4 MACROMOLECULAR COMPLEXES 



ical reactions and the development of the more condensed regions 

 of structure (lamellae, fibrils, etc.) may soon be quantitatively ac- 

 counted for on this basis. 



The biological system is clearly one in which most reactions are 

 carried out in the presence of water. There is, however, apparently 

 an abundance of water, and control of the amount and disposition 

 of water is of paramount importance. The more condensed regions, 

 on whose surfaces or within which much of the molecular traffic 

 will take place, must be prevented from increasing in size (or pre- 

 cipitating out ) to the point where such interferes with the execution 

 of ordered series of linked reactions by the condensed structures and 

 with a reworking of the molecules of the condensed structures them- 

 selves. We suppose that the incorporation or holding of too much 

 water, however, might well interfere with molecular traffic, would 

 certainly reduce mechanical rigidity, and might allow, for example, 

 deformation of the organ or tissue as a whole to produce local ( sub- 

 cellular) deformations in colloidal structure from which the system 

 would not recover. Control of the amount and disposition of water 

 in localities where the condensed structure is organized in the fomi 

 of a continuous network (gel) is most readily understood. How- 

 ever, deformation of a cross-linked network must necessarily involve 

 both flow of the entrapped fluid and a rearrangement of the relative 

 positions of the elements of the network. A network arrangement 

 is most effective where the network supports some biological func- 

 tion such as the transmission of tension, as with collagen and elastin. 

 In the examples chosen, the fraction of the tissue or organ occupied 

 by water is low, and the tissues are relatively inert. 



Many portions of the biological system, however, contain large 

 amounts of water, and interactions of the more condensed structures 

 may be taking place over intervening aqueous regions of consider- 

 able size. Here a distortion of the system need not change the 

 relative positions of the elements of the condensed structures; for 

 example, lamellae might slide past one another without altering the 

 interlamellar distance, thus without altering the local distribution of 

 water and solutes. 



In considering molecular interaction and the formation of con- 

 densed structures, we must first note that the word "molecule" can 

 be given a restricted meaning with respect to small molecules: 

 namely, a group of atoms joined by covalent (infrequently coordi- 

 nation) bonds which are stable with respect to kinetic energy (kT). 



