Increase In Cation Concentration Research Articles

Influence of different nitrogen sources and levels on ion content of cabbage (Brassica oleracea var. capitate)

The objective of this study was to determine the effects of different nitrogen (N) sources and levels on ion content in cabbage (Brassica oleracea var. capitate ’Yalova‐1'). In the process of ion uptake by plants, electro‐neutrality is maintained both by the plant and the nutrient medium in which the plant is grown. N fertiliser not only affects yield but also the quality of the plant. Nitrates convert to nitrite in plant tissues which may cause health problems in infants and also form carcinogenic substances. Higher doses of nitrites change haemoglobin to methaemoglobin and this inhibits the transport of blood oxygen in the human body. Increased rates of N fertilisers also cause the accumulation of oxalic acid in vegetables. Oxalic acid causes acute toxicity if taken with calcium (Ca) and this forms stones in the kidney. Cabbage plants were grown in field conditions with five N fertiliser types, four doses and three replications. The treatments of N consisted of: no added fertiliser; 100, 200, and 400 kg N ha−1 as potassium nitrate, ammonium nitrate, urea, ammonium sulphate, and farmyard manure. The results demonstrate that plant head weight was generally greater when mineral fertiliser rather than organic fertiliser was supplied to plants. In the 400 kg N ha−1 treatment as potassium nitrate, ammonium nitrate, ammonium sulphate, urea, and farmland manure application, plant yields were 66%, 61%, 40%, 49%, and 44% higher than with no added fertiliser, respectively. When the data were subjected to multiple regression analysis, effects of ammonium nitrate fertiliser application at 342 kg N ha−1 on plant yields (3333 g plant−1) were higher than that of other fertiliser applications. With increasing NO3– nutrition, the bulk of anion charge appeared as organic anion accumulation in the plants. The increase in organic anion accumulation was paralleled by an increase in cation concentration (K+, Ca+2, Mg+2, Na+). Total inorganic anion levels (NO3–, SO4 2–, H2PO4–, Cl–) were relatively constant. The effects of increasing NO3– nutrition in stimulating organic anion accumulation such as glutamate, malate, and oxalate were far greater than ammonium and the other nutrient forms. Nitrate content of plants increased with the increasing N application, especially with nitrate fertiliser. The increases observed in the plant were highest in the plots with potassium nitrate applied and lowest in plots with no added fertiliser treatment. The results also indicated that farmyard manure was much more suitable than mineral fertilisers for plant quality according to lower total N, nitrate, and oxalic acid contents of cabbage plants. However, ammonium sulphate application at the rate of 250 kg N ha−1 was the most suitable fertiliser application rate for plant quality according to lower risk yields of plants (2650 g plant−1) for human nutrition.

Identification of the Type I Collagen-binding Domain of Bone Sialoprotein and Characterization of the Mechanism of Interaction

Bone sialoprotein (BSP) is an anionic phosphorylated glycoprotein that is expressed almost exclusively in mineralized tissues and has been shown to be a potent nucleator of hydroxyapatite formation. The binding of BSP to collagen is thought to be important for the initiation of bone mineralization and in the adhesion of bone cells to the mineralized matrix. Using a solid phase assay, we have investigated the interaction between BSP and collagen. Initial studies showed that raising the ionic strength, decreasing the pH below 7, or introducing divalent cations diminishes but does not abolish the binding of BSP to collagen, indicating that the interaction is only partly electrostatic in nature. Both bone-extracted and recombinant (r)BSP exhibited similar binding affinities, indicating that post-translational modifications are not critical for binding. To identify the collagen-binding domain, recombinant peptides of BSP were studied. Peptide rBSP-(1-100) binds to type I collagen with an affinity similar to that of full-length rBSP, whereas peptides containing the sequences 99-201 or 200-301 do not bind. Further studies showed that rBSP-(1-75) competitively inhibits the binding of rBSP-(1-100), whereas rBSP-(21-100) inhibits binding to a lesser extent, and rBSP-(43-100) does not inhibit binding. These results suggest that the collagen-binding site of rat BSP is within the sequence 21-42, with residues N-terminal of this region likely also involved. This site was confirmed by the demonstration of collagen-binding activity of a synthetic peptide corresponding to residues 19-46. The collagen-binding domain, which is highly conserved among species, is enriched in hydrophobic residues and lacks acidic residues. We conclude that residues 19-46 of BSP represent a novel collagen-binding site.

