The Chemical and Physical Structure of Protoplasm - 71 



The newer knowledge of atomic physics also 

 provides a key to other problems. It explains, for 

 example, the source of the energy that is liberated 

 or bound whenever a chemical reaction takes 

 place (see p. 78). Each atom is a system super- 

 charged with energy— the kinetic energy of die 

 vibrating and revolving electrons, and the poten- 

 tial energy of the electrical attractions between 

 the protons and electrons. No two different atomic 

 combinations, or molecules, ever possess pre- 

 cisely equal quantities of intramolecular energy. 

 Therefore, it follows that any alteration in 

 atomic combination, such as occurs during every 

 chemical reaction, must involve either gain or loss 

 of energy. Either there is an energy surplus, 

 which is put forth, or there is a deficit, which is 

 absorbed, with reference to the environment. 

 Accordingly, when we witness an expenditure of 

 energy in any cell— such as the beating of cilia or 

 the contracting of muscle— we know that the 

 energy arises from the chemical reactions that 

 constitute the metabolism of the cell. 



CHEMICAL COMPOSITION OF PROTOPLASM 



Organic vs. Inorganic Substances. The 



many substances present in protoplasm fall 

 naturally into two great classes: organic and 

 inorganic substances. Early in the nineteenth 

 century it was thought that some "vital fac- 

 tor" was distinctive of organic substances, 

 because up to that time no organic sub- 

 stance had ever been obtained except from 

 the bodies of living organisms. But in 1828, 

 Woehler first synthesized an organic com- 

 pound (urea), and since then a wide variety 

 of organic compounds have been manufac- 

 tured in the laboratory — many of which, like 

 aspirin and sulfanilamide, never existed 

 previously. Nevertheless an important dis- 

 tinction differentiates organic from inorganic 

 substances. Excluding artificial synthesis, or- 

 ganic compounds, such as sugar, are found 

 only in living bodies, or in their products 

 and remains; whereas inorganic substances, 

 such as water, are found both in living and 

 in nonliving bodies. 



Inorganic Subsfances. Inorganic substances 

 make up the bulk of living as well as non- 

 living matter. The rocks, soil, atmosphere, 



and waters of the earth are composed almost 

 entirely of a wide variety of inorganic ma- 

 terials. And in living matter also, the inor- 

 ganic components greatly preponderate. This 

 is due mainly to the high proportion of 

 water in all protoplasm, although lesser 

 quantities of the inorganic salts, acids, bases, 

 and gases are likewise always present in 

 protoplasm. 



Water. Water is by far the most abundant 

 single compound in all protoplasm. The 

 proportion of water — in terms of the weight 

 of the protoplasm — varies between 70 and 90 

 percent in different cells, and this water is by 

 no means inert and unimportant. Without 

 water, there is no such thing as protoplasm; 

 and the life structure of any cell is immedi- 

 ately destroyed if the cell loses a significant 

 proportion of its water content. 



Water is so familiar that it is difficult to 

 appraise its functions scientifically. Never- 

 theless, the unique physical and chemical 

 properties of water give it a dominant role 

 in determining protoplasmic structure and 

 activity. In fact, water is the chief dispersion 

 medium of the protoplasm. In other words, 

 water is the liquid that dissolves, suspends, 

 or otherwise disperses most of the various 

 other substances present in the cell. 



Solvent Properties of Water. One important 

 property of water is its high efficiency as a 

 solvent. No other single liquid substance is 

 capable of dissolving so many other sub- 

 stances. Water is the most effective solvent 

 for inorganic compounds generally; also, 

 many important organic compounds are sol- 

 uble in water. 



When a substance (the solute) dissolves in an- 

 other substance (the solvent), the solute tends to 

 become dispersed throughout the solvent. The 

 ideal state of true solution is reached when all the 

 molecules of the solute have become individually 

 separated from the dissolving mass and have 

 scattered freely throughout the solvent. Thus 

 when a crystal of sugar is dropped into a glass of 

 water, the disappearance of the crystal indicates 

 that the sugar molecules, which previously formed 

 a compact and tangible mass, have become indi- 



