THE NATURE OF PROTOPLASM 73 



Colloids are disperse systems of the " solid in liquid " and " liquid 

 in liquid " types, that is, suspensions and emulsions, in which the 

 particles of the disperse phase are extremely minute. The dis- 

 persal of material in minute masses produces a very great increase 

 in the surface of the dispersed phase. Since many phenomena, 

 both chemical and physical, are dependent upon surfaces, it may 

 be said that the characteristic properties of colloids are due to the 

 enormous surfaces present in such systems. 



Chemical Nature of Protoplasm. — When protoplasm is analyzed 

 chemically it is found to contain carbon, hydrogen, oxygen, nitro- 

 gen, phosphorus, sodium, chlorine, magnesium, iron, potassium, 

 sulphur, calcium, and sometimes other elements, such as siUcon, 

 copper, aluminum, manganese, iodine, fluorine, and bromine. 

 These chemical elements are combined to form the four great 

 groups of organic compounds, carbohydrates, fats, lipins, and pro- 

 teins, in addition to organic substances known as enzymes, inor- 

 ganic salts, and water. Organic compounds are so called because 

 they occur in nature only as the products of the chemical activities 

 of living organisms. 



Carbohydrates are compounds of carbon, hydrogen, and oxygen, 

 and are so called because, typically, the hydrogen and oxygen 

 occur in the proportion of two to one, as in water. The great 

 majority of the carbohydrates arc made up of structural units 

 known as the simple carbohydrate or saccharide groups, which 

 have the empirical formula CeHioOo. The starches and sugars 

 are the most commonly knovv-n representatives of this class of sub- 

 stances, although cellulose, gums, dextrines, and glycogen are 

 widely distributed members. The simplest carbohydrates are 

 easily oxidized, and furnish, indeed, the ultimate source of proto- 

 plasmic energj'. 



Among organic compounds is found a very heterogeneous group, 

 soluble in ether, alcohol, and chloroform, and having a greasy 

 feeling. This is the class of lipoid substances and may be sub- 

 divided for our purposes into fats and lipins. Fats, under v/hich 

 we include neutral fats and fatty oils, are composed of carbon, 

 hydrogen, and oxygen, but contain much less oxygen in proportion 

 to the carbon than is found in the carbohydrates. These three 

 elements occur in the form of structural units known as glycerol 

 (glycerine) and fatty acids, which are combined in different ways 

 to give rise to the different fats. Examples of fats are to be 



