Article

Factors Affecting the Fluxes of Potassium and Chloride Ions in Nitella translucens

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Abstract

A more complete study of ionic concentrations and fluxes in the giant internodal cells of Nitella translucens has been made. The vacuolar concentrations were 76 mM K and 170 mM Cl. The content of the chloroplast layer was 135 mmicromoles K/cm(2) and 215 mmicromoles Cl/cm(2); in a layer 9 micro thick these correspond to concentrations of 150 mM K and 240 mM Cl. Such a high level of chloride requires active transport of chloride into the cytoplasm, either at the plasmalemma or at the membranes bounding the cytoplasmic particles; it cannot be achieved by active transport of chloride only at the tonoplast. With concentrations of 0.1 mM K and 1.3 mM Cl outside, the fluxes into the cytoplasm had mean values of 1.0 to 1.4 micromicromoles K/cm(2)sec. and 2.1 to 2.8 micromicromoles Cl/cm(2)sec.; the corresponding fluxes from the cytoplasm to the vacuole were about 110 micromicromoles K/cm(2)sec. and 175 micromicromoles Clcm(2)sec. The transfer of both potassium and chloride to the vacuole under different conditions appeared to be correlated with the uptake of chloride into the cytoplasm. It is suggested that two separate processes are involved in the active accumulation of salts in the vacuole-an active uptake of chloride in the cytoplasm and a subsequent transfer of salt to the vacuole. It may be that the second process involves the formation of small vesicles in the cytoplasm and their subsequent discharge into the central vacuole.

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... Efflux Analysis. The procedure used for estimating the amounts of isotope in each compartment and calculating the fluxes is basically that of Pitman (16); a similar procedure has been used in several other studies on algae (4,14,15) and on higher plant tissues (2,5,13,16,17). With the use of the efflux data, the logarithm of the amount of isotope remaining in the tissue is plotted against time (Fig. 2). ...
... Although the graphical analysis gives values of radioactivities of compartments and their efflux constants, it does not reveal the amounts of the stable species. The procedures for arriving at estimates of amounts and fluxes have been presented in some detail by various authors (2,4,5,(13)(14)(15)(16). Figure where Sc and S, are, respectively, the specific radioactivity of the ion in the cytoplasm and the vacuole expressed as a fraction of So . ...
... From these equations it can be shown that the following relationships can be obtained (2,14): krQv = Jeo.j Jct + Jcto (4) where k, is the efflux constant (rate constant of exchange) of the slowest phase during a tracer washout expeliment (sec-1). ...
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... There is very good indirect evidence for a sodium-potassium-activated ATPase, with similar properties to that found in red blood cells, located at the plasmalemma of cells of Nitrella translucens (MacRobbie, 1962(MacRobbie, , 1964(MacRobbie, , 1965(MacRobbie, , 1966Spanswick and Williams, 1964), although it must be pointed out that this enzyme has yet to be isolated and characterized. ...
... There is very good indirect evidence for a sodium-potassium-activated ATPase, with similar properties to that found in red blood cells, located at the plasmalemma of cells of Nitrella translucens (MacRobbie, 1962(MacRobbie, , 1964(MacRobbie, , 1965(MacRobbie, , 1966Spanswick and Williams, 1964), although it must be pointed out that this enzyme has yet to be isolated and characterized. ...
