24 GENERAL CONCEPTS 



phosphate and suliate aie the important anions (negatively charged 

 ions). 1 he body fluids ot land vertebrates resemble sea water in the 

 kinds ol salts piesent and in their relative proportions, but the total 

 concentration ol salts is only about one-filth as great as in sea water. 

 Most biologists now believe that life originated in the sea. The early 

 protoplasm became adapted to function optimally in the presence of 

 this pattern of salts. As larger animals evolved and developed body 

 fluids, this pattern of salts was maintained, even as some of the de- 

 scendants migrated into fresh water or onto the land. 



Although the concentration of salts in cells and in the body fluids 

 is small, this amount is of great importance for normal cell functioning. 

 The concentrations ot the respective cations and anions are kept re- 

 markably constant under normal conditions; any marked change results 

 in impaired function and finally in death. A great many of the enzymes 

 w'hich mediate the chemical reactions occurring in the body require one 

 or another of these ions— for example, magnesium, manganese, cobalt, 

 potassium— as cofactors. These enzymes are completely unable to func- 

 tion in the absence of the ion. Normal nerve function requires a certain 

 concentration ot calciiun in the body fluids; a decrease in this results 

 in convulsions and death. Normal muscle contraction requires certain 

 amounts ot calcium, potassium and sodium. If a frog heart, for example, 

 is removed from the body and placed in a solution of sodium chloride, 

 it soon stops beating and remains in the relaxed state. If placed in a 

 solution of potassium chloride, or in a mixture of sodium and calcium 

 chloride, it ceases beating in the contracted condition. But it it is placed 

 in a solution of the three salts in proper proportion it will continue to 

 beat for hours. Under the proper conditions, the strength of the heart 

 beat is proportional to the concentration ot calcium ions in the fluid 

 bathing the heart; this method is sensitive enough to be used to measure 

 the concentration ot calcium ions. In addition to these several specific 

 effects ot particular cations, mineral salts serve an important function 

 in maintaining the osmotic relationships between protoplasm and its 

 environment. 



8. Organic Compounds of Biological Importance 



The major types of organic substances found in protoplasm are 

 the carbohydrates, proteins, tats, nucleic acids and steroids. Some of 

 these are required tor the structural integrity of the cell, others to supply 

 energy tor its functioning, and still others are of prime importance in 

 regulating metabolism within the cell. The basic pattern of the types of 

 substances, and even their relative proportions, is remarkably similar for 

 cells from the various parts of the body and for cells from different 

 animals. A bit of human liver and the protoplasm of an ameba each 

 contain about 80 per cent water, 12 per cent protein, 2 per cent nucleic 

 acid, 5 per cent fat, 1 per cent carbohydrate and a fraction of 1 per cent 

 of steroids and other substances. Certain specialized cells, of course, have 

 unique patterns ot chemical constituents; the brain, for example, is rich 

 in certain kinds of fats. 



