The Chemical and Physical Structure of Protoplasm - 95 



form the colloidal framework of a gel is not 

 very clearly understood. The strength and 

 number of the interconnections, and conse- 

 quently the firmness of the resulting gel are 

 sensitive to many factors. Some of these fac- 

 tors are chemical, such as changes in the 

 concentration of hydrogen ions and other 

 substances in the solution; and some are 

 physical, such as changes in temperature. 

 Apparently gelation in protoplasm is under 

 the control of metabolism, which continually 

 alters local conditions in various parts of the 

 cell. All protoplasmic gels that have been 

 studied behave oppositely to gelatin, at least 

 insofar as temperature is concerned. Instead 

 of undergoing solation as the temperature is 

 raised, protoplasm undergoes gelation. Such 

 behavior is also exhibited by gels composed 

 of actomyosin, the main protein present in 

 muscle tissue. In any event, once a gel frame- 

 work has been assembled in the protoplasm, 

 contractility can develop as a result of the 

 folding of the interlinked molecules or other 

 particles, and such folding processes are par- 

 ticularly characteristic of actomyosin and 

 other elongate protein complexes. 



Protoplasmic Surfaces. It is important to 

 realize that a colloidal system such as proto- 

 plasm possesses a tremendous expanse of sur- 

 face area. Each colloidal particle, globule, 

 membrane, or fiber exposes a large propor- 

 tion of free surface at which some one (or 

 more) metabolic reaction may be catalyzed, or 

 speeded up (p. 101). In fact, most metabolic 

 reactions can scarcely proceed at all in the 

 absence of such catalytic surfaces. Conse- 

 quently the extent of the protoplasmic sur- 

 faces, and the fact that their pattern may be 

 changed from moment to moment play a 

 very important role in determining metabolic 

 activity in the cell. 



The stupendous surface of the protoplas- 

 mic system can best be visualized by thinking 

 of a small cube, measuring perhaps 1 centi- 

 meter per edge, that is being cut progressively 

 into smaller and smaller cubes. Initially the 

 total surface is, of course, only 6 square centi- 

 meters; but as the pieces become smaller and 



smaller, the free surface increases tremen- 

 dously. Indeed, if finally the pieces measure 

 only 0.01 micron per edge, which is about 

 average as a colloidal dimension, the total 

 surface of the subdivided mass would be 

 more than 6 million square centimeters. In 

 other words, if we visualize all the free sur- 

 faces in a cell — the surfaces of the micro- 

 somes, mitochondria, canaliculae, chloro- 

 plasts, grana, vacuoles, protein fibers, emul- 

 sion globules, and so forth — we begin to 

 realize that the protoplasmic surfaces are 

 extremely extensive, complex, and adaptable. 



SUMMARY 



Protoplasm represents a very complicated 

 polyphasic dispersion in which each phase 

 displays a complex composition. Simultane- 

 ously, protoplasm is a crystalloidal, colloidal, 

 and coarse dispersion, partly solution, partly 

 emulsion, and partly suspension. Sometimes 

 protoplasm is in a gel state and sometimes it 

 is a sol. 



The complex structure of living matter is 

 not static; that is, protoplasm ceaselessly 

 changes, and these changes underlie the vital 

 activities of the cell. The lipoid phases may 

 alternately undergo coalescence and separa- 

 tion in a fashion that may or may not in- 

 volve phase reversal. The aqueous phases 

 may remain homogeneous, as in the sol con- 

 dition, or they may give rise to colloidal net- 

 works, as when gelation occurs. Such changes 

 continually demand an expenditure of 

 energy, which is supplied by metabolism. 

 When energy-liberating chemical reactions 

 cease, the finer protoplasmic structure breaks 

 down, becoming less complex, more stable — 

 in other words, dead. 



But even as protoplasmic structure is de- 

 pendent on metabolism, so metabolism is 

 dependent on the living structure. The sepa- 

 rate phases of the protoplasmic system con- 

 stitute a series of microscopic and ultra- 

 microscopic reaction foci. The separate films 

 and surfaces at which a variety of the react- 

 ing components may be adsorbed and con- 



