PROTOPLASM AND CELLULAR ORGANIZATION 



17 



Compounds are made up of one or more 

 molecules of the same kind; for example, 

 water, sugar, and carbon dioxide are com- 

 pounds. Molecules are so small that one 

 computation shows that there are about 

 1,000,000 molecules in a single bacterium. A 

 molecule is the smallest particle of a sub- 

 stance that possesses the chemical nature of 

 that substance. For example, a molecule of 

 water can be subdivided, but it ceases to be 

 water when it is broken down into the 2 ele- 

 ments, hydrogen and oxygen, of which it is 

 composed. Elements, such as hydrogen and 

 oxygen, are known as atoms. More than 100 

 different elements or kinds of atoms are 

 known. Many of these atoms can combine 

 in various ways to form molecules; for ex- 

 ample, 2 atoms of hydrogen, combined with 

 1 atom of oxygen, produce 1 molecule of 

 water; 1 atom of carbon and 2 atoms of 

 oxygen combine to form 1 molecule of car- 

 bon dioxide. Evidently there are vastly 

 greater numbers of different kinds of mole- 

 cules than of different kinds of atoms. Like- 

 wise, molecules of different kinds may be 

 mixed together in various combinations so 

 as to produce many more different kinds of 

 substances than there are different kinds of 

 molecules. Atoms and molecules are ordi- 

 narily indicated by means of symbols which 

 provide a sort of chemical shorthand. Thus 

 hydrogen is indicated by the letter H, and 

 oxygen by the letter O. The molecular for- 

 mula of water is written as H^O, since each 

 molecule of water is made up of 2 atoms of 

 hydrogen and 1 of oxygen. Carbon is indi- 

 cated by the letter C, and the molecular 

 form of carbon dioxide is COo. The com- 

 binations of atoms, or molecules, are usu- 



ally written in the form of chemical equa- 

 tions, such as the following: 



Hydrogen Oxygen Water 



2U2 -f 02 -> ZHsO 



Carbon 

 Sugar Oxygen Dioxide Water 



CeHi^Oe + 6O2 -> 6CO. + 6H0O 



This reaction is reversible, as indicated by 

 the following equation: 



6CO2 4- 6H«0 -^ C«Hi206 + 6O2 



Reversible reactions are indicated bv two 

 arrows as follows: 



CaHiaOe + 6O2 ^ 6CO2 + 6H,0 



Water is the most common compound 

 in protoplasm, making up from about 60 to 

 96 per cent of it. Water is ingested in greater 

 amounts than all other substances com- 

 bined, and it is the chief excretion. It is the 

 vehicle of the principal foods and excretion 

 products, for most of these are dissolved as 

 they enter or leave the body. Actually, there 

 is hardly a physiologic process in which water 

 is not of fundamental importance. 



Inorganic salts are essential for life proc- 

 esses. They are present in solution in the 

 protoplasm and in body fluids. In body 

 fluids they are very similar in concentration 

 to the salts in sea water. Although small in 

 amount, they are important since certain 

 salts in certain proportions are necessary for 

 normal life activities. For example, if the 

 calcium content of the blood is lowered suf- 

 ficiently, convulsions and death ensue; and 

 if sodium, calcium, and potassium are not 

 properly balanced, the muscles of the heart 

 do not function normally. The presence of 

 certain salts is quite obvious to us, since 

 calcium phosphate and calcium carbonate 

 make up about 65 per cent of bone. 



The three principal classes of organic 

 compounds in protoplasm are known as 

 carbohydrates, fats, and proteins. Carbo- 

 hydrates and fats are composed entirely of 

 carbon, hydrogen, and oxygen; protein has 



