Physical Properties Of Biological Membranes Research Articles

The evolution of membranes

Consideration of the influence of cholesterol on the physical properties of biological membranes leads to the conclusion that cholesterol increases the thickness of fluid membrane bilayers without appreciably increasing the microviscosity component of membrane fluidity. At sufficiently high cholesterol concentrations, the gel–liquid crystalline phase transition is completely eliminated in phospholipids–cholesterol mixtures and the system has the properties of a two-dimensional liquid over a wide range of temperatures. It is proposed that with the evolution of cholesterol and related sterols in an oxygen-rich atmosphere, the resulting modification of physical constraints on membrane properties made it possible for new evolutionary driving forces to manifest themselves leading to the peculiar properties of plasma membranes of eucaryotic cells.

Fluorescence studies on prokaryotic membranes.

Some of the more common techniques used to study the physical properties of biological membranes include electron microscopy, x-ray and neutron diffraction, Raman spectroscopy, electron spin resonance (ESR), nuclear magnetic resonance (NMR), infrared spectroscopy, fluorescence spectroscopy, and differential scanning calorimetry (DSC). The uses and limitations of these various analytical tools were examined in a number of recent reviews (Ax-elrodet al., 1976; Andersen, 1978; Seelig and Seelig, 1980; Jacobs and Oldsfield, 1981; Yguerabide and Foster, 1981; Amey and Chapman, 1983; Bach, 1983; Davis, 1983; Devaux, 1983,1985; Hoffmann and Restall, 1983; Verma and Wallach, 1983; Chapman and Benga, 1984; Makowski and Li, 1984; Bergelson et al., 1985; Blaurock, 1985; Bloom and Smith, 1985; Mühlethaler and Jay, 1985; McElhaney, 1986; Restall and Chapman, 1986).KeywordsElectron Spin ResonanceOuter MembraneFluorescence StudyPolarization RatioExogenous LipidThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Physical properties of biological membranes determined by the fluorescence of the calcium ionophore A23187

Interactions between the divalent cation ionophore, A23187, and the divalent cations Ca 2+, Mg 2+, and Mn 2+ were studied in sarcoplasmic reticulum and mitochondria. Conductance measurements suggest that A23187 facilitates the movement of divalent cations across bilayer membranes via a primarily electroneutral process, although a cationic form of A23187 does carry some current. On the basis of fluorescence excitation spectra, A23187 can form either a 1:1 or 2:1 complex with Ca 2+ in organic solvents. However, in biological membranes, only the 1:1 complexes with Ca 2+, Mg 2+, or Mn 2+ are detected. A23187 produces fluorescent transients under conditions of Ca 2+ uptake in sarcoplasmic reticulum, which appear to represent changes in intramembrane Ca 2+ content. Changes in A23187 fluorescence due to mitochondrial Ca 2+ accumulation are much smaller by comparison and fluorescence transients are not detected. Studies of A23187 fluorescence polarization and lifetimes in biological membranes allow a determination of the rotational correlation time (ρh) of the ionophore. In mitochondria at 22 °C, ρh is 11 nsec in the presence of Ca 2+ and Mg 2+, and less than 2 nsec in the presence of excess EDTA. The present results are consistent with a model of ionophore-mediated cation transport in which free M 2+ binds with A23187 at the membrane surface to form the complex M(A23187) +. Reaction of this complex with another molecule of A23187 at the membrane surfaces results in the formation of electrically neutral M(A23187) 2, which carries the divalent cation through the membrane. These results are discussed in terms of physical properties of biological membranes in regions in which divalent cation transport occurs.