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However, at low temperatures this stimulation of sulphate uptake by light is considerably higher than at high temperatures.Variation of external concentrations of phosphate and sulphate ions results in similar isotherms of uptake for both anions. The isotherms exhibit a linear dependence at low concentrations, a saturation curve at intermediary concentrations and a further rise at high concentrations. Using enzyme kinetics one obtains hyperbolic curves indicating that at least two different processes are taking place.The dependence on temperature within a limited range can be approximated by an Arrhenius equation and is described by the apparent energies of activation E app which are found to be 15.82.9 and 24.82.8 kcal/mol for the phosphate and sulphate uptake respectively at intermediary concentrations. In the case of phosphate E app is higher for the incorporation into Pu and Pso and considerably lower but very similar for both the incorporation into Plo and Pi. In this respect E app parallels the stimulation by light.The energies of activation of uptake for both anions are strongly dependent on external concentration and reach a maximum at intermediary concentrations.Considering the energetics with respect to internal and external concentrations and the value of the membrane potential, net uptake of both ions must occur with an expenditure of free energy, thus representing an active process in the whole range of external concentrations.However, from the concomitant changes in the stimulation by light and the energies of activation, the isotherms of uptake are interpreted as being composed of three concentration ranges with the following steps being rate-limiting: the diffusion in the unstirred layer at low concentrations, the active uptake at intermediary concentrations and a considerable contribution of diffusive influx at high external concentrations. This diffusive uptake can occur with tracer flux (influx) only, not with net uptake. Thus it is not contradictory to the energetics of anion uptake.On the basis of this interpretation active and passive fluxes at high concentration are separated and permeability coefficients are estimated from the passive fluxes. According to this sparation the energies of activation of diffusive transport in the cellular membrane are calculated as 5 and 8 kcal/mol (Q 10=1.35 or 1.6) for phosphate and sulphate respectively. The phosphate fluxes are considerably higher than the sulphate fluxes.From the values of E app it is deduced that a major component of Plo, possibly ATP, might participate in active phosphate uptake.The isotherms compare well with isotherms of anion uptake by other species so that the results may have some bearing on the interpretation of anion uptake in general.
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A quantitative study has been made of the intercellular movement of chloride in Chara corallina, using pairs of joined internodal cells. One cell of the pair (cell 1) was exposed to a solution containing ³⁶Cl; the distribution of this tracer between the cells was determined at the end of the uptake period. Of the chloride taken up, 0.29 was transported out of cell 1 for all uptake times from 1.8 to 22 ks and 0.57 was transported to the vacuole of cell 1, in experimental series I. In series II, the fraction transported out of cell 1 was 0.43 at 22 and 43 ks, but 0.17 at 600 s. These results represent a rate of transport of 4 to 60 pmol s⁻¹, across an intercellular wall of area 1.5 × 10⁻⁶ m²; the wall has 0.03 to 0.04 of its area occupied by plasmodesmata. The estimate of transport rate is based on an attempt to determine the specific activity of the cytoplasm of cell 1. The electric resistance of the node was found to be 47 mΩ m². The observed transport rate can be explained by diffusion in the plasmodesmata, without the need to postulate active processes. Diffusion in the plasmodesmata is slower than in free solution by an ‘impediment factor’ of 7 to 700, depending on the assumed chloride concentration of the ground-plasm. If the plasmodesmata offer the major conducting path for electric current, the electric impediment factor is 390. Chloride enters the plasmodesmata from the same small kinetic compartment which supplies the flux to the vacuole, or from a smallintermediate compartment.
Article
Movement of Na into cells of Nitella translucens is a ‘downhill’ process; the ions move across the plasmalemma down an electrochemical potential gradient. Nevertheless, measurements of Na influxes under a wide range of experimental conditions have shown that there must be links between Na uptake and processes controlled by metabolism. When Ca ions are present in the bathing solution, Na influxes are greatly increased by light under conditions where photosynthesis can proceed (i.e. when both photosystems are active). In the presence of Ca, the influx of Na increases only slightly when the external Na concentration is raised above 1 mM, and the light-promoted Na influx is considerably inhibited when Cl is removed from the bathing solution. When the Cl concentration is kept constant, the Na influx in light is determined by the concentrations of other cations present in solution (K, Ca, or NH 4 ). In the absence of Ca from the cell wall and solution, the influx is stil enhanced by light, but does not saturate when the external Na concentration is raised above 1 mM. It is suggested that the Na influx in light is partly linked to the inward Cl pump, but there is also a separate (Cl-independent) effect of light on the permeability of the plasmalemma to Na. Links between Na and Cl uptake could be maintained by effects of Cl on electrochemical driving forces controlling Na entry; alternatively, chemical coupling between the two processes may be involved.
Article
By measuring the number of water molecules per ion which were moved electro-osmotically through cells of Nitella translucens and N. flexilis it has been shown that a significant difference existed between samples of these species in 1965. In each species the electro-osmotic efficiency was greater with Na+ than with K++. Also 10-4 to 10-5 M IAA tended to decrease electro-osmotic efficiencies while IAA, after 30 min. treatment, produced a significant increase in water flow into the treated end of a living cell. Calculations based on this work suggest that about 108–109 pore sites per cm2 exist on the surface membranes for Na+ or K+ ion transport.
