The Chemical and Physical Structure of Protoplasm - 67 



or physics, without reference to matter and 

 energy, would be like trying to appreciate the 

 beauty of a landscape in a world devoid of light. 

 Molecular Composition of Matter. Each appreci- 

 able mass of matter, regardless of its specific kind, 

 is composed of a great number of subvisible unit 

 particles, the molecules. This knowledge has be- 

 come so commonplace that many students do not 

 pause to question its origin, although there is a 

 large body of experimental evidence from every 

 field of science that firmly establishes the molecu- 

 lar nature of matter. 



Let us choose any sample of matter, such as 

 water. As everyone knows, water exists in three 

 states: as a solid (at temperatures below the freez- 

 ing point, 0° C), as a liquid (at temperatures be- 

 tween 0° and 100° C), and as a gas (above 100° 

 C)— assuming that the water sample is pure and 

 that other conditions, especially the atmospheric 

 pressure, are standardized. In the gaseous state, 

 of course, the water is not visible. As soon as the 

 water molecules, which in the solid and liquid 

 states are quite closely packed into a tangible 

 mass, have absorbed enough heat energy to escape 

 as individuals from the common mass, their sub- 

 visible smallness is revealed. But water was chosen 

 at random to exemplify matter generally. All 

 other samples of matter— all other specific chem- 

 ical substances— am undergo similar changes of 

 state, although in some cases the practical condi- 

 tions are hard to realize. In the solid state the 

 molecules of a substance are relatively closely 

 packed and the intermolecular attractions are 

 strong enough to prevent the free migration of 

 the individual molecules within the solid mass, 

 although each is free to vibrate (due to heat 

 energy) in the region of a relatively fixed locus. 

 In the liquid state the intermolecular distances 

 are greater and consequently the forces of attrac- 

 tion between the molecules are significantly 

 smaller. Under these conditions, the individual 

 molecules are free, not only to vibrate, but also 

 to migrate (more and more rapidly as the ab- 

 sorbed heat energy increases) through the body of 

 the liquid, although they are not free to escape 

 en masse from the definitive boundary of the 

 liquid. Finally, when a liquid has been energized 

 by heat beyond a certain critical point, the mole- 

 cules of the mass begin to move so rapidly that 

 they separate themselves more or less completely 

 from the intermolecular attractions; and in the 

 gaseous state, the subvisible smallness of the indi- 

 vidual molecules becomes apparent. 



Molecules vs. Atoms. There are thousands of 

 different kinds of matter, such as water, sugar, 

 oxygen, nitrogen, etc., or— as the chemist expresses 

 it— thousands of specifically different substances, 

 each characterized by a different kind of mole- 

 cule. The problem of identifying these many spe- 

 cific molecules belongs, of course, to the field of 

 chemistry. But the molecules present in proto- 

 plasm also belong to biology; and it so happens 

 that some of the protoplasmic molecules are ex- 

 tremely complex in their chemical structure. 



The chemist designates each specific molecule 

 by its formula, which specifies the atoms that 

 are combined in definite proportion in each dif- 

 ferent molecule— as, for example; water = H„0; 

 table sugar = C 12 H 2 20 U ; and oxygen = 2 . If 

 such examples were repeated indefinitely, it would 

 be found that the molecular formulas of all 

 known substances can be given by using the sym- 

 bols of only 92 different atoms (elements), to- 

 gether with their relatively rare isotopes. In other 

 words, the same elements enter into a multitude 

 of specific chemical combinations in forming the 

 molecules of all substances in our universe. 



The methods used by the chemist in determin- 

 ing that a sugar molecule is to be designated as 

 C 12 H 22 O n , or the water molecule as H 2 0, are 

 difficult and indirect; but taken as a whole, they 

 are absolutely convincing. However, only the 

 slightest indication of these methods can be 

 given here. 



In the case of sugar, it is not difficult to demon- 

 strate that carbon (C) is present in the molecule. 

 Anyone who has heated sugar and allowed it to 

 char has demonstrated this point. At high tem- 

 perature sugar molecules begin to decompose, 

 liberating carbon, the familiar black solid, which 

 is easily recognized. The presence of hydrogen 

 (H) and oxygen (O) in the sugar molecules can 

 also be shown by heat decomposition, at very high 

 temperature, in a sealed retort. Under these con- 

 ditions, the water (H a O) that is liberated can be 

 collected and identified. And finally, the water 

 can be decomposed by means of a strong direct 

 current, and the liberated hydrogen and oxygen 

 can be identified (Fig. 4-1). 



The molecules of a substance are the smallest 

 unit particles of that substance; and if a substance 

 be subdivided into particles smaller than its mole- 

 cules, it no longer remains the same substance. 

 Water, for example, becomes hydrogen and 

 oxygen; sugar becomes carbon, hydrogen, and 

 oxygen. Atoms, on the other hand, are the 



