Cell membrane phase transition temperature

One important factor when drying yeast cells is intracellular trehalose. The intracellular trehalose content is considered a critical determinant of stress tolerance in yeast (Nishida et al., 2004). Trehalose is a nonreducing a-linked disaccharide commonly found in any hydrobiotic organisms. When phospholipid membranes are dried, the temperature at which the gel to hquid crystal phase transition occurs increases (Crowe et al., 1984). It has been shown that trehalose interacts with model membranes during drying and lowers the dry membrane phase transition temperature (Crowe et al., 1986). 

Cells can draw on a large number of types of raw material to modify the functional properties of membranes in temperature-adaptive manners. In the most general of terms, there are two fundamental ways in which phase transition temperatures and localized order can be altered. These are (1) intrinsic changes in the lipid composition of the membrane, notably in lipid class, molecular species, and cholesterol content, and (2) extrinsic changes, alterations in the solution bathing the inner and outer surfaces of the bilayer. The latter changes include... 

The occurrence of cholesterol and related sterols in the membranes of eukaryotic cells has prompted many investigations of the effect of cholesterol on the thermotropic phase behavior of phospholipids (see References 23-25). Studies using calorimetric and other physical techniques have established that cholesterol can have profound effects on the physical properties of phospholipid bilayers and plays an important role in controlling the fluidity of biological membranes. Cholesterol induces an intermediate state in phospholipid molecules with which it interacts and, thus, increases the fluidity of the hydrocarbon chains below and decreases the fluidity above the gel-to-liquid-crystalline phase transition temperature. The reader should consult some recent reviews for a more detailed treatment of cholesterol incorporation on the structure and organization of lipid bilayers (23-25). 

SM is the major phosphosphingolipid in mammalian tissues and lipoproteins, and has been studied extensively with respect to its role in the formation of sterol-enriched ordered membrane domains and cell signaling (J. Slotte, 2006). Like Cers (and most other complex sphingolipids), SMs typically have high phase-transition temperatures (>37°C), which is a factor in their tendency to associate with rafts. 

This is of interest because the effect of such additives on the rate of membrane fusion in model systems often correlates with the effect of these additives on the bilayer-to-hexagonal phase transition temperature [75]. The increase in the bilayer-to-Hn-phase transition temperature generally inhibits cell fusion, and several compounds which stabilize bilayers show antiviral activity [76]. 

Membranes are composed of phospholipids and proteins. The fatty acid composition of the phospholipids in a membrane influences how it is affected by the cold. In general, as the temperature of a cell is lowered the lipids in the membrane bilayer undergo a phase transition from a liquid crystalline (fluid) state to a gel (more solid) state. The temperature at which this transition takes place is very narrow for phospholipids composed of a simple mixture of fatty acids, but is quite broad for the phospholipids in cellular membranes. It is usually implied from various methods... 

For most tissues, cells and organs, the effects of cold on the cellular membrane are fully reversible. Cells cooled to 1 °C to 4 °C for short periods of time (about four hours) can regain normal cellular functions, including membrane-linked functions, when rewarmed. This seems to suggest that the phase transition in the membrane-bound phospholipids is reversible when the temperature is elevated to normothermia. 

Fundamental membrane research has benefited greatly from the study of monolayers. One of the most important discoveries from this sort of research is the very existence of two-dimensional phases and phase transitions. Generally, studies of the sort that can be carried out with monolayers and bilayers cannot be directly extended to living cells, but some exceptional cases have shown that the extrapolation is valid. For example, it is known from monolayer studies that the presence of unsaturated hydrocarbon chains in lipid monolayers prevents some phase transitions from occurring as the temperature is lowered. Certain mutants of Escherichia coli are unable to synthesize fatty acids and hence can be manipulated through the compounds they are provided as nutrients. Abnormal levels of saturated hydrocarbon can... 

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