Article
The salt content of the chloroplasts of Tolypella intricata has been found to be very high. To maintain this it seems that there must be some form of ionic control at the outer boundary of the chloroplasts.
Article
A new nuclear magnetic resonance (NMR) technique developed by Conlon and Outhred (1972. Biochim. Biophys. Acta, 288: 354–361) to measure diffusional water permeability was applied to the multicellular plant system Elodea Nuttallii (Planch) St. John leaves. This technique involves measuring a transverse relaxation time (T2) in the absence (T2 = Ta) and in the presence (T2 = Ta′) of extracellular paramagnetic cations. A valid estimate of Ta was measured for Elodea leaves. The value of Ta′ was found to decrease continuously with time. Evidence is presented that the decrease of Ta′ with time is initially related primarily to the time required for the paramagnetic ion to diffuse throughout the extracellular space and then later related to influx of the paramagnetic ion into the cells. By extrapolating to zero time to correct for paramagnetic-cation influx into the cells it was possible to estimate the value of Ta′ required to calculate the water exchange time out of the cells. It was estimated from the NMR data that Mn2+ (the paramagnetic ion used) flux into the cells occurred at a rate of 3.0 × 10−14 mol cm2 s−1. A procedure to determine whether the water-exchange time is controlled by intracellular unstirred layers or by membrane water permeability or by both is given. The water-exchange time of Elodea leaves is predominantly controlled by the intracellular unstirred layers. Thus it was only possible to set a lower limit on the diffusional water permeability coefficient (Pd) of Elodea leaf membranes of 3 × 10−2 cm s−1 at 20 °C.
Chapter
Fresh water giant algal cells have long provided favoured experimental material for studies of processes of ion transport in plant cells. Characean cells played a major role in the early development of the subject, in the classic work of Osterhout et al., in the period 1920–40, and after a period of relative neglect they have been much used in more modern studies. They have been used in their own right, with the aim of understanding the processes involved in maintaining the internal ionic composition of a single plant cell, and its intracellular organelles, but have also been studied as potential models for the behaviour of more typical cells in higher plants, where the small cell size, the heterogeneity of cells and the complexities of tissue organisation make it difficult to study well-defined membrane processes. While it is clear that not all the transport and metabolic processes involved in the activities of higher plant cells are reproduced in the simpler algae, there are reasonable indications that the ion transport processes identified in giant algal cells can be observed also in higher plant cells under suitable experimental conditions, and are likely to form a part of the range of normal activities of such cells. The problems faced by any plant cell, in acquiring its complement of osmotic solutes for growth, largely to be sequestered in the central vacuole, and in maintaining ionic environments in its cytoplasm and various constituent organelles that are suitable for their respective ranges of metabolic activities, are basically similar. Understanding of membrane activities in cells in which such processes are accessible to well-defined experiment must certainly throw light on more complex systems, and may be directly transferable. Freshwater conditions are more nearly comparable to the ionic environment sampled by cells in most parts of normal terrestrial plants, with relatively low external ion concentrations (in soil or in the free space supplied by the transpiration stream) than are marine conditions; it is probably only in halophytes growing in extreme saline conditions that higher plant cells face conditions similar to those of the marine giant cells. The evidence available bears this out, and the transport activities of higher plant cells and of fresh water giant algal cells do seem to have much in common.
Article
An internode of Chara was permeabilized as described by Shimmen and Tazawa [(1983) Protoplasma 117:93]. The Cl- efflux of the permeabilized cell increased when the extracellular Ca2+ concentration was increased, and the degree of the increase was dependent on the Ca2+ concentration. This suggests that the Cl- channel in the tonoplast was activated by Ca2+.
Chapter
Other Chapters are devoted to the relationships between fluxes of ions and the driving forces that produce them—concentration differences, electric potential differences, flows of solvent or solute, metabolic flows. This Chapter is concerned with the problem of measuring such fluxes across cellular membranes, and with the related problem of determining the distribution of ions between the various cellular compartments.
Chapter
The use of giant algal cells has been very important in recent advances in our understanding of the mechanism of solute and water transport in plants. This research has viewed giant algal cells as convenient material for the study of membrane properties per se; the involvement of solute transport in the life of the plant particularly with respect to nutrition, has been relatively neglected.
