330 VETEEINAEY PHYSIOLOGY 



One gramme of fat yields twice as much energy as the same 

 amount of protein or carbohydrate. The combustion of 1 

 gramme of fat yields 9'3 Calories of Energy. 



4. Carbohydrates (for tests for different carbohydrates, see 

 "Chemical Physiology"}. The carbohydrates starches and 

 sugars form a group of bodies which do not occur largely 

 in animals, but are abundant constituents of plants. 



They contain carbon, hydrogen, and oxygen, the carbon atoms 

 of the molecule usually numbering six or some multiple of six, 

 and the hydrogen and oxygen being in the same proportions 

 in which they occur in water. They are aldoses or ketoses 

 and derivations from these, of the hexatomic alcohol, C 6 H 14 6 

 (p. 452). A group of carbohydrates having five carbon atoms, 

 and hence called Pentoses, have been found in the animal body, 

 but they are of minor importance. 



The simplest carbohydrates are the monosaeeharids, of 

 which dextrose, the aldose of mannite, is the most important. 

 Dextrose is the sugar of the animal body. It has been called 

 glucose, grape sugar, and blood sugar. 



Closely allied to dextrose in chemical composition is the 

 ketose Icevulose, a sugar which, instead of rotating the plane 

 of polarised light to the right, rotates it to the left, but 

 which in other respects behaves like dextrose. It occurs in 

 certain plants, and in the foetus and foetal tiuids of certain 

 animals. 



The other monosaccharid of importance is galactose, a sugar 

 produced by the splitting of milk sugar. 



These monosaeeharids, when boiled with a solution of cupric 

 acetate in acetic acid (Barfoed's solution), are oxidised, taking 

 oxygen from the cupric salt and reducing it to the cuprous 

 state. When boiled with caustic potash, they, along with 

 certain of the double sugars, are oxidised, and if a metallic 

 salt be present which can readily give up its oxygen, it becomes 

 reduced, the sugar appropriating the oxygen. On this depends 

 Fehling's and many other tests for glucose. 



Under the influence of yeast they split into ethyl alcohol and 

 carbon dioxide. 



They also form crystalline compounds, osazones, with phenyl- 

 hydrazin. These have proved most useful in distinguishing 

 different sugars. 



