44 ORGANIC SUBSTANCES : CHEMICAL CHARACTERS 



plasm, as the essential part of its structure, proteins 

 occur in more or less solid aggregations as grains (called 

 " aleurone grains ") of non-living substance in the 

 cells of certain parts of plants, such as seeds. Here 

 they represent a store of protein substance used as 

 food by the young seedling, and are comparable in 

 this respect with starch grains or oil (fat) globules, 

 which similarly form stores of non-nitrogenous sub- 

 stance. 



There are of course many other classes of organic 

 substances besides the three mentioned. In plants, for 

 instance, the so-called " aromatic substances " occur 

 universally. They contain carbon, hydrogen and 

 oxygen, and are founded on the structure known as 

 the benzene ring, or a similar ring, but our knowledge 

 of their role in the economy of plants is still obscure. 

 The three classes of organic substances — the carbo- 

 hydrates, fats and proteins — briefly described comprise 

 the most inportant organic substances entering into 

 the composition of organisms, and they can all be 

 used as foods by animals. 



Enzymes. — These are a very important class of 

 substances, whose exact chemical composition is still 

 unknown, but many of them can easily be isolated. 

 Their importance consists in the fact that they act as 

 catalysts to many of the chemical changes occurring 

 in living protoplasm. A catalyst is a substance that 

 facilitates a chemical change without itself entering 

 into the final products of the change. It probably 

 works by temporarily entering into combination with 

 the substance or substances in reaction, causing a 

 disruption of their molecules, and ultimately separating 

 from the products. Owing to the fact that the enzyme 

 itself is not involved in the fijial product, a very small 



