Section N - Cell specialization

Each cell within vertebrate striated muscle contains within its sarcoplasm many parallel myofibrils which in turn are made up of repeating sarcomere units. Within the sarcomere are the alternating dark A band and light I band, in the middle of which are the H zone and Z line, respectively. A myofibril contains two types of filaments the thick filaments consisting of myosin which are present only in the A band, and the thin filaments consisting of actin, tropomyosin and troponin. When muscle contracts, the thick and thin filaments slide over one another, shortening the length of the sarcomere. 

the major constituent of the thin filaments, can exist as monomeric globular G-actin or as polymerized fibrous F-actin. The actin filaments are connected to the thick filaments by cross-bridges formed by the SI heads of myosin. 

The cyclic formation and dissociation of complexes between the actin filaments and the SI heads of myosin leads to contraction of the muscle. On binding to actin, myosin releases its bound Pj and ADP. This causes a conformational change to occur in the protein which moves the actin filament along the thick filament. ATP then binds to myosin, displacing the actin. Hydrolysis of the ATP returns the SI head to its original conformation. 

Tropomyosin is an elongated protein that lies along the thin filament and prevents the association of myosin with actin in the resting state. Troponin is a complex of three polypeptide chains TnC, Tnl and TnT. Ca2+ ions released into the sarcoplasm from the sarcoplasmic reticulum in response to a nerve stimulation bind to TnC and cause a conformational change in the protein. This movement is transmitted by an allosteric mechanism through Tnl and TnT to tropomyosin, causing the latter to move out of the way and allowing the actin and myosin to associate. 

Both actin and myosin are found in nonmuscle cells where they are involved in cell movement, the movement of organelles around the cell, and cell division. 

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As discussed in Section 8.2 the relation between the chemical diffusion coefficient and diffusivity (sometimes also called the component diffusion coefficient) is given by the Wagner factor (which is also known in metallurgy in the special case of predominant electronic conductivity as the thermodynamic factor) W = d n ajd In where A represents the electroactive component. W may be readily derived from the slope of the coulometric titration curve since the activity of A is related to the cell voltage E (Nernst s law) and the concentration is proportional to the stoichiometry of the electrode material ... 

In the method developed by Exerowa, Cohen and Nikolova [144] the insoluble (or slightly soluble) monolayers are obtained by adsorption from the gas phase. A special device (Fig. 2.28) was constructed for the purpose a ring a in the measuring cell of Scheludko and Exerowa for formation of microscopic foam films at constant capillary pressure (see Section 2.1.2.). The insoluble (or slightly soluble) substance from reversoir b is placed in this ring. Conditions for the adsorption of the surfactant on either surface of the bi-concave drop are created in the closed space of the measuring cell. The surfactant used was n-decanol which at temperatures lower than 10°C forms a condensed monolayer. Thus, it is possible to obtain common thin as well as black foam films. The results from these studies can be seen in Section 3.4.3.3. 

Let us consider the conformation of a single chain in the special case of a disordered state with no vacancy. Fixing Ao=0, Ai = 1 in the theory developed above, the number Wh (n) of paths that visit all lattice points (cells) without overlap, referred to as Hamiltonian path, is found [21]. Within the theoretical framework (2.146) described in the preceding sections, the entropy of Hamiltonian paths is estimated by... 

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