210 NUTRITION AND METABOLISM 



The disaccharides or double sugars, having the chemical formula 

 CizH-zzOn are broken up into single sugars, monosaccharides, by the 

 following process: 



The two molecules of CeH^Oe are different with different sugars. 

 If the disaccharide is saccharose, the two monosaccharide molecules 

 are dextrose and levulose. Lactose will yield dextrose and galactose, 

 and maltose will give two molecules of dextrose. For each of these 

 sugars, there is a special enzyme which can hydrolyze only its par- 

 ticular sugar and none of the others; like a key, made for one lock, 

 it will not open another lock. Maltase will split only maltose mole- 

 cules, not lactose, while the lactase cannot attack the maltose. /- 

 vertase (or sucrase) will decompose nothing but saccharose. This 

 decomposition of the complex sugars into the simple sugars was be- 

 lieved to be necessary because only sugars of the type CeH^Oe can 

 be fermented directly by the fermenting enzyme in the cell, be it an 

 alcoholic or lactic or gassy fermentation. This explains why beer yeast 

 cannot ferment lactose; it produces no lactase, and therefore cannot 

 attack the lactose molecules; they would be easily attacked, if besides 

 the yeast, some lactase were added. Certain lactic bacteria cannot 

 ferment saccharose, because they do not form invertase. Recent 

 experiments have shown that bacteria exist which ferment lactose 

 and saccharose but not dextrose or levulose. An explanation for this 

 cannot be given. 



Invertase is, like diastase, a very common enzyme in green plants. 

 It is also produced by most molds and yeasts, and bacteria. Maltase 

 is not quite so common, and lactase is limited to a few species of 

 microorganisms. A few organisms are known which do not secrete 

 these enzymes but retain them within the cell. This is especially 

 true of lactase, but is also known, in a few instances, of invertase. 

 The enzyme can be obtained from the broken cells. Such enzymes 

 are called endo-enzymes . 



The decomposition of carbohydrates has been followed from the 

 most complex representatives to the simplest ones, the monosacchar- 

 ides. If these are decomposed further, the resulting product is no 

 longer a carbohydrate. The simplest sugars are decomposed by zy- 

 mases, inside the microbial cell, into compounds which are generally 



