40 PRINCIPLES OF ANIMAL BIOLOGY 



surprise, therefore, if in the following chapter the activities of protoplasm 

 must be stated with caution and reserve. 



Lipoids. — The term lipoids is applied loosely to a certain group of 

 substances whose chemical and physical properties resemble those of 

 fats. They are often sticky and waxy in consistency. The lecithins are 

 some of the very common lipoids, found in quantity in the yolk of eggs; 

 cholesterin is also usually included in this class of substances; and there 

 are many others. 



The structure of the lipoid molecule is simpler than that of the 

 proteins, and in many cases it is well understood. Thus the lecithins 

 are compounds of glycerophosphoric acid, substituted by two fatty 

 acid radicals, with a base called choline. Since there are many kinds 

 of fatty acids with which to make the substitutions mentioned there 

 are also various kinds of lecithins. Although the lecithin molecules are 

 large, containing often a hundred atoms, their empirical formulas (stating 

 merely the number of atoms of each of the elements present) have been 

 definitely determined. Moreover, their chemical relations are so well 

 understood that there is no hesitation in ascribing to them definite 

 structural formulas. 



The principal elements in the lipoids are carbon, hydrogen, and oxygen; 

 but phosphorus and nitrogen are often also present. On the whole they 

 tend to lower the surface tension of cells, and therefore for reasons which 

 cannot be explained here collect at the surface, forming part of the cell 

 membrane. 



Carbohydrates. — These substances are composed of carbon, hydrogen, 

 and oxygen, and no others. The hydrogen and oxygen atoms are charac- 

 teristically present in the ratio of 2 to 1, as in water. From this latter 

 fact comes the name carbohydrate, which implies that the substances are 

 hydrates of carbon. Ordinary cane sugar, glucose, the various starches, the 

 cellulose of which cell walls are composed, and others are carbohydrates. 



The soluble members of this class, as the sugars and allied substances, 

 may exist in any part of the protoplasm, and may diffuse readily from one 

 part of a cell to another, or from cell to cell. The insoluble carbohydrates 

 are stored as solid particles in the cells, as glycogen (animal starch) in the 

 liver and muscles, or starch grains in plant cells. 



Fats. — The fats and oils are compounds of carbon, hydrogen and oxy- 

 gen, as are the carbohydrates; but oxygen is usually less abundant than 

 in the starches and sugars. Chemically the fats are compounds of glyc- 

 erol and one or more fatty acids. They are not soluble in water, and 

 are not thus capable of diffusion through the general protoplasm, but 

 are stored. Most cells contain small quantities of fat in the form of 

 droplets, while in the larger animals connective tissue may be heavily 

 laden with it.' 



'Further characterization of proteins, carbohydrates and fats is given in the next 

 chapter. 



