CARBOHYDRATES. 27 



Natural and artificial sugars which reduce Fehling's solution (includ- 

 ing milk sugar and maltose) give the above reaction. The products 

 obtained are very characteristic, and are the most valuable means that 

 we possess for recognizing and separating the sugars. 



An important property of the natural sugars which has been mentioned 

 already is their optical activity. Quantitative methods for the analysis 

 of sugars are based upon this property, and it serves for classifying the 

 sugars as well. Thus the dextro-rotary dextrose was first designated as 

 d-glucose, and laevo-rotary Isevulose as Z-fructose. Emil Fischer then 

 proposed a different method of nomenclature which shows the relation of 

 the compounds to one another. The monoses derived from a d-, 1-, or 

 t-monose are also designated by the letters d-, 1-, and i-, even when they 

 possess a rotary power opposite in sign to that indicated by these letters. 

 In this way laevulose is now designated as d-fructose, because of its close 

 relation to d-glucose, or dextrose. 



The most interesting reaction of sugar from a biological standpoint is 

 its ability to undergo fermentation. Thus different yeasts cause dextrose 

 to break down into alcohol and carbon dioxide (alcoholic fermentation) : 



C 6 Hi 2 O 6 = 2 C 2 H 5 OH + 2 CO 2 . 



On the other hand, bacterium lactis converts it into ordinary lactic 

 acid (lactic acid fermentation) : 



C 6 H 12 6 = 2 C 3 H 6 3 . 



Finally dextrose may be converted into butyric acid, carbon dioxide, and 

 hydrogen by the action of certain microbes (butyric acid fermentation) : 



C 6 Hi 2 O 6 = C 4 H 8 O 2 + 2 CO 2 + 2 H 2 . 



At this place we shall not take up these processes in further detail. We 

 shall later on find opportunity for showing how great an influence the 

 stereo-configuration of the different sugars has upon their fermentability, 

 and how Emil Fischer by the aid of fermentation studies was able to 

 formulate hypotheses and arrive at conclusions which are of great 

 biological importance and form the foundation of our whole knowledge 

 concerning fermentation reactions. 



It remains still to show certain relations between the sugars and two 

 other classes of compounds, namely, their reduction products (the corre- 

 sponding alcohols) and their oxidation products (the acids) . The former 

 relation is at once apparent from the following summary: glucose is the 

 aldehyde of sorbite, mannose of mannite, and galactose of dulcite, while 

 fructose is the ketone of mannite. By oxidation we obtain: 



From glucose, first the monobasic gluconic acid, then by further 

 oxidation the dibasic saccharic acid. 



