Membranes, in living cells

Chen, Y. Pant, A. C. Simon, S. M., P-glycoprotein does not reduce substrate concentration from the extracellular leaflet of the plasma membrane in living cells, Cancer Res. 61, 7763-7768 (2001). 

Contrary to the accumulated knowledge on the static or quasi-static characteristics of thin lipid films at air/water interface, less attention has been paid to the dynamical or nonequilibrium behavior of the film. Studies on the dynamical characteristics of thin lipid films may be quite important, because the life phenomena are maintained under nonequilibrium conditions. According to the modern biochemistry [11,12], thin lipid membrane in living cells is not a rigid wall but a thermally fluctuating barrier with high fluidity. In the present section, we will show that thin lipid film exhibits the various interesting dynamical tc-A characteristics, such as the "overshoot hump", the "zero surface pressure", and the "flat plateau". 

Because the lipid components of membranes must be in a fluid state to function as membranes in living cells, it is reasonable to assume that primitive membranes in the first forms of cellular life must also have had this property. Straight-chain hydrocarbons have relatively high melting points due to the ease with which van der Waals interactions can occur along the chains. Any discontinuity in the chains interrupts these interactions and markedly decreases the melting point. As an example, stearic acid contains 18 carbons in its alkane chain and melts at 68 °C, while oleic acid, with a cis-double bond between carbons 9 and 10, has a melting point near 14 °C. If cellular life today requires fluid membranes, it is reasonable to assume that the earliest cell membranes were also composed of amphiphilic molecules in a fluid state. 

The lipid bilayer membrane in living cells is a fluid membrane and, therefore, has no shear rigidity. However, within the cell and subjacent to the membrane lies an intricate network of the cytoskeleton that is attached with some regularity to the lipid bilayer that constitutes the cell membrane via proteins that are anchored in the bilayer. The shear rigidity of cell membranes is thus provided in a large measure by the cell cytoskeleton. 

Polymer Membrane Ion-Selective Electrodes Similar to Liquid Membranes in Living Cells... 

Figure 8.29. Ion transport mechanisms through lipid membranes in living cells. There are principally two kinds of transport protein (a) channel proteins, that is, a channelforming ionophore, and (b) carrier proteins, that is, a mobile ion carrier ionophore. The phenomena observed in living cells have much in common with those in artificial polymer membrane ion-selective electrodes. (From Widmer, 1993.)...

Membranes in Living Cells Membranes are an essential feature of living cells. Most biochemical processes occur in or near these dynamic and complex supramolecular structures. 

Controlled-release drug delivery systems mimic nature. Molecules called lipids are found in fats and also form the membranes of living cells. A lipid molecule is similar in struc-... 

A dual isotope labeling technique [85] has been used to measure membrane permeability in plant cells, based on the selective permeabiHty of the membranes of living cells to tritiated water and carbon-14 labeled mannitol. Kieran [29] showed that the results of the dual isotope labeling and Evan s Blue staining methods correlated well as indicators of cell viability however, the latter was preferable in terms of reagent cost and ease of analysis. 

Fig. 6.16. The membrane-permeant probe CCF2/AM monitors lactamase activity in living cells after removal of the bioactivatable protecting groups by endogenous esterases. 

Piljic, A. and Schultz, C. (2006). Annexin A4 self-association modulates general membrane protein mobility in living cells. Mol. Biol. Cell 17, 3318-28. 

Yeow, E. K. L. and Clayton, A. H. A. (2007). Enumeration of oligomerization states of membrane proteins in living cells by homo-FRET spectroscopy and microscopy Theory and application. Biophys. J. 92, 3098-104. 

In summary, we may thus conclude that PGLa and GS do not form stable, NMR-observable pores in native membrane as readily as they do in model bilayers. The corresponding tilted and/or inserted states of our two representative MAPs could only be comprehensively characterized in DMPC-based samples, where the peptides could be trapped in a uniform state. In living cells, on the other hand, these states would seem to be only of a transient nature, i.e. at the very moment when the antimicrobial peptide attacks the membrane and passes through the lipid barrier along its concentration gradient towards the cytosol. 

Sharma, P., Varma, R., Sarasij, R. C., Gousset, K., Krishnamoorthy, G. and Rao M, Mayor S. Nanoscale organization of multiple GPI-anchored proteins in living cell membranes. Cell 116 577-589, 2004. 

Fundamentally, the eel is simply a living battery. The tips of its head and tail represent the poles of the eel s battery . As much as 80 per cent of its body is an electric organ, made up of many thousands of small platelets, which are alternately super-abundant in potassium or sodium ions, in a similar manner to the potentials formed across axon membranes in nerve cells (see p. 339). In effect, the voltage comprises thousands of concentration cells, each cell contributing a potential of about 160 mV. It is probable that the overall eel potential is augmented with junction potentials between the mini-cells. 

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