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1.
Studies on mitochondria has revealed apparent violation of the laws of osmosis, probably related to ion transport. In this context, we studied changes in the content of potassium, sodium, magnesium, and calcium in mitochondria from wheat (Triticum aestivumL.) roots incubated in the buffer without the substrate that contained 0.3 or 0.5 M sucrose as well as potassium, and phosphate. The most pronounced changes were observed for potassium. Potassium inflow into mitochondria was more active at 0.5 than 0.3 M sucrose. The direction of the cation transport depended on its content in the initial buffer. At the certain threshold level, the inflow was replaced with the outflow; this threshold was lower at 0.3 than 0.5 M sucrose. Magnesium was also released from mitochondria; its stable outflow was primarily observed at its lower initial content in the suspension at 0.3 rather than 0.5 M sucrose. These data showed that the apparent violation of laws of osmosis was false and resulted from ion transport. Passive ion transport as a possible mechanism of adaptation to osmotic properties of the external medium is discussed.  相似文献   

2.
It has been shown previously that biogenic amines and a number of pharmaceutical agents can redistribute across vesicle membranes in response to imposed potassium ion or proton gradients. Surprisingly, drug accumulation is observed for vesicles exhibiting either a pH gradient (interior acidic) or a membrane potential (interior negative), implying that these compounds can traverse the lipid bilayer as either the neutral or charged species. This interpretation, however, is complicated by the fact that vesicles exhibiting a membrane potential (interior negative) accumulate protons in response to this potential, thereby creating a pH gradient (interior acidic). This raises the possibility that in both vesicle systems drug redistribution occurs in response to the proton gradient present. We have therefore compared the uptake of several lipophilic cations by reconstituted cytochromec oxidase vesicles and by similar vesicles exhibiting a potassium ion diffusion potential. While turnover of the oxidase generates a membrane potential of comparable magnitude to the potassium ion diffusion system, it is associated with a proton gradient of opposite polarity (interior basic). Both systems show rapid uptake of the permanently charged lipophilic cation, tetraphenylphosphonium, but only the potassium ion diffusion system accumulates the lipophilic amines doxorubicin and propranolol. This provides compelling evidence that such weak bases redistribute only in response to pH gradients and not membrane potential.  相似文献   

3.
Electrical Properties of Mitochondrial Membranes   总被引:2,自引:2,他引:0       下载免费PDF全文
The electrical capacity of the membrane of rat liver mitochondria is 0.5 to 0.6 µ./cm2. This membrane capacity is obtained from the analysis of the frequency dependence of the admittance of a suspension of swollen mitochondria. In potassium chloride media the mitochondrial membrane capacity does not depend on the ion concentration. The internal conductance of the mitochondria was approximately one-half that of the external medium; the same applies if the mitochondria are equilibrated in a medium with a 10-fold difference in potassium chloride concentration. Hence the swollen mitochondria investigated here appear to be able to adjust their internal ion concentration in proportion with that of the external phase. The similarity of the membrane capacity of isolated mitochondria with the range of values known for other membranes suggests a common molecular structure. The analysis of experimental data suggests an anisotropic electrical behavior of the interior of mitochondria. This anisotropy is readily explained by the existence of internal membranes.  相似文献   

4.
Certain aspects of the secretion of solutes into, and removalfrom, the sieve tubes of isolated stem segments and rooted cuttingsof Salix viminalis have been studied. Sieve-tube sap was obtainedeither as honeydew from whole individuals or via the severedstylets of the aphid Tuberolachnus salignus (Gmelin). It was shown that interference occurred between the chemicallyunrelated solutes, sucrose and the cations potassium and rubidium.On raising the potassium concentration in the sieve-tube sapby passing a solution of this ion through the xylem, the sucroseconcentration declined. When the sucrose concentration fellover a period of days due to respiratory loss of carbohydratesfrom an isolated stem segment, a concomitant rise in eitherthe potassium or rubidium level in the sap occurred. When a solution of sodium was passed through the xylem, theconcentration of this ion in the sieve-tube sap rose, whilstthat of potassium fell at first, but later rose higher thanits initial value, indicating that both antagonism and synergycan occur between these ions. On introducing both these cationsinto the xylem simultaneously, more sodium than potassium wastaken up by the segment, though the increase in the sodium concentrationin the sieve-tube sap was less than that of the potassium. Perfusingthe xylem with a calcium solution had no effect upon the concentrationof potassium in the sieve tube. It has been shown that the rate of translocation of a solutealong the sieve tube, as measured by the two colony technique,depends upon the rate of removal of this solute from the sievetube. The amount of such lateral loss from the sieve tube isrelated to the potential gradient for a solute between the sievetube and surrounding cells.  相似文献   

