Biodynamics of living cells: the molecule-contraction theory.

Living cells can carry out work, by transforming chemical to mechanical energy. It has been found that such biodynamic activities are carried out by filaments built of proteinaceous subunits. Since work in general involves contractions, the proteins which are involved in the execution of work may thus be called "contractile proteins". There seem to be three such proteins: flagellin, actin and tubulin, forming bacterial flagella, actin filaments and microtubules, respectively. Myosin is also a filament-forming protein, important for the activity of actin filaments, notably in striated muscle cells, but it is not a contractile protein. When a muscle works, that is, contracts, then it is observed that the actin filaments slide in between the myosin filaments, and this observation has been used to develop a theory of contraction, the "sliding filament" theory. But it should be emphasized that this theory is essentially descriptive, not explanatory, and the main point in muscular activity, the presence of two kinds of filament, is not found in most other cases when cells are performing physical work. In order to solve this dilemma and reach a general theory of biodynamics the molecule-contraction theory, as presented here and on some earlier occasions, was created. The essential aspect of the theory is the existence of "contractile" proteins, which also have the property of forming filaments. This theory is based on four premises, out of which three are supposed to be involved in all kinds of biodynamic activity, while the fourth is required only in those instances where polymerization and depolymerization is a constituent part of the performance of mechanical work.