Polar Pore Research Articles

Diffusion and Electric Mobility of KCI within Isolated Cuticles of Citrus aurantium

Fick's second law has been used to predict the time course of electrical conductance change in isolated cuticles following the rapid change in bathing solution (KCI) from concentration C to 0.1 C. The theoretical time course is dependent on the coefficient of diffusion of KCI in the cuticle and the cuticle thickness. Experimental results, obtained from cuticles isolated from sour orange (Citrus aurantium), fit with a diffusion model of an isolated cuticle in which about 90% of the conductance change following a solution change is due to salts diffusing from polar pores in the wax, and 10% of the change is due to salt diffusion from the wax. Short and long time constants for the washout of KCI were found to be 0.11 and 3.8 hours, respectively. These time constants correspond to KCI diffusion coefficients of 1 x 10(-15) and 3 x 10(-17) square meters per second, respectively. The larger coefficient is close to the diffusion coefficient for water in polar pores of Citrus reported elsewhere (M Becker, G Kerstiens, J Schönherr [1986] Trees 1: 54-60). This supports our interpretation of the washout kinetics of KCI following a change in concentration of bathing solution.

Water permeability of plant cuticles

Using the system vapor/membrane/liquid, permeability coefficients of cuticular transpiration (P ct) were determined as functions of water activity in the vapor (a wv). Enzymatically isolated cuticular membranes (CM) of Citrus aurantium L. and nonisolated CM of onion bulb scales and eggplant fruits were investigated. P ct of Citrus and eggplant CM decreased with decreasing a wv, while permeability coefficients of CM of onion were independent of a wv. Extraction of soluble cuticular lipids (SCL) from the CM of Citrus increased permeability coefficients by a factor of approximately 500. This extraction had no effect on the dependence of P ct on a wv.Treating cuticular membranes as a resistance network consisting of SCL and the polymer matrix, it is shown that the permeability of onion CM is determined by the resistance of the SCL arranged in series with the polymer matrix. In this type of CM liquid and vapor are separated by a continuous, nonporous layer of SCL, and the driving force of transpiration is the gradient of partial pressure of water vapor across the SCL layer. In the CM of Citrus and eggplant, the SCL layer is traversed by polar pores that swell or shrink depending on a wv. However, liquid continuity is maintained across these membranes down to a wv=0.22, the lowest value used. In this type of membrane the driving force of transpiration is the water potential gradient across the membrane.

Water permeability of isolated cuticular membranes: The effect of pH and cations on diffusion, hydrodynamic permeability and size of polar pores in the cutin matrix

The upper astomatous cuticle of Citrus aurantium L. leaves was isolated enzymatically or chemically, extracted with lipid solvents and used for the determination of water diffusion (P d ) and osmotic water permeability (P f ). The water permeability was strongly dependent on the pH value and the cations of the buffer solutions. In presence of monovalent alkali metal ions P d increased almost five fold between pH 3 and 11. The shape of the plot P d vs. pH suggests the presence of 3 different dissociable groups fixed to the membrane matrix. They are tentatively identified as two carboxyl groups dissociating between pH 3 to 6 and 6 to 9, respectively, and as phenolic hydroxyl groups dissociating above pH 9. The carboxyl group dissociating between pH 6 and 9 discriminated between alkali metal ions according to their ionic radius. Water permeability was lowest in the Li(+) from and increased in the order Li(+)<Na(+)<K(+)<Rb(+). The water permeability of membranes in Ca(2+) form was only slightly higher than that of membranes in H(+) form and little dependent on pH. The energy of activation which amounted to 13 kcal mol(-1) was constant over the temperature range of 5 to 40°C and pH independent. Since P f was greater than P d it was concluded that the cutin matrix contained polar pores and that water transport caused by a chemical potential gradient was both by diffusion and by viscous flow. The porous nature of the membranes was also confirmed by the fact that they are permselective according to size of the permeating molecule. Using the empirical equations of Paganelli and Solomon (1957) and Nevis (1958) the equivalent radius of the pores was estimated to be 0.46 and 0.45 nm, respectively. This estimate is in good agreement with the observations that (a) [(14)C]urea (molecular radius r s =0.264 nm) and [(3)H]glucose (r s =0.444 nm) penetrated the membranes and (b) the reflection coefficient was equal to one for raffinose (r s =0.654 nm) and sucrose (r s =0.555 nm) but 0.95 for glucose and 0.78 for urea. Both, the reflection coefficient and the pore radius estimates were pH independent, hence the increase in water permeability with increasing pH was due to an increase in the number of pores per unit area (1 cm(2)) from 5x10(10) at pH 3 to 15.8x10(10) at pH 9.

Microsporidan spore structure as revealed by scanning electron microscopy

Scanning electron microscopy (SEM) was used to study spore surface morphology of the microsporidans Nosema necatrix, N. whitei, Parathelohania opacita, P. legeri, and Pleistophora sp. Nosema and Pleistophora spores were similar, having spores with either smooth or highly rugose surfaces. The surface of Parathelohania spores was characterized by four to six parallel ridges joining to form an apical ring. Polar filaments extruded from N. necatrix spores, by pressure, demonstrated the nature of the polar filament, anchor of the polar filament, terminal pore, and sporoplasm. The demonstration of different morphological characters by SEM makes evident its applicability to the identification of Microsporida.

The influence of lipid composition and of some adsorbed proteins on the capacitance of black hydrocarbon membranes

It is known from earlier work that the capacitance of a bimolecular egg lecithin + n-decane membrane containing less than 50 % by volume of n-decane is approximately 0·38 microfarad/cm 2. Cell membrane capacitances are usually greater than 0·5 microfarad/cm 2, and an average value of 1 microfarad/cm 2 is often quoted. This paper is concerned with the explanation of these two capacitances in terms of differences in molecular structure of the synthetic and natural membranes. Two possible explanations have been examined. The first concerns the importance of the nature of the lipid. The second concerns the influence of protein associated with the membrane by adsorption, so as to give a sandwich structure. It is concluded that by the inclusion of appropriate lipids the capacitance of the synthetic membranes may rise to a maximum value of about 0·6 microfarad/cm 2. The adsorption of the proteins used in the present work does not affect the capacitance, at least, between 5 × 10 −3 and 10 7 c/s. It is pointed out, on the other hand, that if up to 2 to 3 % of the area of a synthetic membrane was made up of “polar pores”, its capacitance could be increased by a factor of two or more, thereby becoming more closely in line with the cell membrane values.