5.
The effects of phloretin on membrane ionic conductances have been studied in the giant axon of the squid, Loligo pealei. Phloretin reversibly suppresses the potassium and sodium conductances and modifies their dependence on membrane potential (Em). Its effects on the potassium conductance (GK) are much greater than on the sodium conductance; no effects on sodium inactivation are observed. Internal perfusion of phloretin produces both greater shifts in GK(Em) and greater reductions maximum GK than does external perfusion; the effect of simultaneous internal and external perfusion is little greater than that of internal perfusion alone. Lowering the internal pH, which favors the presence of the neutral species of weakly acidic phloretin (pKa 7.4), potentiates the actions of internally perfused phloretin. Other organic cations with dipole moments similar to phloretin's have little effect on either potassium or sodium conductances in squid axons. These results can be explained by either of two mechanisms; on postulates a phloretin "receptor" near the voltage sensor component of the potassium channel which is accessible to drug molecules applied at either the outer or inner membrane surface and is much more sensitive to the neutral than the negatively charged form of the drug. The other mechanism proposes that neutral phloretin molecules are dispersed in an ordered array in the membrane interior, producing a diffuse dipole field which modifies potassium channel gating. Different experimental results support these two mechanisms, and neither hypothesis can be disproven.  相似文献   

6.
When the ambient atmosphere of Acer pseudoplatanus cells in suspension culture is rapidly changed by opening the culture flasks and gently stirring (‘mild gas-shock’) or by filtering and suspending in new medium (‘strong gas-shock’), drastic modifications of the rates of leucine, methionine, glucose, adenine, sulphate and phosphate uptake are observed. Following the gas-shock, rates of uptake rapidly decrease within a few minutes. Subsequently the rates increase again to the intial level within several hours. The uptake of potassium, which is known to be passively distributed between the medium and the interior of many plant cells, at least at high external concentrations, is apparently independent of gas-shock. The shock and recovery kinetics are similar for all solutes investigated (except K+), in particular for different solutes studied in double labelling experiments with the same batch of cells. At the maximum of the after-effect of shock, i.e. at minimum rates of uptake, uptake shows a highly reduced dependence on temperatures. Gas-shock probably inactivates, denatures, structurally alters or releases membrane macromolecules engaged in transport. These molecules are then re-synthesized and re-incorporated into the membrane during recovery.  相似文献   

7.
Glass micropipette electrodes have been employed to study the transsurface potential difference of Neurospora crassa. For mature hyphae grown in agar cultures, the internal potential is large and negative, often exceeding -200 mv. The potential is sensitive to the concentrations of extracellular potassium, sodium, hydrogen, and calcium ions, but does not vary in a manner which is readily explained by ionic diffusion potentials. With extracellular solutions containing only potassium chloride (or sulfate) and sucrose, the internal potential shifts toward zero (becomes less negative) at 45 mv per tenfold increase of potassium, over the range 0.1 to 10 mM. A similar result has been found with sodium, though the slope is only 33 mv/log unit. Calcium (1 mM) diminishes the influence of potassium and sodium by 60 to 70 per cent. As potassium or sodium is raised above 20 mM, the slope of the internal potential increases sharply to 85 to 90 mv/log unit, both in the presence and absence of calcium. With increasing hydrogen ion concentration, too, the internal potential shifts toward zero; in this case the slope is about 12 mv/pH unit at pH 9 and rises smoothly to 33 mv/pH unit at pH 3. All these phenomena are probably properties of the plasma membrane. The polysaccharide cell wall contains few fixed negative charges, has a low transverse resistance, and supports very little potential difference when separated from the plasma membrane.  相似文献   

