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Summary On a mole basis, water is by far the most abundant molecule in any living cell. Furthermore, the internal surface area of a cell is enormous, and thus a very large fraction of cell water occurs near the interfaces. Classical cell physiology takes into account the effects of both ionic and nonionic solutes (for instance, in calculations of osmotic properties). Only fairly recently, however, has it become clear that physico-chemical parameters based on bulk measurements fail to account for many of the properties of large surface-to-volume-ratio systems. The explanation for these differences, which can be pronounced, must be sought in the long-range effects of interfaces on the structure and properties of water and aqueous solutions. The geometric arrangements in bulk water still defy a rigorous description, and less is known about the structure of aqueous solutions (of small solutes, both ionic and nonionic). Even less is known about the long-range geometric effects on the aqueous structure induced by proximity to an interface. The water (or solution) affected by an interface is referred to as vicinal water. Its properties often differ from the corresponding bulk properties; although the energetics of vicinal and bulk water do not differ greatly, some properties are notably different, among them the dielectric constant and relaxation properties, viscosity, density, and particularly the specific heat. Although estimates of the extent of vicinal water vary greatly among different investigators, the thickness of modified layers is at least 10 or 20 molecular diameters of a water molecule, and perhaps as high as 50 (to 100) molecular diameters. One of the most characteristic features of vicinal water is the occurrence of anomalies in its properties as a function of temperature. These anomalous changes occur approximately 15°C apart, i.e., near 14–16°, 29–32°, 44–46°, and 59–62°C. The changes at these transition temperatures are quite abrupt and in many cases very pronounced. The effects of vicinal water and the thermal transitions on cell biology are frequently dramatic and surprising. The functional role of vicinal water in cell biology is the subject of the next chapter. |
