Cell stretch, reversal

Using optical traps, Cui and Bustamante [76] stretched isolated chicken erythrocyte fibers, and Bennink et al. [77] pulled on fibers directly reconstituted in the flow cell from X-DNA and purified histones with the help of Xenopus extracts (see Fig. 10a for a schematic of the latter experiment). Up to 20 pN, the fibers underwent reversible stretching, but applying stretching forces above 20 pN led to irreversible alterations, interpreted in terms of removal of histone octamers from the fibers with recovery of the mechanical properties of naked DNA. 

The plasma membrane of the cell is a lipid bilayer sheet in which membrane-bound proteins are embedded. Steps 4B-6B of Figure 1.21 illustrate some events in the production of a membrane-bound protein. After synthesis of the protein, the ribosome on which it was formed dissociates from the membrane but the protein remains bound to the membrane (Step 4B). This binding is mediated by a short stretch of lipophilic amino acids that may occur near the C terminus, as shown in Figure 1.21, or near the N terminus in the case of other proteins. Subsequently, part of the ER membrane forms a bud that breaks off (Step 5B) to form a secretory vesicle (Step 6B). The continued association of the entire membrane-bound protein during the budding process and during subsequent events is maintained by the special lipophilic sequence. Eventually, the secretory vesicle fuses with the plasma membrane in a process that resembles a reversal of Steps 4B-6B. After completion of the insertion of the membrane-bound protein into the plasma membrane, its N terminus is in contact with the extracellular fluid and its C terminus is in contact with the cytoplasm, at least for the protein depicted in Figure 1.21. 

Gupta, V., J. A. Werdenberg, B. D. Lawrence, J. S. Mendez, E. H. Stephens, and K. J. Grande-Allen. 2008. Reversible secretion of glycosaminoglycans and proteoglycans by cyclically stretched valvular cells in 3D cultme. Ann Biomed Eng 36 1092-103. 

Most time-resolved measurements involve the investigation of reversible reactions such as stimulation of a compound to an excited state by a laser pulse followed by a relaxation to the ground state over a period of time. Alternatively, physical stress may be applied to a sample that reversibly changes a property of a sample (e.g., the rapid application of a strain to a polymer film by stretching it to an amplitude within the elastic limit). The concept of reversibility is important because the sample must be in the same state when the interferometer mirror is stepped to its next position and the reaction is reinitiated. A few workers have developed methods of recharging the cell at each step so that irreversible reactions can be studied. Examples of each of these processes are given in the following section. 

Figure 2. Examples of numerical solutions for the cathodic current distribution on a plate electrode immersed in a cell with the counter electrode at the bottom. Three cases are compared (a) (/ column) completely reversible kinetics (primary distribution) (b) center) irttermedrate kinetics (Ub 0.2) (c) (right column) irreversible kinetics (Wa 10). The top row provides a comparison of the current distribution or the deposit profile on the cathode (cross-hatched region). The center row provides the current distribution along the electrode ( stretched ). The bottom row provides the corresponding poterrtial distributions. It is evident that the current distribution uniformity increases as the electrode kinetics become more passivated (Cell-Design software simulations ).

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