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  • Author: Josef Weigl x
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The exchange of tissue water of corn roots with ambient water was investigated using D2O, a gaschromatographic method, and a technique which avoids prolonged contact of tissue water with atmospheric water.

The exchange experiments were performed at 10°, 20°, and 25 °C and the activation energies for different exchange phases were calculated by a method involving a graphical determination of the relative exchange rates at certain H2O/D2O gradients. Log10 of the rates were plotted versus 1/T as usual and the activation energies were calculated from the slopes of the straight lines. The energy of activation of the exchange process increased from 4.4 kcal-mol-1 in an initial phase (exchange of surface water and free space water) to 6.3 kcal·mol-1 in later phases which represent the processes of permeation through plasma and plasma membranes. This suggests that the hydrogen bonds of permeating water have a mean energy of 6 —7 kcal·mol-1 resulting from interaction with membrane (and plasma) constituents.

The theory is proposed that cell membranes contain water phases with hydrogen bonds stronger than those in pure liquid water. These water phases are assumed to be located mainly within apolar portions of globular membrane proteins. Not solely a continous lipid layer, but a specific arrangement of polar and apoiar portions of globular membrane proteins is regarded to be essential for semipermeability and other membrane properties. Results from various authors were considered in establishing the general working hypothesis that agents like apoiar compounds which increase water structure decrease water permeability, and agents like salts which disrupt water structure increase water permeability.

Durch Vergiftung mit Arsenat in steigender Konzentration wird die Atmung von Maiswurzeln zunehmend erhöht und ihr Gehalt an labilem Nucleotidphosphat und organischen Phosphatverbindungen in zunehmendem Maße herabgedrückt. Gleichlaufend damit wird ihre Fähigkeit zur aktiven Ionenaufnahme gelähmt. Die Befunde werden als Beweis für die Wirkung von „energiereichen“ Phosphatverbindungen (letztlich von ATP!) beim Vollzug der aktiven (endergonischen) Ionenaufnahme angesehen.

Xenon (1—3 atm added to air) was found to bring about a marked diminuation or total inhibition in the response of Mimosa pudica to seismonic stimulation. Since the most conspicuous property of xenon is to form hydrate crystals of the clathrate type4, the result may stress the importance of water structure for the properties of cellular structures like membranes 16, 22. Associations of globular macromolecules (proteins or lipoproteins) with water molecules which may play an essential role in the structure of membranes (and protoplasm) and in certain functional changes of membranes 16, 20, 21 may be visualized as rather labile and (in specialized cells particularly) susceptible to changes in the environment. Incorporation of apolar gases may increase the rigidity of the intramolecular hydrogen bonds of globular membrane proteins and of the intermolecular hydrogen bonds of water phases and therefore prevent the sudden loss of water from the pulvinus of Mimosa which is reported to be due to, 1. a sudden increase in membrane permeability and, 2. a loss in the “water brinding power” of protoplasmatic components 13, 14. Alternative hypotheses such as the inhibition of oxygen diffusion by inert gases 24 are considered.

Using the device of Fig. 1 (this paper and 1. c. 3) efflux of Cl across the plasma membrane of root cortex cells was found to increase with increasing external concentration of various salts. We conclude that non-specific (passive) plasma membrane permeability (!) to anions increases with increasing external concentration of salts. As a primary result passive efflux of anions, which is downhill in the electrical gradient, increases.

Efflux of Rb across the plasma membrane is only slightly stimulated by external KCl and is inhibited by Ca2⊕.

The decrease in electrical potential with increasing external salt concentration is a consequence of the increase in passive plasma membrane permeability to anions.

Interaction of salts with charged groups of the plasma membrane is not unlikely to be the reason for the increase of the permeability to anions and for the multiple hyperbolic influx isotherm observed at high external concentrations (1-100 mM). We want to emplasize that a multiple hyperbolic isotherm may be considered a feature of passive ion movement across a barrier of different charged groups.

Efflux of ions across the plasmalemma of root cells was studied in order to understand how the plasmalemma acts as a barrier to passive ion distribution.