8.
Spherical droplets, derived from Physarum plasmodia by incubation in 10 mM caffeine, seemed to be an excellent system for electrophysiological studies because they were large (less than or equal to 300 micrometer in diameter) and because they tolerated intracellular electrodes filled with 3 M KCl and 10 mM EDTA for a few hours. Intact plasmodia, by contrast, gave valid records for only a few minutes. Under standard conditions ([K+]o = 1 mM, [Na+]o = 5 mM, [Ca++]0 = 0.5 mM, [Mg++]o = 2 mM, and [Cl-]o = 6 mM at pH 7.0), the potential difference across droplet membranes was -80 to -120mV, interior negative. The membrane potential was only slightly sensitive to concentration changes for the above-mentioned ions, and was far negative to the equilibrium diffusion potentials calculated from the known internal contents of K, Na, Ca, Mg, and CL (29.4, 1.6, 3.7, 6.5, and 27.8 mmol/kg, respectively). Variations of external pH did have a strong influence on the membrane potential, yielding a slope of 59 mV/pH between pH 6.5 and 5.5. In this pH range, however, the equilibrium potential for H+ (assuming 6.2 less than or equal to pHi less than or equal to 7.0) was greater than 75 mV positive to the observed membrane potential. Membrane potential was directly responsive to metabolic events, being lowered by potassium cyanide, and by cooling from 25 to 12 degrees C. This ensemble of results strongly indicates that the major component of membrane potential in plasmodial droplets of Physarum is generated by an electrogenic ion pump, probably one extruding H+ ions.  相似文献   

9.
  • 1.1. Respiratory trees of Holothuria glaberrima exposed to solutions in which sodium has been replaced by choline, Tris pH 6.1, Tris pH 8.0 or lithium show a net loss of intracellular water, potassium, sodium and chloride. Intracellular content of neutral orgainc osmotic effectors remains unmodified.
  • 2.2. Extracellular lithium and Tris pH 8.0 decrease intracellular potassium concentration to half that in sodium, choline and Tris pH 6.1. Intracellular sodium concentration falls markedly while that of chloride falls moderately in sodium-free solutions. Sodium substitutes appear to enter the cells.
  • 3.3. A model based on Donnan considerations accounts for the patterns of ion and water distribution.
  相似文献   

10.
Summary The transport of potassium, sodium and various anions in rat-liver mitochondria was studied mainly by analysis of ion content and water compartmentation of the mitochondrial pellet. A comparison of spontaneous transport with valinomycin- or gramicidin-stimulated transport is made. The rate or extent of uptake, the internal concentrations and the concentration ratio (Cin/Cout) are calculated and compared to test existing models for ion transport in mitochondria.Several models of ion transport in mitochondria are based on a cation-pump which is directed inward. This hypothesis is rejected because of the following findings: (1) Valinomycin stimulates the rate of potassium uptake but does not increase the potassium concentration ratio that can be actively maintained in a steady state (in which there is no potassium flow). (2) Valinomycin greatly stimulates the efflux of42K from mitochondria during the process of potassium accumulation. When potassium accumulation is stimulated the flux ratio, i.e. influx/efflux, decreases; in the presence of valinomycin this ratio approaches 1. (3) In the presence of gramicidin, the concentration ratios of potassium and sodium are about the same under a variety of conditions. These findings indicate that potassium and sodium transport are passive processes of relaxation towards electro-chemical equilibrium (of the potassium and sodium). In high external potassium concentrations the extent of potassium uptake is limited by the permeation of anions; of the permeating anions multivalent acids support a higher extent than monovalent acids. It was found that succinate, acetate and oxalate which are transported together with potassium are distributed in accordance with the pH and without any relation to the potassium concentration ratio. These findings are compatible with the hypothesis that an outward-directed proton pump creates an electrical potential gradient, which shifts the equilibrium state for the cations and drives sodium and potassium inward, and also creates proton gradient that is the driving force for anion transport.  相似文献   

