MOLECULAR MORPHOLOGY 59 



Fibrinogen molecules are recruited into fibrin fibers in the presence of 

 small amounts of thrombin. The actin-myosin system of muscle responds 

 to changes of potassium ion concentration or to ATP in a dramatic 

 manner. Such processes are readily appreciated in the light of the rich 

 physiological background in blood clotting and muscle contraction. 



But the general concepts of the lability of macromolecular complexes, 

 their sensitivity to their chemical environment, is perhaps not fully 

 appreciated by students of cellular physiology. The heritage of classical 

 morphology conditions the investigator to static rather than to dynamic 

 concepts. 



Many examples might be cited to illustrate the point. Investigators of 

 permeability phenomena in searching for the structure of the plasma 

 membrane are prone to regard this organelle as a fixed mosaic of 

 protein and lipid components. The small amount of evidence at hand 

 indicates that the protein component consists of a feltwork of very thin 

 protein filaments. Is it not possible that such proteins may be susceptible 

 to changes in ionic environment, to enzymes or to materials like ATP, 

 as is the case with the fibrous muscle proteins, and that such changes 

 may result in alterations of the protein which may profoundly affect 

 permeability properties? The protein of the erythrocyte envelope, 

 stromatin, may be isolated and studied for such properties. Perhaps this 

 would be a favorable system in which to test the suggested hypothesis. 



The case of neurofibrils may also be mentioned. Clearly demonstrable 

 by suitable cytological techniques, these structures have been regarded 

 by physiologists as mere fixation artifacts. Polarization optics, however, 

 has demonstrated the existence of longitudinal submicroscopic strands 

 which form the framework of neurofibrils. This molecular lattice is 

 extremely sensitive to its chemical environment. In what ways is this 

 sensitivity involved in the propagation of the nerve impulse? In the 

 normal maintenance, growth, and repair of the neuron? These questions 

 cannot be answered until the neuronin complex is better characterized 

 biochemically and structurally. 



The chromosome is of course the final and best illustration of the 

 importance of macromolecular ecology. On this structure is imposed 

 the task not only of maintaining through countless duplications a rigid 

 structural integrity — a process which physicists like Schrodinger find 

 difficult to understand — but also of presiding over the multitudinous 

 determinative processes of growth and development. The chromosome 

 is capable of a determinative role not only by virtue of its specific 

 intrinsic structure but also because of the very particular chemical 



