Lecture 1 _3 — Survey 



ated, including those of the highest degree of intricacy 

 and of largest molecular weights among organic mole- 

 cules. The familiar fact will be recalled that upon 

 this synthetic power and upon the processes by which 

 the energy of sunlight is absorbed and fixed all life 

 is ultimately dependent. The green plant winds up 

 living systems as a whole, which otherwise would run 

 down and disappear. 



Even in a single microscopic green cell reside 

 potentialities for chemical reactions of extraordinary 

 diversity. Smoothly, rapidly, at low temperatures, 

 without the strong reagents of the chemist, the cell 

 carries on its remarkable syntheses. The chemical 

 compounds built up from the simple substances are 

 generally of the finest specificity. We think of giant 

 protein molecules of biologically determined architec- 

 ture, and we remember that even relatively small mole- 

 cules, like those of the sugars, may exist in right and 

 left handed forms of equal chemical stability, with as 

 nearly as possible an even chance that either form 

 will be synthesized unless a directional agency inter- 

 venes; and that the plant cell nearly always synthe- 

 sized only one of the forms. Some investigators be- 

 lieve that this is invariably true of the molecule 

 freshly synthesized by the protoplasm, although under 

 some circumstances, a reversion may later occur to 

 racemic forms. All this is, of course, not new knowl- 

 edge. There are, however, several aspects of the 

 functions of plant cells that have been illumined by 

 the new kind of experimental evidence that can be 

 gained through use of the tool of chemical isotopes, 

 both stable and radioactive forms. It appears from 

 recent work by several groups of investigators that 

 the chemical organization of either the metabolizing 

 plant or animal cell is far more dynamic in nature 

 than was previously imagined. The concept evolved 

 is that of a continuous flux of interdependent chemical 

 processes, with complex molecules built up and broken 

 down with great rapidity, all under the orienting 