Article
This chapter outlines the rationale, methodology, and present applications of the measurement of intracellular ionic activities with ion-selective microelectrodes. The theoretical description and experimental determination of the thermodynamic state of small ions, even in homogeneous polyelectrolyte solutions, is not a simple problem. In heterogeneous multicompartment polyelectrolyte systems, the problem becomes much more complex. Despite these complications, it has long been apparent that the influence of the polyelectrolyte components of cytoplasm on the physical state of intracellular ions may be highly significant and should be taken into consideration. At present, the only direct method available for determining the ionic composition of the ground cytoplasm of cells is the measurement of intracellular ionic activities with ion-sensitive microelectrodes. The position, with respect to cellular ultrastructures, of the tip of the ion selective electrode following impalement of a cell is usually not exactly known, and one cannot prove beyond doubt that the medium surrounding the electrode tip is in fact ground cytoplasm. However, it is clear that the position of these microelectrodes is exactly the same as that of ordinary capillary microelectrodes used for transmembrane potential difference measurements. Therefore, ion activities found by the method of ion-sensitive microelectrodes are plausible for the description of electrochemical plasma membrane properties and transmembrane ionic fluxes.
Article
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Cl⊖ and K⊕ uptake by cells of variegated leaves of Oenothera and Tradescantia having normal and mutated chloroplasts, respectively, have been investigated. Under our conditions light has no effect on ion uptake. Cl⊖ uptake is highly sensitive to FCCP in both green and mutated cells, while K⊕ uptake is inhibited only in the green cells. Rates of ion uptake are higher in the green than in the mutated cells. The results allow two alternative explanations: (I) mutation may affect carrier-proteins and hence alter ion accumulation capacity of the cells; (II) light-independent metabolically controlled ion accumulation may be correlated with chloroplast differentiation.
Article
The influence of DCMU on the photosynthesis and the uptake of chloride and rubidium ions by Vallisneria leaves was studied.The photosynthesis was more reduced by DCMU than was the ion uptake. The rubidium uptake is somewhat more sensitive to DCMU than the chloride uptake.The provisional conclusion is that both the rubidium and chloride uptake can be supported by cyclic photophosphorylation.
Article
SUMMARYA survey is given of the present state of the theory of symplasmic transport. The symplasmic movement of organic and inorganic substances in Vallisneria leaves shows polarity. The polarity is not a stable factor of the tissue, since it can be changed in different ways. Data regarding the movement of auxin in Vallisneria leaves have been compared with data about auxin transport in coleoptiles. Attention has been given to the localisation and the function of the plasmodesmata. The conclusion is arrived at that there are two fluxes in coleoptiles, a longitudinal flux which uses the plasmodesmata in the transverse walls and a transverse flux which uses the plasmodesmata in the longitudinal walls.Movement in the symplasm is caused by concentration differences in combination with electric potential gradients and by gravity. Active transport requires the maintenance of these potential gradients by cell metabolism. The forces presumably influence the transit of the substances through the plasmodesmata, from cell to cell.At a cut surface cells are opened and substances are released to the exterior. The exit of auxin at the basal wound surface is the consequence of the continuous polar movement of auxin through the plasmodesmata in the transverse walls of the cells and the opening of the adjoining cells by cutting. The movement of endogenous and exogenous auxin are different.Endogenous auxin is moved by electric potential gradients in the tissue and by gravity, while exogenous auxin is moved by the same factors and moreover by the concentration difference which is the result of the active uptake of auxin in the symplasm.
Measurements were made of ionic fluxes in cells of the marine algae G. monile and G. pulvinata. The data for sodium and chloride fluxes appear to be consistent with the usually proposed model for the plant cell in which the vacuolar, cytoplasmic, and external compartments are in series, with the cytoplasm bounded on the inside by the tonoplast and on the outside by the plasmalemma. However, there are difficulties in explaining the results for potassium fluxes on this model. At the plasmalemma, the passive fluxes of potassium, sodium, and chloride were 50-380, 15, and 5 p.moles cm-2 sec-I respectively. There was also an active chloride influx of 0-35 p·moles cm-2 sec-I and an active sodium efflux of about 15 p.moles cm-2 sec-I. The fluxes at the tonoplast, on the basis of the compartments-in-series model, were potassium 300-1000, sodium 10 p-moles cm-2 sec-I, and chloride at least 30 p-moles cm-2 sec-I. Electrochemical data (Findlay, Hope, and Williams 1969) indicate that there is an active flux of potassium across the tonoplast from the cytoplasm into the vacuole. The large fluxes of potassium and chloride at the tonoplast are difficult to reconcile with the electrical resistance of 5000 Qcm2, which suggests that a large part of the tonoplast potassium flux is exchange diffusion, or that vesicular movement of potassium and chloride ion pairs occurs. The influxes ofK+, Rb+, Cs+, and Na+ ions at the plasmalemma were measured and the following order of permeability obtained: P K > P Rb > PCs > P Na? This order agrees with that obtained from measurements of depolarization of the plasmalemma potential.