High external concentrations of salts (KCl, CaSO4 etc.) were found to increase the permeability of the plasmalemma to Cl. The decrease in resistance of the plasmalemma to anion permeation was suggested to be due to binding of cations to negatively charged groups of the membrane. We have stressed the significance of binding of cations (Ca2⨁, H, K, Na) to phospholipid films10 for the mechanism of ion permeation across the plasmalemma of plant cells1-4. H-efflux from salt-loaded roots is likely to be stimulated with increasing external salt concentration. Efflux of Cl was stimulated by externally applied H2SO4. The different effect of Ca2⨁-salts on Cl- and Rb® (K®)-permeability of salt-loaded roots suggests that it is not only he H-ions which decrease the charge density of the membrane but also the Ca2O-ions (and monovalent cations at sufficient concentrations?). Thus Ca2⨁ increases passive anion permeability by decreasing the density of negative charges of the membrane and decreases passive (nonspecific) monovalent cation permeability. Ca2⨁ and H (K, Na) appear to act synergistically on Cl-permeability and antagonistically on monovalent cation permeability.

The high Rb® (K®)-efflux across the plasmalemma (fig. 1) may contribute to the electric conductance and potential of the plasmalemma. The plasmalemma is to be expected to depolarize as the pH of the medium is lowered. However, this would not unequivocally point to H-conductance as a major component f plasmalemma conductance (KITASATO 16), since permeability of the plasmalemma to Cl(PCl) appears o be increased as the pH is lowered.

It is concluded that the plasmalemma of plant cells acts as an electrostatic barrier against anion diffusion. The lipid of the membrane favours the permeability to apoiar molecules as suggested by OVERTON 5 and COLLANDER 6. The general barrier action to the movement of polaf and charged molecules is believed to arise from a compact arrangement of cholesterol and apoiar hydrocarbon chains of phospholipids (and apoiar amino acid residues). Small polar molecules penetrate the lipid lattice easier than big polar molecules. However, the charged groups of the phospholipids play an important role in permeability to ions rather than the plasmalemma being an electrically neutral lipid filter.

Trans-stimulation by salts of the passive efflux of Cl across the plasma membrane of plant cells was established previously. In this paper the trans-effect of salts is compared with the effect of nystatin on ion efflux. It is further shown that the influx of anions is also stimulated by external salts. Influx of Cl was stimulated by K2SO4 (>~1 mM), influx of SO4 2⊖ was stimulated by KCl (>~lmM). This suggests that with increasing external salt concentration not only the electrical potential across the plasmalemma is lowered (due to preferential permeability to monovalent cations) but alsoth e permeability (i. e. the permeability coefficient) of the plasmalemma to anions is increased. According to the model proposed for the salt-stimulated decrease in the resistance to passive anion permeation the plasmalemma may be considered a lipid lattice-electrofilter. The nature of the coupling of the counter fluxes is discussed.

A strong physical association of indoleacetic acid. 2.4-dichloro-phenoxyacetic acid, indolepropionic acid and indolebutyric acid with lecithin was found which might have physiological significance (regulation, polar mobility). The association is assumed to be mainly due to bonding between the complementary charged groups of the phospholipid and auxin molecules and to specific interaction of the more hydrophobic parts of the molecules.

The following interactions were established:

Lecithin dissolved in CCl4 moves indoleacetic acid and 2.4-dichloro-phenoxyacetic acid out of an aqueous phase. Cholesterol, long chain fatty acids and amines did not give this interaction with indoleacetic acid and 2.4-dichlorophenoxyacetic acid 4, 5.

1 mole lecithin was found to bind up to 0.8 mole indoleacetic acid. Cephalin and phosphatidylserin exhibit a weaker interaction. Indolepropionic acid and indolebutyric acid were found to compete with indoleacetic acid. There was no effective competition of benzoic acid, phenoxyacetic acid, phenylacetic acid, cholesterol and several fatty aids with indoleacetic acid for the binding sites on the lecithin molecule. 2,4-dichlorophenoxyacetic acid appears to be bound stronger than indoleacetic acid and phenoxyacetic acid. Indoleacetic acid and 2.4-dichlorophenoxyaetic acid were incorporated into swollen lecithin lamellae.

Similar interactions are to be expected for other hormones and phospholipids. The lipoprotein structures of cell membranes may be visualized to interact even more specificly with growth hormones than our model system. It is suggested that interaction of hormones with membranes should be considered in theories on regulation. Experiments on ion permeability indicate an influence of indoleacetic acid on cell membranes.