CHAPTER 4 

 THE FUNCTIONS OF PROTOPLASM AND CELLS 



The living substance whose functions we are to study differs from 

 nonhving matter in certain characteristic ways. It has certain types of 

 chemical structure, not easily defined, but not duplicated in inorganic 

 bodies. It is arranged in unit masses, the cells, which are usually 

 recognizable by their form and such nearly universal features as the 

 nucleus. This living matter moves spontaneously, that is, from causes 

 arising within itself. It grows by taking up new material throughout 

 its interior, not just by additions on the outside. It is irritable; that is 

 it responds in some way to changes in the environment, or changes 

 within itself, which are great enough to act as stimuli. And finally, 

 individual living things are capable of producing other individuals of 

 their own particular kind; that is, they reproduce. 



These statements are not intended as a definition of life, or of living 

 things, because there are exceptions to them, or situations in which the 

 criteria could not be practically applied. They are meant merely to 

 indicate the general types of functions which must be examined in a 

 survey of life activities. 



Protoplasm is not a chemical compound, the structure of Avhich may 

 be expressed by a chemical formula, but is an elaborate mixture of 

 chemical compounds in water. A bit of protoplasm large enough to 

 analyze, from any source, always yields carbon, hydrogen, nitrogen, 

 oxygen, phosphorus, sulfur, sodium, potassium, magnesium, calcium, iron, 

 and chlorine. Additional elements that frequently occur in such analyses 

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

 Naming these elements tells very little, however, concerning protoplasm, 

 since it does not suggest the manner in which the elements are combined, 

 and it is the compounds, not the elements, that are of real importance. 

 These compounds in protoplasm are of a variety of kinds, which are 

 partly organic (produced in living things) and partly inorganic. The 

 latter are described first; they are water and the various salts. 



Water and Salts. — Water is the most abundant constituent of proto- 

 plasm, making on the average about 80 per cent of the total mass. The 

 properties and activities of protoplasm are quite as dependent upon the 

 remarkable properties of water as upon the properties of its other con- 

 stituents. Some of these properties of water are its power to absorb or 

 give off great quantities of heat without changing much in temperature, 



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