11.
The changes in the membrane permeability to sodium, potassium, and chloride ions as well as the changes in the intracellular concentration of these ions were studied on frog sartorius muscles in Ca-free EDTA solution. It was found that the rate constants for potassium and chloride efflux became almost constant within 10 minutes in the absence of external calcium ions, that for potassium increasing to 1.5 to 2 times normal and that for chloride decreasing about one-half. The sodium influx in Ca-free EDTA solution, between 30 and 40 minutes, was about 4 times that in Ringer's solution. The intracellular sodium and potassium contents did not change appreciably but the intracellular chloride content had increased to about 4 times normal after 40 minutes. By applying the constant field theory to these results, it was concluded that (a) PCl did not change appreciably whereas PK decreased to a level that, in the interval between 10 and 40 minutes, was about one-half normal, (b) PNa increased until between 30 and 40 minutes it was about 8 times normal. The low value of the membrane potential between 30 and 40 minutes was explained in terms of the changes in the membrane permeability and the intracellular ion concentrations. The mechanism for membrane depolarization in this solution was briefly discussed.  相似文献   

12.
Seed coats of Phaseolus vulgaris L. unload photosynthetic products,mineral ions and acid into the apoplastic space surroundingthe embryo. We report measurements, on detached seed coats,of the rates of unloading of photosynthates, ions and acid atdifferent external pH and in the presence of treatments intendedto alter the rate of proton pumping. We also report measurementsof membrane potential difference (PD) and of cytoplasmic pHunder the same conditions, measurements which have allowed usto validate the treatments we used and to investigate functionalrelationships between membrane processes. A chemiosmotic model of the seed-coat cell membrane is proposed,in which sucrose efflux and acid efflux are both driven by theproton pump. Sucrose efflux is proposed to occur by sucrose/protonantiport driven by the proton-motive force (PMF), and acid effluxto occur by pumped protons accompanied by a passive efflux ofanions. We use our measurements to estimate the net efflux ofsucrose on the antiporter and the total efflux of protons onthe pump. We have tested the model by using experimental treatments designedto manipulate the pump rate as the independent variable. Underthese conditions, and assuming the model is correct, the pumprate determines the cytoplasmic pH. Over the range covered byour experiments the net sucrose efflux is dependent on externaland cytoplasmic pH, the latter having the major role. The effluxof acid, under the same treatments, depends primarily on theproton pump rate, and was found to be well fitted by a quadraticfunction of pump rate. This means that, as pump rate increases,an increasing proportion of the pump output is used by acidefflux and a decreasing proportion by sucrose antiport. The membrane PD, although an important component of the PMF,does not appear to function in rate control of net sucrose orof acid efflux, since neither efflux is correlated with membranePD under our treatments which vary the pump rate. The PD correlateswell with external potassium concentration, and seems largelydetermined by the diffusion of potassium ions and anions. Key words: Phaseolus vulgaris L, photosynthate efflux, proton pump, sucrose/proton antiport, seed coat, membrane transport model  相似文献   

13.
Light-driven potassium ion uptake in Halobacterium halobium is mediated by bacteriorhodopsin. This uptake is charge-balanced by sodium ions and not by proton release. Light-induced shifts in concentrations of divalent cations were found to be negligible. The transient changes in extracellular pH (alkaline overshoot) can be understood by the concomitant processes of ATP synthesis, proton/sodium exchange and potassium uptake. The driving force of potassium ion uptake is the membrane potential, no ATP-dependent potassium transport process is found. Fluorescence measurements indicate a high permeability of the membrane to potassium ions compared to sodium ions. Therefore the potassium ion diffusion potential contributes to the membrane potential (about 30 mV/decade) and thereby influences the ATP level. Sudden enhancement of the diffusion potential by the potassium ionophore monactin leads to the expected transient increase in cellular ATP level. Due to the large size (up to 100-fold) of the potassium ion gradient and its high capacity (intracellular concentration up to 3 M) the potassium ion gradient can well serve the cell as a long term storage form of energy.  相似文献   

14.
Angelo Azzi  Giovanni F. Azzone 《BBA》1967,131(3):468-478
1. Water uptake coupled to ion movement has been studied in respiratory-inhibited liver mitochondria, of which the permeability to cations was increased by valinomycin, gramicidin or EDTA, and to anions by raising the pH of the medium. The movement of water was accounted for by the osmotic pressure of the penetrating solutes.

