to represent in a formula a com- 

 pound of C and H.,O, we find that it must be the following : 



H 



I, 



H C=O, which is known as formaldehyde. Looked at from 

 another point of view, it is an aldehyde group (CHO) combined 

 with H. It is the starting point of a number of hexoses, including 

 glucose, which have the properties of aldehydes, and are hence 

 called " aldoses" Adding further carbons and waters so as to form 

 a chain, they must go between the carbon and one of the hydrogens 

 of formaldehyde, and consist of a series of carbons united to H on 

 the one side, and to OH on the other. Thus : 



HCO 



! 



HCOH 



HO 



HCOH 



:OH 



HCOH 



HCOH 

 H 



Note that there is now an aldehyde group at one end, and an 

 alcohol group at the other end. Aldehydes have powerful reducing 

 properties, taking up oxygen to become acids, so that the CHO 

 group becomes COOH. This latter group is known as " carboxyl," 

 and confers acidic nature on the compounds in which it is 

 present. 



Looking at the general formula given above, we see that one 

 or more of the Hs or OHs may be changed from one side to the 

 other of the central line. Thus a number of different sugars are 

 possible, many of which are known, although of the aldoses only 

 glucose, galactose (in milk sugar), and mannose (a rare hexose) 

 are of use to the organism. The capacity of dealing with the 

 others is absent. Also, to avoid error, it should be pointed out that 

 there is reason to believe that the simple chain formula, as given, 

 is that produced by the action of reagents, and that the usual state 

 of these sugars is in that of a closed ring by union of the aldehyde 

 carbon to the oxygen of the fourth carbon below, the hydrogen 