Possible influence of cell walls upon ion concentrations at plasma membrane surfaces. Toward a comprehensive view of cell-surface electrical effects upon ion uptake, intoxication, and amelioration.

Plant uptake of ions, intoxication by ions, and the alleviation of intoxication by other ions often correlate poorly with ion concentrations in the rooting medium. By contrast, uptake, intoxication, and alleviation correlate well with ion concentrations at the plasma membrane (PM) surface computed as though the PM were bathed directly in the rooting medium with no effect from the cell wall (CW). According to two separate lines of analysis, a close association of CWs and PMs results in a slight increase in cation concentrations and a slight decrease in anion concentrations at the PM surface compared with concentrations when the CW is separated or has no effect. Although slightly different, the ion concentrations at the PM surface computed with and without close association with the CW are highly correlated. Altogether, the CW would appear to have a small effect upon ion uptake by the PM or upon intoxication or alleviation of intoxication originating at the PM surface. These analyses have been enabled by the recent evaluation of parameters required for the electrostatic models (Gouy-Chapman-Stern and Donnan-plus-binding) used to compute electrical potentials and ion concentrations in CWs and at PM surfaces.

Heat Capacity Measurements of U1-yGdyO2(y=0–0.27)from 325 to 1,673 K

Molar heat capacities were measured for U1-yGdyO2 (y=0–0.27) by differential scanning calorimetry (DSC) in the temperature region from 325 to 1,673 K. The molar heat capacities of U1-yGdyO2 (0<y&;lt;0.27) were close to, or slightly lower than, the values calculated by using the summation rule, and anomalous increases of heat capacities were not clearly observed at temperatures above 1,000 K. The heat capacity analysis for (U, Gd)O2 showed that the excess heat capacities of (U, Gd)O2 tended to decrease with increasing cation concentration excluding U4+ ions in crystals. This suggested that the Schottky term contribution to the heat capacity, which was mainly caused by the excitation of f electrons in the 5f 2 configuration in U4+ ions, decreased in (U, Gd)O2, compared to that in undoped UO2 due to the formation of U5+ and Gd3+ ions in (U, Gd)O2 by addition of Gd2O3. Thermal functions of (U, Gd)O2 were also evaluated.

Heat Capacity Measurements of U1-yGdyO2 (y=0-0.27) from 325 to 1,673K

Molar heat capacities were measured for U1-yGdyO2 (y=0–0.27) by differential scanning calorimetry (DSC) in the temperature region from 325 to 1,673 K. The molar heat capacities of U1-yGdyO2 (0<y&;lt;0.27) were close to, or slightly lower than, the values calculated by using the summation rule, and anomalous increases of heat capacities were not clearly observed at temperatures above 1,000 K. The heat capacity analysis for (U, Gd)O2 showed that the excess heat capacities of (U, Gd)O2 tended to decrease with increasing cation concentration excluding U4+ ions in crystals. This suggested that the Schottky term contribution to the heat capacity, which was mainly caused by the excitation of f electrons in the 5f 2 configuration in U4+ ions, decreased in (U, Gd)O2, compared to that in undoped UO2 due to the formation of U5+ and Gd3+ ions in (U, Gd)O2 by addition of Gd2O3. Thermal functions of (U, Gd)O2 were also evaluated.

Synthesis and characterisation of cationically modified phospholipid polymers

Phospholipid-like copolymers based on 2-(methacryloyloxyethyl) phosphorylcholine were synthesised using monomer-starved free radical polymerisation methods and incorporating cationic charge in the form of the choline methacrylate monomer in amounts varying from 0 to 30 wt%, together with a 5 wt% silyl cross-linking agent in order to render them water-insoluble once thermally cured. Characterisation using a variety of techniques including nuclear magnetic resonance spectroscopy, high-pressure liquid chromatography and gel permeation chromatography showed the cationic monomer did not interfere with the polymerisation and that the desired amount of charge had been incorporated. Gravimetric and differential scanning calorimetry methods were used to evaluate the water contents of polymer membranes cured at 70°C, which was seen to increase with increasing cation content, producing materials with water contents ranging from 50% to 98%. Surface plasmon resonance indicated that the coatings swelled rapidly in water, the rate and extent of swelling increasing with increasing cation level. Dynamic contact angle showed that coatings of all the polymers possessed a hydrophobic surface when dry in air, characteristic of the alkyl chains expressed at the surface (>100° advancing angle). Rearrangement of the hydrophilic groups to the surface occurred once wet, to produce highly wettable surfaces with a decrease in advancing angle with increasing cation content. Atomic force microscopy showed all polymer films to be smooth with no features in topographical or phase imaging. Mechanical properties of the dry films were also unaffected by the increase in cation content.

