164 PRACTICAL PHYSIOLOGY 



/. Fermentation with Yeast. By allowing yeast to grow on a solution 

 of dextrose, the latter is split up into alcohol and carbon dioxide, 

 C 6 H 12 6 = 2C 2 H 5 OH + 200 2 



Dextrose. Ethylic alcohol + carbon dioxide. 



All carbohydrates do not give this reaction. It is of great value as 

 a test for the presence of dextrose in the urine. Commercially it is 

 the agency employed in the preparation of alcoholic beverages. 



To ascertain whether the addition of yeast to any solution produces 

 fermentation, the process should be allowed to proceed in an inverted 

 tube, so that any carbon dioxide gas which develops may be collected, 

 and if necessary tested for (see p. 274). 



//. Rotation of Polarised Light. All simple carbohydrates (mono- 

 saccharides) rotate light to the right except laevulose, which rotates it 

 to the left. (See Advanced Course.) 



III. Moore's Test. When heated with caustic potash a dark substance 

 called caramel is produced. This is also produced when sugar is burnt. 

 Caramel contains several chemical bodies, the most important of 

 which is an acid called levulinic acid (CH 3 - CO - CH 2 - CH 2 - COOH). 



EXPERIMENT VI. Mix equal quantities of a 1 % solution of dextrose 

 and 40% NaOH in a test-tube ; heat. A yellow to brown colouration 

 results, and a smell of burnt sugar (caramel) is evolved, especially on 

 adding weak H 2 S0 4 . 



The Chief Monosaccharides are dextrose, laevulose, and galactose. 



Dextrose, Grape Sugar or Glucose (C 6 H 12 6 ) is found in many fruits 

 and is an important food stuff. In the healthy animal body it occurs 

 in minute traces in blood and muscle, and in disease its percentage may 

 rise in the blood, and it then also appears in the urine (see p. 271). 



It is soluble in water and in alcohol. It has only a slightly sweet 

 taste. It rotates polarised light to the right. 



Laevulose (C 6 H 12 6 ) is found along with dextrose in fruits, and 

 results from the hydrolysis of cane sugar (see disaccharides). It is very 

 rarely found in animal tissues. It is crystallisable with great difficulty, 

 being usually obtained as a putty-like mass. It is laevo-rotatory. 



Galactose (C 6 H 12 6 ) is a dextro-rotatory sugar produced, along with 

 dextrose, by hydrolysing lactose (see Disaccharides). Certain gums 

 also yield it on hydrolysis. It differs but slightly from dextrose in its \j 

 reactions. 



II. DISACCHARIDES. 



Chemically each molecule of a disaccharide consists of two molecules 

 of a monosaccharide minus one molecule of water. 

 2C 6 H 12 6 -H 2 = C 12 H 22 U . 



