Temperature-dependent Phase Change Research Articles

Temperature dependence of mast cell histamine secretion.

A diverse set of direct experimental observations and theoretical considerations supports the existence of fluidity in cell membranes (1) . Several membrane-dependent functions of Escherichia coli have been shown to exhibit biphasic Arrhenius plots with transition temperatures that are determined by the degree and configuration of the unsaturation of fatty acids incorporated into the membrane phospholipids (2, 3) . Biphasic plots and even discontinuities in the plots have been demonstrated for several mitochondrial enzymes in their membrane-bound states (4, 5) . It has been proposed that the transitions in activation energy for these reactions are related to a phase change in the organization of the membrane phospholipids (2-5) . In mammalian cells the mixing of surface antigens in newly formed heterokaryons (6) and the capping of surface antigens in the presence of specific antibodies (7) have been interpreted as evidence for lateral movement of proteins in a more or less fluid membrane . Both these phenomena have a temperature dependence that is consistent with a transition in membrane structure occurring in the vicinity of 15°C, but the available data do not permit distinction between phase transitions and high activation energies for lateral diffusion of membrane proteins (7) . Secretion by exocytosis would be expected to be dependent on the state of the cell membrane and thus substantially affected by any temperaturedependent phase change in the membrane. This reasoning led to study of the effect of temperature on the secretion of histamine by mast cells .

Fundamentals of ridge crest topography

A linear relationship between the sea floor depth and the square root of age has been found for ocean lithosphere spreading from mid-ocean ridges. The asymptotic solution of depth as a function of age for the thermally contracting lithosphere predicts a linear dependence of depth ontwith a proportionality involving the initial lithosphere temperature, the thermal diffusivity, and the isostatic expansion coefficient averaged to include any temperature dependent phase changes. Empirical depth observations, when plotted as a function of the square root of age, bear out this prediction well, but there is a variation in the gradient,ht, along the ridge on a fine scale (up to 20% over 200 km). This implies a fundamental variation of the contraction parameter over the same scale, most probably of compositional origin. Details of a more complete cooling model near the ridge crest, including a crust of different thermal parameters than those of the mantle, predict a crestal height about 0.2 km below that of the simplified model. Individual profiles from the southeast Pacific show no such crestal deviation, and it is concluded that by quickly cooling the new crust, hydrothermal circulation may remove any effects of the crust which would be seen in the topography of a lithosphere cooled totally by conduction. The straightness of depth versust for older ocean data (to 80 m.y.) precludes any basal isothermal boundary shallower than 100 km.