FOOD 287 



By further polymerisation of monosaccharids with the 

 further loss of water, molecules of greater size are produced 

 and form the set of substances known as the polysaccharids. 



a;(C6Hi.,06 - H,0) or ^^(CeHioOs). 



The polysaccharids are distinguished from the sugars by 

 being precipitated from their solutions by alcohol. They 

 do not reduce Fehling's solution, nor do they ferment with 

 yeast. In cold neutral or acid solution most of them strike 

 a blue or brow-n colour with iodine. On hydrolysis they 

 split into monosaccharids. Some can be split by dilute 

 mineral acids, or by enzymes. Others can be acted upon 

 by strong acids. Most are insoluble in water. 



Polysaccharids form a series of bodies of which the 

 following are the most important : — 



Starch is built up of a large number of dehydrated mono- 

 saccharid molecules. Common starch seems to have a mole- 

 cular weight of 20,000 to 30,000. On hydrolysis it yields 

 glucose. 



Glycogen occurs mainly in the livers of animals. On hydro- 

 lysis it yields glucose. It gives an opalescent solution, and 

 strikes a brown colour with iodine. 



Cellulose is the basis of the cell walls of plants. It can 

 be hydrolysed only by strong acids, and is not acted upon 

 by the body ferments. There is in plants, however, a 

 ferment cytase which can act upon it. On hydrolysis it 

 yields glucose. It is attacked and disintegrated by bacteria 

 in the first stomach of the ruminant and in the colon of the 

 horse. On bacterial disintegration it yields lower fatty acids, 

 and the sjases methane and carbon dioxide. 



Pentosans. — In addition to the carbohydrates described 

 above, which all contain either six, or some multiple of six, 

 atoms of carbon in the molecule, there is a group having five 

 carbon atoms, and hence called pentoses. The polysac- 

 charids of this group are called pentosans. They are 

 represented by gums, pectins, mucilages, and other sub- 

 stances in plant bodies where they occur in great variety. 

 It is supposed that in digestion they are disintegrated 

 by bacteria, yielding the same end products as cellulose. 