2. The rate of movement of water was inversely proportional to the concentration of solutes in the medium, and was dependent on the presence of permeating cations and anions. The above findings are interpreted within the concept of an osmotic movement of water.

3. The flow of anions through the membrane was inhibited by Ca2+ and Mn2+ Mitochondrial swelling was inhibited by sucrose.

4. Ion movement was independent of energy supply from metabolism. The nature of the force driving the ion movement is discussed.  相似文献   


15.
Apical membrane vesicles were prepared from confluent monolayers of LLC-PK1 cells grown upon microcarrier beads. The final membrane preparation, obtained by a modified divalent cation precipitation technique, was enriched in alkaline phosphatase, leucine aminopeptidase and trehalase (8-fold compared to the initial homogenate). Analysis of phosphate uptake into the vesicles identified a specific sodium-dependent pathway. Lithium and other cations were unable to replace sodium. At 100 mmol/l sodium and pH 7.4, an apparent Km for phosphate of 99 +/- 19 mumol/l and an apparent Ki for arsenate of 1.9 mmol/l were found. Analysis of the sodium activation of phosphate uptake gave an apparent Km for sodium of 32 +/- 12 mmol/l and suggested the involvement of two sodium ions in the transport mechanism. Sodium modified the apparent Km of the transport system for phosphate. The rate of sodium-dependent phosphate uptake was higher at pH 6.4 than at pH 7.4. At both pH values, an inside negative membrane potential (potassium gradient plus valinomycin) had no stimulatory effect on the rate of the sodium-dependent component of phosphate uptake. It is concluded that the apical membrane of LLC-PK1 cells contains a sodium-phosphate cotransport system with a stoichiometry of 2 sodium ions: 1 phosphate anion.  相似文献   

16.
The process of sucrose transport was investigated in sealed putative tonoplast vesicles isolated from sugarbeet (Beta vulgaris L.) taproot. If the vesicles were allowed to develop a steady state pH gradient by the associated transport ATPase and 10 millimolar sucrose was added, a transient flux of protons out of the vesicles was observed. The presence of an ATPase produced pH gradient allowed [14C]sucrose transport into the vesicles to occur at a rate 10-fold higher than the rate observed in the absence of an imposed pH gradient. Labeled sucrose accumulated into the sealed vesicles could be released back to the external medium if the pH gradient was dissipated with carbonylcyanide-m-chlorophenyl hydrazone (CCCP). When the kinetics of ATP dependent [14C]sucrose uptake were examined, the kinetic profile followed the simple Michaelis-Menten relationship and a Michaelis constant of 12.1 millimolar was found. When a transient, inwardly directed sucrose gradient was imposed on the vesicles in the absence of charge compensating ions, a transient interior negative membrane potential was observed. This membrane potential could be prevented by the addition of CCCP prior to sucrose or dissipated by the addition of CCCP after sucrose was added. These results suggest that an electrogenic H+/sucrose antiport may be operating on the vesicle membrane.  相似文献   

17.
离子转运蛋白在维持细胞内pH稳态、离子动态平衡等方面发挥着重要作用。钠离子转运体和钾离子转运体在嗜盐耐盐微生物中广泛存在,其"保钾排钠"机制是微生物抗盐胁迫的两大策略之一。近年来,嗜盐耐盐微生物中许多新型钠、钾离子转运体被陆续发现,如RDD蛋白、UPF0118蛋白、DUF蛋白和KimA蛋白等;Fe3+、Mg2+等其他金属离子的转运蛋白也被证实可通过影响微生物胞内相容性溶质的合成起到渗透调节的作用。本文综述了嗜盐耐盐微生物中抗盐胁迫相关的各类离子转运蛋白,分析其分子结构和工作机理,并对这些蛋白在农业方面的应用进行了展望。继续发现新的离子转运蛋白,探究抗盐胁迫相关离子转运蛋白的结构和机理,解析各转运系统的协同作用及分子调控机制,将进一步加深对嗜盐耐盐微生物抗盐胁迫调控的认识,并为盐碱地农作物的改良等提供新的思路。  相似文献   

