42 The Structure of Protoplasm 



of the organic substances, sugar, fats, and allied compounds, are 

 "small molecules"; the proteins are to be thought of as "large 

 molecules"; while aggregates of an indefinite, although a small 

 number, of protein and other molecules, are considered as "small 

 submicroscopic" particles. The "large particle" may be composed 

 of many hundreds or thousands of these three. It is obvious that the 

 four size classes are merely arbitrary conveniences, although they 

 may have vague boundary lines. 



The small molecules are, in size, about 5 to lOA; the large mole- 

 cules, about 20 to 50A; while the small particles range from 50 to 

 lOOA or larger; and the large particles, from say 500A to 5,000A. 

 Particles of 5,000A, or 0.5 micron, in diameter are at the limit of 

 microscopic visibility. Each of these size classes may be thought of 

 as furnishing structural units, or members, for particles of the next 

 larger class. A particle at the limit of visibility could conceivably 

 consist of approximately a million small 50A particles. This concep- 

 tion of multiple aggregates leads to the necessity for thinking in 

 terms of dimensional levels, since it is practically impossible to 

 visualize the details of one of the component molecules and at the 

 same time to visualize the whole structure containing a million of 

 these and to recognize the internal organization and many complex 

 details of structure which undoubtedly exist. 



The particulate nature of cytoplasm is well known from micro- 

 scopic and ultramicroscopic observations (2, 3, 4, 5, 6) , which have 

 recognized the existence of particles ranging in size from the visible 

 far down into submicroscopic regions. Properties of these particles 

 are, however, very inadequately known. It seems reasonable to 

 think that properties, such as weight, shape, size, electrical features, 

 and other characteristics, must derive from their molecular compo- 

 sition, and that their capacity to take part in metabolic activities of 

 any sort is a consequence of these properties. Considerable evidence 

 from chemical and physical investigation (7, 8, 9) has accumulated 

 concerning the molecular components, especially the protein mole- 

 cule, which should be useful in attaining a comprehension of the 

 nature of these particles and their properties. A particle composed 

 of a thousand or more 50 A protein particles may be readily con- 

 ceived as having considerable internal organization and many 

 complex details of structure, which give it catalytic properties and 

 make of it a functional aggregate. 



By the term, functional aggregate, we mean that the relative 

 arrangement of the constituents is such that certain reactions can 





