PROTOPLASM 33 



binations. Water is also necessary for the continuance of those 

 processes of metabolism, i.e., the building up and the breaking 

 down of proteins and carbohydrates, which are characteristic of 

 living protoplasm. Salts usually found in protoplasm are the 

 chlorides, carbonates and phosphates of sodium, potassium, am- 

 monium, calcium, magnesium, and iron. 



Of the organic compounds, the proteins are the most complex 

 and the most variable, but they always contain some nitrogen 

 and a carbohydrate. In the nucleus the proteins are particularly 

 complex, especially in the substance known as chromatin, the 

 material from which definite bodies called chromosomes* are 

 formed at periods of cell division (see Fig. 2, page 9). It 

 is most remarkable that these chromosomes are always of the 

 same number in a given species of plant or animal. They are 

 the seat of the genes — those hypothetical genetic units which are 

 commonly believed to bear the hereditary traits. 



The carbohydrates and fats are combinations of carbon, 

 hydrogen, and oxygen in which the hydrogen is always twice that 

 of oxygen, just as in water. They are present in great variety 

 in protoplasm. Those carbohydrates which are least easily 

 broken down serve as the structural materials of cellulose (the 

 covering of plant cells), of wood, etc., while others, less stabile, 

 are sources of energy for the organism — an energy which is made 

 available through oxidation and by reducing agents generally. 

 Fats have a much higher energy content, because their larger 

 molecules have more carbon and hydrogen atoms capable of being 

 oxidized. Their ability to unite with electrolytes* to form soap- 

 like compounds with varying water-soluble properties is particu- 

 larly significant. 



Organization 



These fundamental substances of protoplasm should be kept 

 in mind if we wish to gain even a glimmer of the nature of vital 

 processes. We speak of a certain combination of such sub- 

 stances as the fundamental organization of a species, and recog- 

 nize the fact that no two such combinations are alike. The 



* See "Heredity and Variation" in this Series. 



t An electrolyte is a compound substance which can be decomposed (separated) 

 into its elements by an electric current. Salts, as a rule, disassociate readily, acids 

 in various degrees. Substances which do not conduct an electric current are called 

 nonelectrolytes. 