Cation effects on sol–gel and gel–sol phase transitions of κ-carrageenan–water system

Sol–gel and gel–sol phase transitions of κ-carrageenan in pure water and in KCl solution were studied using photon transmission technique. Photon transmission intensity, I tr, was monitored against temperature to determine the sol–gel and gel–sol temperatures ( T sg and T gs) and activation energies (Δ H sg and Δ H gs). It was observed that T gs was notably higher than T sg due to the hysteresis on the phase transition loops. T gs and Δ H gs values were also higher for gels containing KCl than for those without KCl. The increase in carrageenan content caused an increase in both critical temperatures and activation energies for the gels prepared in pure water and in KCl solution. Increases in the KCl/carrageenan ratio, raised both T gs and T sg. Similarly Δ H sg was elevated by the increase in cation content of the gel. These results were interpreted as the formation of stronger gels in the presence of KCl in water.

Cation diffusion in cartilage measured by pulsed field gradient NMR.

In this study, the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique was used for the investigation of (1) concentration and compression effects on cation self-diffusion, and (2) restricted diffusion of cations in cartilage. Since physiologically relevant cations like Na+ are difficult to investigate owing to their very short relaxation times, the cations tetramethylammonium (TMA) and tetraethylammonium (TEA) were employed for diffusion studies in samples of explanted cartilage. Results indicated that the diffusion of monovalent cations shows strong similarities to observations already made in studies of the diffusion of water in cartilage: with increasing compression, i.e. decreasing water content, the diffusion coefficient of the cation decreases concomitantly. The diffusion coefficients also showed a decrease with increasing cation concentrations, basically reflecting the corresponding decrease in the water content. Both results could be explained by the well-established model of Mackie and Meares. This, together with the similarity of the diffusion coefficient D in cartilage relative to free solution (about 50%) for both cations, is consistent with the view that the water content and not the charge is the most important determinant of the intratissue diffusivity of monovalent cations. Diffusion studies with increasing observation times showed strong evidence of restricted diffusion, allowing the estimation of the geometry of barriers within cartilage.

The reversible condensation and expansion of the rotavirus genome.

Understanding the structural organization of the genome is particularly relevant in segmented double-stranded RNA viruses, which exhibit endogenous transcription activity. These viruses are molecular machines capable of repeated cycles of transcription within the intact capsid. Rotavirus, a major cause of infantile gastroenteritis, is a prototypical segmented double-stranded RNA virus. From our three-dimensional structural analyses of rotavirus examined under various chemical conditions using electron cryomicroscopy, we show here that the viral genome exhibits a remarkable conformational flexibility by reversibly changing its packaging density. In the presence of ammonium ions at high pH, the genome condenses to a radius of approximately 180 A from approximately 220 A. Upon returning to physiological conditions, the genome re-expands and fully maintains its transcriptional properties. These studies provide further insights into the genome organization and suggest that the observed isometric and concentric nature of the condensation is due to strong interactions between the genome core and the transcription enzymes anchored to the capsid inner surface. The ability of the genome to condense beyond what is normally observed in the native virus indicates that the negative charges on the RNA in the native state may be only partially neutralized. Partial neutralization may be required to maintain appropriate interstrand spacing for templates to move around the enzyme complexes during transcription. Genome condensation was not observed either with increased cation concentrations at normal pH or at high pH without ammonium ions. This finding indicates that the observed genome condensation is a synergistic effect of hydroxyl and ammonium ions involving disruption of protein-RNA interactions that perhaps facilitate further charge neutralization and consequent reduction in the interstrand spacing.