18.
The physicochemical conditions of the environment in which life arose are discussed, along with the appearance of protocells, their membranous envelope and the subsequent appearance of plasma membranes. The hypothesis that the first cells originated in reservoirs where potassium and magnesium salts (necessary for protein synthesis and thus for the formation of a cellular membrane) dominated, is substantiated. This was followed by adaptation of these cells to an external ocean-like environment, where sodium salts were prevalent. This stage of evolution required a plasma membrane capable of providing ion asymmetry between the cell’s cytoplasm and the external environment. At this stage of evolution in the predecessors of animals, the process of removal of sodium ions and accumulation of potassium ions began functioning in the plasma membrane. The problem of multicellular organisms was solved differently by animals and plants: animals developed a system of the extracellular fluids that provided stable physicochemical conditions on the external surface of the plasma membrane. Sodium ions were the stimulus for the formation of the polar cell, where sodium channels are situated on one side of the plasma membrane, and sodium pumps on the other, allowing the development of the absorption, excretion and breathing functions. The formation of fluids of the internal environment enabled the development of homeostasis and facilitated the biological progress of the animal kingdom.  相似文献   

19.
Claviceps purpurea PRL 1980 grew on partially dissociated succinic acid (pH 4) but not on fully dissociated succinic acid (pH 7.2). Myeclium suspended in 42 mM solution of partially ionized succinic acid (pH 4; 60.1% nonionized, 39% monoanion, and 0.9% dianion, K+ salt) over a period of 25 min accumulated more succinic acid carbon than mycelium suspended in highly ionized solution (pH 6.8; 0.01% nonionized, 4.8% monoanion, and 95% dianion). The greater accumulation from partially ionized solution was not attributable solely to metabolism of succinic acid nor to the lower external concentration of potassium ion. Rate of uptake by sodium azide and iodoacetate-treated mycelium was proportional to external concentration at least up to 200 mumol/ml. External potassium or sodium ion was not required for uptake by inhibited or uninhibited mycelium and external sodium ion and glucose did not allow concentration of succinic acid. The internal concentrations of succinic acid carbon expressed as succinic acid in cell water were about the same as the external concentrations. Uptake was not appreciably affected by extent of ionization of external succinic acid but accumulation was markedly affected. A plot of accumulated succinic acid carbon against external pH produced a bimodal curve with the two maxima corresponding to the maximal concentrations of nonionized and monoanion succinic acid. The bimodal curve probably results from overlapping of two separate curves; the nonionized form accumulating efficiently because of one interaction with the cell and the monoanion form accumulating efficiently because of another interaction. Uptake from concentrated solution is by diffusion and efflux is rapid but not complete. Efflux is not retarded by presence of phosphate in the external solution.  相似文献   

20.
When turgor was increased, by decreasing the concentration of mannitol bathing discs of sugar beet storage root tissue, the rates of sucrose and potassium uptake into the vacuole were decreased. At all external mannitol concentrations the rate of sucrose and potassium uptake across the plasma membrane was an order of magnitude greater than the rate of quasi-steady uptake into the vacuole, implying a very large efflux. Efflux of both sucrose and potassium was increased at high turgor. However, while increasing turgor decreased the rate of K+ uptake, the rate of sucrose uptake at the plasma membrane increased with time. Compartmental analysis of tracer exchange kinetics was used to determine unidirectional K+ fluxes. From these results, it was estimated that the increase in K+ efflux accompanying a 1.5 MPa increase in turgor could lead to a net increase of 140mol?3h?1 in the external potassium concentration. It is suggested that the turgor-imposed increase in solute efflux is a means of regulating intracellular osmotic pressure and/or turgor in sugar beet storage roots, but that sucrose is preferentially retrieved from the apoplast, even under conditions of excessively high turgor. However, much of this sucrose is probably lost from the cell, implying a ‘futile’ sucrose transport cycle at the plasma membrane. The turgor-stimulated leak of potassium could play a major role in the regulation of turgor pressure in sugar beet storage root tissue.  相似文献   

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