Abstract.-Both membranes of Mycoplasma laidlawii and water dispersions of protein-free membrane lipids exhibit thermal phase transitions that can be detected by differential scanning calorimetry. The transition temperatures are lowered by increased unsaturation in the fatty acid residues, but in each case they are the same for membranes and lipids. The transitions resemble those observed for synthetic lipids in the lamellar phase in water, which arise from melting of the hydrocarbon chains within the phospholipid bilayers. Such melts are cooperative phenomena and would be greatly perturbed by apolar binding to protein. Thus the identity of membrane and lipid transition temperatures suggests that in the membranes, as in water, the lipids are in the bilayer conformation in which the hydrocarbon chains associate with each other rather than with proteins. Observations of morphological changes indicate that osmotic imbalance occurs when the membrane transition temperature exceeds the growth temperature, and that for transport processes to function properly the hydrocarbon chains must be in a liquid-like state.After many years of research, knowledge of the molecular organization of biological membranes remains meager. Although the concept of a phospholipid bilayer bounded on each side by protein is accepted by some investigators, others question the basic assumptions of the bilayer model and suggest alternative models in which the association between lipid and protein is hydrophobic rather than polar.1 If in fact lipids exist in membranes as bilayers, some unique property of a bilayer array might be detectable in membranes by a direct physical technique. Such a property is the reversible thermotropic gel-liquid crystal phase transition observed in phospholipid myelin forms in water. It has been studied by differential scanning calorimetry, differential thermal analysis, nuclear magnetic resonance spectroscopy, X-ray diffraction, and light microscopy; it arises from the melting of the hydrocarbon interiors of lipid bilayers.2-' Unlike transitions between liquid-crystalline phospholipid mesophases,5 the melt does not result in a molecular rearrangement and the lipids exist in the lamellar conformation both above and below the transition temperature. As in the case of bulk hydrocarbons, the melting point varies with unsaturation and chain lengths of the fatty acids in the phospholipids. Because cholesterol interferes, to detect a phase change above the ice point in a membrane an organism containing rather saturated fatty acids but little or no cholesterol must be chosen.The membranes of Mycoplasma laidlawii satisfy these requirements. Previous studies of this organism have shown that the cell membrane contains no choles-104

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The interactions of cholesterol and dipalmitoyl-phosphatidylcholine in bilayers were investigated by differential scanning dilatometry and related techniques. Dipalmitoyl-phosphatidylcholine bilayers ranging from 0 to 50 mol % cholesterol were studied over a temperature range of 0-50 degrees C. These investigations allowed construction of a three-dimensional surface with dimensions of mole fraction of cholesterol, temperature, and apparent partial specific volume. Much of the phenomenology reported for dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine-cholesterol bilayers appears and can be interrelated on this surface. In addition to the thermotropic events associated with the system, two cholesterol-induced events at 17.5-20 and 29 mol % cholesterol are particularly in evidence.

We propose a new approach to the determination of elastic and anelastic parameters of the Earth's structure from seismic data. Instead of measuring such functionals of the medium response as the phase delay or spectral ratios, we perturb parameters of the model to satisfy the observed waveforms. The advantage of our waveform inversion technique (WIT) is that it uses the properties of the Earth as a smoothing filter. Also, because the Earth's structure is the common denominator, the method allows simultaneous interpretation of different functionals of this structure; for example, the waveforms of Rayleigh and Love waves for the same source-receiver pair.Following extensive testing of the method on synthetic and actual data, we subject results of our analysis of 37 recordings for various sources and stations to 'pure path' decomposition. The period range of analysis extends from 160 to 630s. We distinguish four types of regions: stable continents, areas tectonically active within the last 400Myr, ocean floors younger than 38 Myr, and old ocean floors older than 38 Myr.The results indicate significantly different responses of the oceanic and continental areas at long periods. Stable continental and tectonic regions have nearly the same dispersion for periods greater than 300-35Os, and then diverge rapidly, with stable continents showing higher velocities. The young and old oceans, on the other hand, become distinct at periods as long as 500 s; the old oceans are faster. In terms of shear velocity models, the data are consistent with a difference of about 4 per cent between the young and old oceans in a depth range from 400 to 670 km, while the continental regions are similar and close to the global average. At shallower depths our structures are conceptually similar to those inferred from previous 'pure path' analyses and short-period surface wave studies.While we cannot, yet, establish statistically significant differences in Q for the four regions, such differences are obtained if one distinguishes only between continents and oceans: the Q for the latter is 10-20 per cent lower in the period range of the analysis.