Cell Constitution 



59 



cell membrane is the incorporation into the 

 film of molecules which have diffused from 

 the cytoplasm to the region of the surface 

 film. Experiments with monofilms show that 

 molecules from the svibsolution may readily 

 penetrate the film, increasing the film pres- 

 sure. Since the tension at the surface of the 

 cell is very low, sensibly zero, penetration 

 from the subsolution may increase the sur- 

 face area and possibly the shape of the cell. 

 This was shown in model systems by Lang- 

 muir and Waugh ('38). Conversely, mole- 

 cules in the surface film may be ejected 

 either by increase in surface pressure or by 

 a decrease in their affinity for film molecules. 

 The degree to which forces in the surface 

 film may determine the shape of cells, es- 

 pecially free cells, cannot yet be accurately 

 assessed. The matter will be considered more 

 in a subsequent section. 



An important factor not thus far men- 

 tioned is the surface charge of the cell mem- 

 brane. Electrophoretic measurements show 

 that the net charge is negative on the sur- 

 face of most cells. The red cell membrane is 

 negative over the entire range in which the 

 cell is stable (down to pH 4). This nega- 

 tivity may be due in part to ionization of 

 phosphoric acid grovips in phospholipids 

 (chiefly cephalin), although proteins and 

 possibly also acid polysaccharides may also 

 be involved. 



Dan ('47) has made electrophoretic stud- 

 ies of the sea iirchin egg treated in various 

 ways. The surface charge is negative even 

 at pH's as low as 2. Fertilization is said to re- 

 duce the negativity. Dan also studied the ef- 

 fect of Ca*% Ce**"^ and other ions on the elec- 

 trokinetic potential and the role of this poten- 

 tial in surface adhesiveness and agglutination 

 phenomena. Such quantitative studies, rela- 

 tively rare in the literature, are to be encoiir- 

 aged. They give information of the net 

 charge, but not of the particular types of 

 ionized groups in the exterior surface of the 

 cell. While the electrokinetic potential is of 

 importance in determining cell to cell inter- 

 action; the entire constellation of charges 

 and ion atmosphere must also be considered. 



SOME PHYSICAL CHEMICAL 



CONSIDERATIONS OF MORPHOGENETIC 



PROCESSES 



STRUCTURAL PATTERNS AT VARIOUS 

 LEVELS OF ORGANIZATION 



Chemical analytical and crystallographic 

 data, as well as biological properties, support 

 the view that there is very precise regularity 



of structure in protein molecules, reaching to 

 the atoms themselves. To understand this 

 regularity of pattern, it is necessary to sup- 

 pose that each native protein molecule is 

 composed of a specific number of amino acid 

 residues arranged in a specific sequence of 

 residues or residue types and that the specific 

 configuration of the polypeptide chains is 

 that which has maximum stability under any 

 particular conditions. 



We are still far from an understanding of 

 the mechanisms by which protein molecules 

 are formed. Much work is currently being 

 done on the biosynthesis of peptides utilizing 

 energy-covipling reactions revealed in recent 

 years. However, the process by which the 

 specific sequences of amino acid residues are 

 joined and the chains characteristically 

 folded remains a matter of speculation. 



Precise patterns of organization exist also 

 at the level of the giant macromolecular com- 

 plexes. Illustrative are the fibrous proteins 

 (Table 1) which manifest axial periodic 

 structure so regularly repeating as to give 

 dozens of orders of x-ray diffraction. This 

 regularity depends in turn upon a precise 

 sequence of amino acid types along the 

 chains, giving rise to alternating regions of 

 relative order and disorder in the adjacent 

 chains which form the fibrils. These regions 

 are thought to correspond to the bands, or 

 cross-striations, seen in the EM. 



Important light on the processes by which 

 such fibrous patterns are formed is thrown 

 by experiments made some years ago by 

 Nageotte and Faure-Fremiet, in which it 

 was demonstrated that collagen fibers may be 

 dissolved in dilute acid and reconstituted by 

 neutralization or addition of salt. The re- 

 constitvited fibrils have the same period 

 Tea. 650 A) and fine structure as the native 

 fibrils, as seen in the EM (Schmitt, Hall and 

 Jakus, '42). Appropriate adjustment of ionic 

 strength and pH causes the dispersed, solvated 

 chains to aggregate again in perfect register 

 with respect to the axial discontinuities. 

 When serum acid glycoprotein or certain 

 other substances are added to the acid solu- 

 tion of collagen, dialvsis yields fibrils with 

 a new axial period (Highberger, Gross and 

 Schmitt, '51) several times greater (2000 to 

 3000 A) than that of native collagen. Ap- 

 parently the added substance combines with 

 the collagen chains to produce a new pattern 

 of structure. Bizarre two-dimensional pat- 

 terns have been observed in which grids 

 were formed by the intersection of lona;- 

 spacing fibrils radially directed from several 

 centers. 



