Baitsell • The Cell as a Structural Unit 



223 



individual elements or from any known 

 association of elements existing out- 

 side the living organism. Even at the 

 present time the scientist has nothing 

 definite to offer as to when or where 

 or how protoplasm and the associated 

 living phenomena originated, and 

 comparatively little as to the basic 

 features of the complex chemistry of 

 metabohsm essential to life. 



THE USE OF THE X RAY IN THE 



DETERMINATION OF MOLECULAR 



PATTERNS 



The structure of anv crystalline sub- 

 stance as revealed by the X rays is es- 

 sentially uniform, that is, there is a 

 definite characteristic unit pattern 

 formed by the constituent atoms and 

 molecules which is continuously re- 

 peated throughout the substance; a 

 pattern which, though ultramicro- 

 scopic, may be determined by the 

 proper treatment of the data obtained 

 from the diffraction of the X rays as 

 recorded on a photographic film. This 

 condition may be visualized by com- 

 paring the ultramicroscopic pattern in 

 a cr}^stalline substance with the visible 

 repeat pattern in wall paper or in a 

 woven textile. A regular arrangement 

 of the molecules in a substance con- 

 stitutes a crystal and it will diffract X 

 rays in a definite pattern which is con- 

 tinuously repeated and can be identi- 

 fied. This repeat pattern in a crystal- 

 line substance is the "unit-cell, that is, 

 the size of the smallest area containing 

 the unit of atomic or molecular pattern 

 which is repeated from end to end of 

 the cr^'stal." 



The molecule of the chemist is usu- 

 ally defined "as the smallest portion of 

 an element or compound that retains 

 chemical identity with the substance 

 in mass." As such, the molecular unit 

 of inorganic substances or even that of 

 a much more complex organic sub- 



stance is far below microscopic visibil- 

 ity. But X ray studies first revealed the 

 basic fact that the molecules do not 

 exist as independent units in solid or 

 crystalline substances. The constituent 

 atoms are all bound together to form 

 larger units of the crystal pattern with- 

 out molecular separation. 



It should be recognized that the 

 association of the independent mole- 

 cules in the development of a charac- 

 teristic cr}'stal pattern occurs only in 

 solids. In a gas the unit molecules are 

 entirely independent except for chance 

 contacts in their rapid movements. 

 Molecules in a liquid are more closclv 

 associated than in a gas but even so are 

 not bound together in a rigid cr\stalline 

 pattern; they are, in general, free to 

 move in any direction and accordinglv 

 are uniformly dispersed in a solution. 



Increasingly in recent years X rays 

 and polarized light have been used to 

 study the ultramicroscopic structure of 

 various organic materials, particularly 

 the proteins. It now looks as if the 

 situation in the study of proteins, the 

 protoplasmic building materials may 

 be compared to that in the physical 

 sciences in earlier years at a time when 

 the results from the early X ray studies 

 were first considered. The results al- 

 ready obtained are of basic importance 

 to workers in the physical and biolog- 

 ical sciences and also to the economic 

 world as well. TThe most extensive re- 

 searches in the structure of the organic 

 substances by X rays have been made 

 by the textile industry in the studies 

 of cotton, wool, silk, and other fibers 

 essential to this great industry. 



It is also clearly established that not 

 only are the nonliving organic ma- 

 terials, such as cellulose and keratin, 

 crystalline in nature, but protoplasm 

 itself, as found in various types of 

 highly differentiated cells such as those 

 of muscle and nerve, has a basic struc- 

 tural plan extending from molecular 