Processes controlling colloid composition in a fractured and karstic aquifer in eastern Tennessee, USA

Groundwater was sampled from a number of wells along recharge pathways between fractured shale and karstic formations to evaluate the chemical and hydrologic mechanisms controlling the nature and abundance of groundwater colloids. The colloids recovered using low flow rate purging and sampling exhibited a composition and abundance consistent with lithology, flow paths, and effects of hydrology and aqueous chemistry on colloid mobilization and stability. In general, the larger-size colloids and Ca-containing colloids were more abundant in the karstic lithologies, while Na-containing colloids were more important in the shales. The composition of the colloids reflected recharge pathways from the fractured shale and dolomite formations on the ridges into the limestone in the valley floor. The Mg-colloids in the limestone reflect the possible contributions from the dolamite, while the Na, K, and Si reflect possible contributions from the shale. However, it was not possible to use the colloid composition as a signature to demonstrate colloid transport from one lithology to another. Mixing of recharge water from the shale with groundwater within the limestone formation and precipitation/dissolution reactions could account for the colloids present in the limestone without invoking transport of specific shale-derived colloids into the limestone formation. The abundance of colloids in groundwater appears to be controlled by both chemical factors affecting colloid stability, as well as physical factors related to hydrology (storm-driven recharge and water velocities). In general, colloids were more abundant in wells with low ionic strength, such as shallow wells in water table aquifers near sources of recharge at the top of the ridges. Increases in cation concentrations due to dissolution reactions along flow paths were associated with decreases in colloid abundance. However, in spite of elevated ionic strength, colloid concentrations tended to be unexpectedly high in karstic wells that were completed in cavities or water-bearing fractures. The higher levels of colloids appear to be related to storm-driven changes in chemistry or flow rates that causes resuspension of colloids settled within cavities and fractures.

HYDROPHOBIZING EFFECT OF CATIONS ON ACIDIC PHOSPHOLIPID MEMBRANES

ABSTRACT By means of contact angle measurements with water and aqueous salt solutions, it is shown that plurivalent cations increase the hydrophobicity of negatively charged phospholipid vesicle membranes (consisting of phosphatidic acid, PA, or of phosphatidylserine, PS), but does not influence the hydrophobicity of neutral phospholipid membranes, (e.g., phosphatidylcholine, PC, at up to 200 mM of CaCl2). The hydrophobizing action of cations on PA and PS membranes is concomitant with the reduction in (negative) zeta potential with increasing cation concentrations. Trivalent cations, La3+, showed more effective in hydrophobizing negatively charged phospholipid membranes than divalent and monovalent cations. Except for hydrogen ions, monovalent cations do not show any appreciable hydrophobizing effect on lipid vesicle membranes at concentrations less than 1 M. The hydrophobizing effect on phospholipid membranes can also be used to explain the induction of lateral phase separation into patches of different phospholipids as well as cell fusion.

Kinetic and chemical mechanisms for the effects of univalent cations on the spectral properties of aromatic amine dehydrogenase.

Univalent cations and pH influence the UV-visible absorption spectrum of the tryptophan tryptophylquinone (TTQ) enzyme, aromatic amine dehydrogenase (AADH). Little spectral perturbation was observed when pH was varied in the absence of univalent cations. The addition of alkali metal univalent cations (K+, Na+, Li+, Rb+ or Cs+) to oxidized AADH caused significant changes in its absorption spectrum. The apparent Kd for each cation, determined from titrations of the spectral perturbation, decreased with increasing pH. Transient kinetic studies involving rapid mixing of AADH with cations and pH jump revealed that the rate of the cation-induced spectral changes initially decreased with increasing cation concentration to a minimum value, then increased with increasing cation concentration. A kinetic model was developed to fit these data, determine the true pH-independent Kd values for K+ and Na+, and explain the pH-dependence of the apparent Kd. A chemical reaction mechanism, based on the kinetic data, is presented in which the metallic univalent cation facilitates the chemical modification of the TTQ prosthetic group to form an hydroxide adduct which gives rise to the spectral change. Addition of NH4(+)/NH3 to AADH caused changes in the absorption spectrum which were very different form those caused by addition of the metallic univalent cations. The kinetics of the reaction induced by addition of NH4+/NH3 were also different, being simple saturation kinetics. Another reaction mechanism is proposed for the NH4+/NH3-induced spectral change that involves nucleophilic addition of the unprotonated NH3 to TTQ. The general relevance of these data and models to the physiological reactions of TTQ-dependent enzymes and to the roles of univalent cations in modulating enzyme activity are discussed.

Effects of pH and sulfate on insects and protozoans inhabiting treeholes.

