3 i6 HUMAN PHYSIOLOGY 



same amount of proteid 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," p. 8, et seq.). 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 mul- 

 tiple of six, and the hydrogen and oxygen being in the same 

 proportions in which they occur in water. They are 

 aldehydes or ketones, and derivatives from these, of the 

 hexatomic alcohol, C C H U O 6 (p. 422). A group of carbo- 

 hydrates 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 monosaccharids, of 

 which dextrose, the aldehyde of mannite, is the most im- 

 portant. 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 

 kevulose, 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. 



The other monosaccharid of importance is galactose, a 

 sugar produced by the splitting of milk sugar. 



These monosaccharids, 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 

 phenylhydrazin. These have proved most useful in dis- 

 tinguishing different sugars. 



