WATER. CARBOHYDRATES 37 



we must have some knowledge of the nature and proper- 

 ties of these before we can understand the nature and 

 functions of protoplasm. 



We must, of course, be careful to distinguish between 

 an organic substance which is a mixture of chemical 

 compounds in various proportions, and a chemical 

 compound which has a perfectly definite chemical 

 composition, i.e. its molecule or ultimate unit of 

 structure consists of a definite number of atoms of 

 different elements arranged in a definite way. Milk, 

 for instance, is an organic substance which has a 

 varying composition, being a mixture in various propor- 

 tions of the chemical compounds water, milk sugar, 

 various definite fats, proteins, salts, etc. Such organic 

 substances as wood, horn, hair, etc., are, similarly, 

 mixtures of chemicM compounds. 



Of the chemical compounds which enter into the 

 composition of the body of an organism, water (HgO) 

 is of first importance, as we shall see in detail 

 later. It is essential to the structure of living proto- 

 plasm, and is found in greater or less proportion in 

 all parts of the body. More than 90 per cent, of the 

 weight of a herbaceous plant is water, as can easily be 

 shown by weighing the plant and then heating it at 

 100° C. till it loses no more water and weighing again. 



The three classes of chemical compounds which play 

 the leading part in organisms are the carbohydrates, 

 the fats, and the proteins, the two former consisting 

 of carbon, hydrogen and oxygen, the latter with nitrogen, 

 sulphur, and sometimes phosphorus in addition. 



The Carbohydrates. 



These are a class of carbon compounds in whose 

 molecules the atoms of hydrogen and oxygen are 