We used laboratory microcosms designed to simulate treeholes to study the effect of changing levels of chemicals found in precipitation in central Pennsylvania (H+ and SO4=) on the treehole insects Aedes triseriatus (a mosquito), Helodes pulchella (a helodid beetle), and Culicoides guttipennis (a ceratopogonid midge), and treehole protozoans. Protozoans were tested in both the presence and absence of insects. We hypothesized that the individual insect species would have differential tolerances to abiotic stresses, and that effects of low pH on protozoans would be especially strong. Survival of helodids was higher than mosquitoes and midges. Emergence and survival of mosquitoes and midges were lower at low pH. Densities of ciliates increased the most at high pH in the absence of insects. Densities of flagellates increased the most at low pH in the absence of insects. The presence of helodids, mosquitoes, and protozoans was associated with higher final [SO4=], as was low pH. Mosquitoes at low pH caused the largest increases in cation concentrations, and protozoans at high pH caused the largest decreases in cation concentrations. It appears that the biota of treeholes are affected by ionic changes in simulated tree stemflow that can be caused by anthropogenic atmospheric deposition. The species studied here were sometimes differentially affected by the common pollutants, [H+] and [SO4=]. The resulting changes in these discrete treehole communities may allow them to be useful bioindicators of the status of forest ecosystems altered by changing atmospheric chemistry.

Spectroscopic properties of low Fe3+-doped silica gels

Trivalent iron (0.21–1.21 mol% equivalent Fe 2 O 3 ) was introduced into silica gel monoliths prepared via sols obtained from tetraethyl orthosilicate and metal salts at different final sol pH values. The UV-visible spectra indicated a change from an octahedral to tetrahedral Fe 3+ environment as the pH increased. An electron paramagnetic resonance spectral resonance at g = 4.22 appeared, in addition to that at g = 2.0, as the final pH increased. The resonance at g = 4.22 was also observed for some gels upon heat treatment. Colloidal precipitation of Fe 2 O 3 was noted with increase in cation concentration and temperature of heat treatment. The effects of processing on light transmission of gels were measured. The results indicated several processing conditions leading to total cut-off of ultraviolet light through the doped gel monoliths.

Compression strength and deformation of gellan gels formed with mono- and divalent cations

The influence of divalent (Ca 2+ and Mg 2+) and monovalent (Na + and K +) cations on the failure stresses and strains in gels formed with gellan polymer were investigated. Gellan gels containing 0.6-2.2% polymer and varying cation concentrations were tested using large compressive deformation until failure. Maximal true shear stresses and corresponding shear strains represented gel strength and extensibility. At small cation concentrations, the strength of gellan gels increased while the extensibility decreased with cation concentrations. The strongest gels were obtained at cation levels corresponding to 0.5 cations per carboxylate group in the repeat gellan unit for gels crosslinked with divalent cations and 10 to 30 cations per repeat gellan unit for gels with monovalent cations. At optimum cation levels, gellan gels with Ca 2+ were stronger than with Mg 2+, and gellan gels with K + were stronger than with Na +. Above the optimum cation levels, gellan gels became brittle, and the strength of the gels decreased with increasing cation concentrations.

5339564 Method for control and destruction of agricultural pests by coherent electromagnetic excitation

A method for the control and destruction of agricultural pest organisms comprising generating a selected EM radiation at a plurality of different frequencies, pulse widths, and intensities for application on pest organisms, coupling the EM radiation to selected cellular organelles, organelle membranes, or cell membranes of the pest organism, disrupting an ionic balance within the cellular organelles, organelle membranes, or cell membranes, and initiating an increase in cation concentration within the membranes or organelles. The cation concentration is sustained until the pest organism is destroyed. A system and apparatus are also disclosed where an oscillator operably coupled to a broad band linear amplifier generate selected EM radiation frequencies to be applied to pest organisms. The oscillator is preferably a frequency-hopping RF oscillator and is communicatively linked to an antenna coupled to a reflector and shield for focusing the EM radiation.

