The state of water in the outer barrier of the isolated frog skin

Summary The flux of water across the outer barrier of the frog skin is generally regarded as the rate-limiting step in the movement of water across the whole membrane. This paper presents some evidence that, at room temperature, the flux of water across the outer barrier occurs through water in a non-liquid state. The organization of water in a non-liquid state lowers the diffusion coefficient of water through water by several orders of magnitude. The study employs a method recently developed in this laboratory which permits measurement of unidirectional fluxes at the outermost part of an epithelial membrane mounted as a flat sheet. Only above 25°C is the activation energy for the flow of tritiated water (4.3 kcal mole⁻¹) similar to the one observed in free water (4.6 kcal mole⁻¹). At temperatures around 15°C, the energy of activation is 8.5 kcal mole⁻¹. At temperatures near 0°C, at which the frog lives only part of the year, the energy of activation is 16.7 kcal mole⁻¹.