42 



Cellular Structure and Activity 



envelope, and containing particulates of vari- 

 ous sizes and shapes, including a tenuous 

 fibrous system capable of sol-gel reversal. 

 In the microcosm of the living cell the various 

 particulates and subcellular systems have in- 

 timate chemical and structural relationship 

 with each other. The properties of the in- 

 dividual components depend on this inter- 

 relationship and the normal physiological 

 function of the cell requires an appropriate 

 organization of the system as a whole. It is 

 improbable that we can fully learn the chem- 

 ical and structural properties of the individ- 

 ual components after their removal from 

 the cell. However, information about such 

 partial systems mvist be obtained before the 

 types of interaction which provide the emer- 

 gent organizational properties can be de- 

 duced. Much of the discussion in this chapter 

 will necessarily deal with partial systems of 

 various sorts. 



For convenience of presentation these sys- 

 tems will be considered according to the 

 geometric form of the components rather 

 than according to chemical composition or 

 supposed function. It is convenient to con- 

 sider three geometric forms: the one-, two- 

 and three-dimensional arrays. These corre- 

 spond, respectively, to the fibrous, membran- 

 ous and globular particles, which will be 

 considered separately, though not in that 

 order. Equally important, of course, is the 

 aqueous milieu with dissolved substances 

 and this will be considered first. 



WATER AND DISSOLVED SUBSTANCES 



The water content of protoplasm ranges 

 from 80 to about 95 per cent or more. In 

 terms of molecular species, water represents 

 over 98 per cent of the molecules present in 

 the cell. If we were to look at a cell with 

 "molecular spectacles" we should see little 

 besides water molecules. 



Water is unique among biological liquids 

 in regard to its physical and chemical prop- 

 erties, owing primarily to its peculiar dipolar 

 structure and the bonding of molecules to 

 each other by means of hydrogen bonds (see 

 Gortner, '49). Similarly, water is bonded to 

 polar groups of organic molecules in cells 

 and this solvation determines many of the 

 colloidal properties of cell structures. Physio- 

 logical functions, such as contractility, de- 

 pend importantly upon the reversible altera- 

 tions of the reactivity of chemical grovipings 

 which, in turn, are conditioned by the shell 

 of water molecules coordinated about the 

 groups. 



Important for the chemical coordination 

 of cell processes is the high solubility of 

 organic and inorganic substances in water. 

 Dissolved in the cell water are the mineral 

 salts, lyoenzymes, intermediary metabolites 

 and certain hormones. Energy-rich com- 

 pounds, such as adenosine triphosphate 

 (ATP), presumably diffuse freely in the 

 aqueous phases of protoplasm from their site 

 of synthesis, primarily in the mitochondria, 

 to the molecular effectors where the energy 

 is liberated. 



In general the water content of cells is 

 greatest in cells in which chemical metabol- 

 ism is high. Embryonic cells have a higher 

 water content and a higher metabolism than 

 do the corresponding cells in the adult or- 

 ganism. Osmotic influx of water resulting, for 

 example, from the hydrolysis of large mole- 

 cules or macromolecules to form a much 

 larger nvimber of smaller molecules may play 

 a role in morphogenesis through change in 

 cell shape (particularly if the water perme- 

 ability of one surface of a cell layer is dif- 

 ferent than that of the other surface). 



PARTICULATE SYSTEMS 



The view that many of the essential re- 

 actions of protoplasm occur within, or at the 

 surfaces of, subcellular, macromolecular sys- 

 tems having a high degree of internal organ- 

 ization is not original with modern analytical 

 biologists. It has been inherent in the think- 

 ing of great physiologists, morphologists, 

 geneticists and embryologists for over a cen- 

 tury.* However, the isolation and analysis 

 of several types of particulate systems has 

 now given valuable clues as to the struc- 

 tures responsible for certain of the major 

 functions of the cell and there is every reason 

 to expect that further important advances 

 will be forthcoming by the vise of fractiona- 

 tion techniques. 



For purposes of convenience we shall con- 

 sider in one category the organized particu- 

 lates, whether they occur as microscopically 

 visible structures in the cell or as submicro- 

 scopic aggregates. This is probably justified 

 since in most cases, where such systems have 



* To realize how fundamental this concept has 

 been, one has only to recall the numerous terms 

 which were invented over the years for the giant 

 colloidal particles in which "life" was thought to re- 

 side: biogens (BuflFon, Verwom), physiological 

 units (Spencer), bioplasts (Altmann), gemmules 

 (Darwin), pangens (DeVries), plastidules (Haeck- 

 el). biophores (Weismann), idiosomes (Naegeli) 

 and many others. The gene, in its original form, is a 

 descendent of such concepts. 