The K+ channel in the plasma membrane of rye roots has a multiple ion residency pore

The permeation of K+ and Na+ through the pore of a K+ channel from the plasma membrane of rye roots was studied in planar 1-palmitoyl-2-oleoyl phosphatidylethanolamine bilayers. The pore contains at least two ion-binding sites which can be occupied simultaneously. This was indicated by: (i) biphasic relationships with increasing cation concentration of both channel conductance at the zero-current (reversal) potential of the channel (Erev) and unitary-current at a specified voltage and (ii) a decline in Erev in the presence of equimolar Na+ (cis):K+ (trans) as the cation concentration was increased. To determine the spatial characteristics and energy profiles for K+ and Na+ permeation, unitary-current/voltage data for the channel were fitted to a three energy-barrier, two ion-binding site (3B2S) model. The model allowed for simultaneous occupancy of binding sites and ionic repulsion within the pore, as well as surface potential effects. Results suggested that energy peaks and energy wells (ion binding sites) were situated asymmetrically within the electrical distance of the pore, the trans energy-well being closer to the center of the pore than its cis counterpart; that the energy profile for K+ permeation differed significantly from that of Na+ in having a higher cis energy peak and a deeper cis energy well; that cations repelled each other within the pore and that vestibule surface charge was negligible. The model successfully simulated various aspects of K+ and Na+ permeation including: (i) the complexities in current rectification of a wide range of contrasting ionic conditions; (ii) the biphasic relationships with increasing cation concentration of both channel conductance at Erev and unitary-current at a specified voltage; (iii) the decline in Erev in equimolar Na+ (cis):K+ (trans) as cation concentrations were increased and (iv) the complex relationships between mole fraction and Erev at total cation concentrations of 100 and 300 mM.

Effects of different cations on the hydrodynamic radius of DNA

The effects of different cations on the hydrodynamic radius (RH) of a 48-bp synthetic DNA are measured by time-resolved fluorescence polarization anisotropy of intercalated ethidium. Relative statistical errors in RH are only approximately 1%. With increasing cation concentration, Na+ causes a small decrease in RH, Cs+ causes a somewhat larger decrease by up to 0.5 A at 100 mM, and (CH3CH2)4N+ causes an increase in RH by approximately 0.5 A at 100 mM. The qualitatively different effects of these monovalent cations indicates that the changes in RH with cation concentration do not arise primarily from electrolyte friction. Divalent cations cause much larger increases in RH with increasing cation concentration. Mg2+ causes an increase in RH by up to 1.0 A at 24.4 mM, and Mn2+ causes an increase in RH by up to 1.6 A at 24.4 mM. These effects are independent of DNA concentration. There is some positive correlation between the order of effects of the different cations on RH and the order of their effects on interhelical hydration forces. It is suggested that these different ions affect RH either by altering the hydration layer or possibly by some effect on DNA structure, such as stabilizing bends.

Kinetics of Divalent Cation-Induced and pH-Induced Aggregation of Dimyristoylphosphatidylserine Vesicles

Kinetic study was carried out on the aggregation of dimyristoylphosphatidylserine (DMPS) vesicles induced by the addition of divalent cations (Mg2+, Ca2+, Sr2+, and Ba2+) as well as by pH jump, using a stopped-flow method with turbidity detection. With the increase in cation concentration, the initial rate of aggregation, V0, increased rapidly above the threshold concentration of about 1 mM, and reached the maximum velocity, V0,max . The increase in V0 toward V0,max with the concentration was appreciably dependent on the cation species, whereas the value of V0,max was not affected significantly by the cation species. This behavior of aggregation kinetics of DMPS vesicles is well described in terms of the intervesicular interaction potential composed of van der Waals attraction and electrostatic repulsion, without introducing an extra force such as hydration force which is regarded as responsible for the stability of phosphatidylcholine vesicles. The aggregation of DMPS vesicles was also induced when the negative charge on the DMPS head group was eliminated by protonating the serine carboxyl group by mixing the vesicle preparation with acid solution. This observation is in accordance with the above interpretation that no hydration force acts between DMPS vesicles to such an extent that the aggregation kinetics are affected appreciably.

Kinetical parameters of monovalent cation uptake in yeast calculated on accounting for the mutual interaction of cation uptake and membrane potential

Kinetical parameters of monovalent cation uptake in yeast are calculated according to a model for mutual interaction of membrane potential and cation uptake. Apparent K m values for monovalent cation uptake obtained from uptake studies are 3–5-times lower than the K m values expected when the membrane potential remains constant at increasing cation concentrations instead of being reduced. The model accounts for varius phenomena as (i) the increase in apparent K m of Rb + uptake accompanying the decrease in maximum rate of uptake found on increasing the cellular K + content, (ii) the decrease in maximum uptake rate and increase in K m in case of simultaneous transport of phosphate and Rb +, (iii) the change in the maximum uptake rate without a change in K m under conditions that the proton pump is effected and (iv) the absence of an increase in K + efflux at high depolarizing external cation concentrations.