THE CARBOHYDRATES AND THEIR METABOLISM 229 



chloral, thymol or phenol, are excreted in the urine of the carnivora and 

 herbivora as conjugated gluctironates. These glucoside r> compounds serve 

 as a means of removing injurious substances from the body. In the 

 plant kingdom, glucuronates have also been found frequently, e. g., in 

 the sugar beet.) 



As one would expect of ketones, the ketohexoses do not yield acids 

 containing the same number of carbon atoms on oxidation. The molecule 

 divides at the ketone group. 



Monosaccharides, and many disaccharides and trisaccharides, are oxi- 

 dized in acid solution, forming products similar to those formed by the 

 action of alkali, but the oxidation occurs much less readily. 



These reducing powers of the simpler carbohydrates are utilized in 

 detecting and estimating them quantitatively. In alkaline solution they 

 will reduce many metallic hydroxides, such as those of copper, mercury, 

 bismuth, silver, gold, etc. Methylene blue, permanganates, bromin, 

 chlorin, etc., are also reduced by sugars, the last three in acid solution as 

 well as in alkaline solution. 



The carbohydrates are usually estimated quantitatively or detected 

 qualitatively by an alkaline cupric tartrate solution, known as Fehling 

 solution or some modification of it. If glucose be heated with cupric hy- 

 droxid [Ctt(OH) J and sodium hydroxid, it will reduce some cupric hy- 

 droxid to cuprous oxid [Cu 2 O]. When much cupric hydroxid is present 

 it will remain partly dissolved and some of it may be dehydrated to form 

 black cupric oxid [CuO]. 



Many substances, usually those having several hydroxyl groups, such 

 as tart rates, citrates, glycerol and sugars, possess the property of dissolving 

 metallic hydroxids, as in the case of sodium tartrate and Cu(OH) 2 form- 

 ing cupric tartrate. If enough sodium tartrate be added to cupric hy- 

 droxid and sodium hydroxid, all the cupric hydroxid will dissolve. When 

 glucose is heated with such a solution reduction of the cupric hydroxid 

 will occur with no danger of formation of cupric oxid, which might obscure 

 the result. Fehling's solution is an alkaline cupric tartrate solution made 

 from copper sulphate, sodium potassium tartrate (Rochelle salt) and sodi- 

 um or potassium hydroxid. When kept for any length of time, the tartrate 

 will reduce the cupric salt. To avoid this the copper sulphate is kept sepa- 

 rate and is known as Fehling's solution "A" and the alkaline tartrate solu- 

 tion as Fehling's solution "B". 



The stages in the reduction of copper by reducing sugars are roughly 

 as follows: the alkali decomposes the sugar into a number of fragments 

 which reduce the cupric salt to insoluble yellow cuprous hydroxid/ first. 

 If heating is continued, the cuprous hydroxid loses a molecule of water 

 and is converted into red cuprous oxid, which is also insoluble., 



8 A glucoside is an ether of glucose (or other sugars) and an cJcohol. On hydrol- 

 ysis with acid, the sugar is liberated. 



