274 UNITY AND DIVERSITY IN BIOCHEMISTRY 



to explain a watch to a Martian by drawing him a small circle, even though 

 it have the agreeable brightness of gold. 



It was necessary to wait until 1940 when Albert Claude had the idea of 

 dissecting the cell into its constituents by means of a centrifugal field 

 applied to a suspension of macerated cells under defined conditions. This 

 mode of penetration into the unexplored world inside the cell gave access 

 to previously recognized organelles and led to the identification of others, 

 as was the case with the microsomes. The parallel use of the electron 

 microscope gave further valuable help in this field. Once more the march 

 of scientific progress continued. Schwann would not have formulated the 

 cellular theory if he had not had the benefit of a microscope incorporating 

 improvements introduced by Amici. Similarly, without the ultracentrifuge 

 and the electron microscope, Claude, and many workers after him, would 

 not have been able to penetrate inside the cell. 



From a biochemical point of view, a cell can be divided into a nucleus and a 

 cytoplasm containing mitochondria and other inclusions. Even with the or- 

 dinary microscope, and better still with the phase-contrast microscope, it is 

 possible to distinguish granular or fibrillar inclusions in the cytoplasm. In 

 cells which respire, mitochondria are found, and in certain cells there are other 

 specialized structures buried in an apparently structureless cytoplasm. 



A. Cytoplasm 

 (a) The Fundamental Material 



The C)rtoplasm has been the subject of numerous theories. The most 

 recent have been thought out with a view to explaining the physical 

 properties of cytoplasm in terms of its structure. The most comprehensive 

 is the theory of bonds, proposed by Frey-Wyssling. 



During the first quarter of this century, ideas about the structure of 

 protoplasm were dominated by a fallacious conception which sought to 

 interpret the properties of protoplasm according to whether the material 

 was in the sol or the gel state, where these terms have the same significance 

 as in colloidal chemistry. Today, a large number of facts make us consider 

 that the cellular elements are formed from macromolecules and not from 

 small molecules associated together by physical forces to form micelles 

 analogous to those found in soaps. 



The cell is made up of a number of organelles bathed in a cellular juice. 

 The cytoplasm contains protein filaments which make up a labile frame- 

 work whose structure is due to bridges between the fibres and whose 

 lability is due to the fact that the bridges are easily broken. The filaments 

 themselves vary between 80 and 200 A in thickness. 



If we consider the number of different amino acids present in proteins, 

 we can understand what a variety of side-chains is possible in a polypep- 

 tide chain. 



