22 GENERA t CONCEPTS 



throughout a hirge mass of protoplasm. Finally, water has an important 

 function as a lubricant. It is present in body fluids wherever one organ 

 rubs against another and in joints where one bone moves on another. 



A mixture is made of two or more kinds of atoms or molecules which 

 may be present in varying proportions. Air is a mixture of oxygen, nitro- 

 gen, carbon dioxide and water vapor, plus certain rare gases such as 

 argon. The proportions of these constituents may vary widely. Thus, in 

 contrast to a pure compound, which has a fixed ratio of its constituents 

 and definite chemical and physical properties, a mixture has properties 

 which vary with the relative abundance of its constituents. 



Molecules may be composed of one, two, or many kinds of atoms. 

 Those of gaseous oxygen or nitrogen are made of two of the same kind 

 of atom— O2 and No. The molecules of table salt, sodium chloride, are 

 composed of one atom of sodium and one of chlorine (NaCl). A common 

 sugar, of great physiologic importance, is glucose, whose molecules con- 

 tain six carbon, twelve hydrogen and six oxygen atoms; its formula is 

 written CgHioOg. 



To learn more about the constituents of protoplasm, biochemists 

 have used very sensitive analytical techniques and have taken great pains 

 to preserve the extremely labile substances present in this enormously 

 complicated system. To prevent the disappearance of certain substances 

 it is necessary to quick-freeze a bit of excised tissue, or even a whole small 

 animal, by dropping it directly into liquid air. Biochemical research has 

 made it abundantly clear that the composition of the protoplasm of any 

 cell is constantly changing, that the cell constituents are in a "dynamic 

 state." There is a continuous synthesis of large, energy-rich molecules and 

 continual decomposition of these into smaller, energy-poor ones. Some 

 of the most important compounds of protoplasm are present only in ex- 

 tremely minute amounts at any given time, although the total amount 

 formed and used in a 24 hour period may be quite large. An apprecia- 

 tion of this may be gained from the following consideration: when sub- 

 stances undergo chemical reactions in sequence (and almost all of the 

 reactions of importance biologically are sequences or "cycles") such as 

 A ^. B ^. C ^^ D, the rate of the whole process is controlled by the rate 

 of the slowest reaction in the chain. For example, if reaction A -> B is 

 10 times as fast as B -^ C, and if C -> D is 100 times as fast as B -> C, 

 then the least reactive substance, B, will tend to accumulate and the most 

 reactive one, C, will be present in the smallest amount. For this reason 

 many of the most active and important substances of protoplasm are 

 present in extremely minute amounts. This, coupled with their chemical 

 instability, has made their detection and isolation difficult. There are 

 probably many such intermediate compounds that remain to be dis- 

 covered. 



The compounds found in protoplasm are of two main types: inor- 

 ganic and organic. The latter include all the compounds (other than 

 carbonates) that contain the element carbon. The element carbon is 

 able to form a much wider variety of compounds than any other element 

 because the outer orbit of the carbon atom contains four electrons, 

 which can be shared in a number of different ways with adjacent atoms. 



