POLYSACCHARIDES. FATS 4I 



into glucose. Hydrolysis of starch also takes place in 

 plant cells (see below). Starch is a very common and 

 widespread substance in plant cells. It bulks largely 

 in the economy of plants, though it stands second to 

 the sugars in this respect. It also forms a very valu- 

 able food for animals, being hydrolysed into sugars 

 (or rather its hydrolysis is catalysed) by the action of 

 the digestive juices of the mouth and of the small 

 intestine. 



Cellulose is a substance of even more complex composi- 

 tion than starch. Its empirical formula is the same, 

 i.e. it has the same proportion of carbon, hydrogen and 

 oxygen atoms in the molecule, though a much higher total 

 number. Thus, if starch be represented by (CgHioOg)^ 

 we may represent cellulose by (CgHjoOs)^. 



Cellulose is of immense importance because it forms 

 the main substance of the cell walls of most plants, 

 and thus the " skeleton " of the plant body. Linen 

 and cotton are nearly pure cellulose, and paper contains 

 a varying amount according to the plant substance 

 from which it is made. Cellulose is formed in a similar 

 manner to starch by the condensation of many mole- 

 cules of sugar. It is not coloured blue with iodine, 

 but it can be altered, e.g. by the action of strong 

 sulphuric acid into a substance which resembles starch 

 in so far as it is so coloured with iodine. 



The Fats. 

 These, like the carbohydrates, consist of carbon, 

 hydrogen and oxygen, but the fat molecule is much 

 larger than the sugar molecule, and contains a much 

 smaller proportion of oxygen. Fats are widely dis- 

 tributed in plant cells, and form a characteristic store 

 of non-nitrogenous food in many seeds, e.g. linseed, 