Article
The uptake of C1 ions by cells of Tolypella intricata is greatly increased by light, and must be an active process. K ions in the cells (at 90–110 mM concentration) are in approximate electrochemical equilibrium with the external solution, but the K influx is affected (directly or indirectly) by cellular metabolism. The K influx is increased by light, and the increase is greater in the presence of C1 than when C1 is removed from the solution. K uptake is inhibited by chemicals which also inhibit the C1 pump.It is suggested that light increases the permeability of the plasmalemma to K, but there must also be links between K and C1 uptake. The possible nature of these links is discussed.The internal Na concentration (3–10 mM) is considerably below the expected equilibrium concentration, but the Na influx is also very low (and is not increased by light). The permeability of the plasmalemma to Na is thus very low, and there can be little active extrusion of Na under normal conditions.
Article
The findings of JACOBI (1959) that glycolate enhances the uptake and the incorporation of (32)P-labelled orthophosphate were re-investigated. In contrast to the findings of JACOBI we found that glycolic acid has no effect at concentrations between 10(-3) and 10(-7)M. The effect reported by JACOBI was seen only when instead of glycolic acid Na-glycolate was used in the experiments, as it was done by JACOBI. Further experiments showed that the enhancement of (32)P-incorporation is due only to the Na(+)-ions and not to glycolic acid. In addition it was found that up to pH 4,3 no glycolic acid is resorbed from the medium by cells of Ankistrodesmus braunii.
Article
This paper deals with the effect of calcium binding in the cell wall on the measured (45)Ca influx in Chara corallina Klein ex Will. esk. R.D. Wood. Calcium in the cell wall was in the range 687-1197 (μmol · m(-2) compared to the sap which contained only 144-256 μmol · m(-2). In dilute culture solutions the calcium content of the cell wall was relatively independent of external calcium at concentrations above about 0.1 mol · m(-3). The half-times for exchange of calcium from (45)Ca-labelled cell walls varied from 45 min at 0.05 mol · m(-3) to less than 2 min at 2 mol · m(-3). The effectiveness of other cations in displacing calcium from cell walls was in the order La > Zn > Co > Ni > Mg. Rinsing of (45)Ca-labelled cell walls in 2 mol · m(-3) LaCl3 for 20 min removed more than 99% of the bound (45)Ca. However, the residual (45)Ca activity in isolated cell walls following La(3+) rinsing was similar to that in whole cells. It is concluded that in whole cells (45)Ca influx cannot normally be distinguished from extracellular binding of calcium. Methods are described for the measurement of (45)Ca fluxes in charophyte cells by isolation of intracellular (45)Ca after the uptake period using techniques which avoid contamination from the large amount of tracer bound in the cell wall. At an external calcium concentration of 1 mol · m(-3), the plasmalemma influx was approx. 0.2 nmol · m(-2) · s(-1) of which about half entered the vacuole and half was effluxed back into the external solution. The cytoplasm filled with calcium with a half-time of 40-50 min with an 'apparent' pool size of 50 mmol · m(-3). After 2 h the net flux to the cell was almost the same as the vacuolar flux. The fluxes reported are an order of magnitude lower than previously reported calcium fluxes in plants.
Article
Active chloride uptake by cells of Chara corallina decreases as the pH of the bathing solution is increased, using a range of buffers. Superimposed on this effect, large stimulations of Cl− uptake can be obtained by the addition of imidazole and tris buffers or ammonium sulphate. ‘Stimulated’ or ‘normal’ Cl− influxes respond similarly to the presence of metabolic inhibitors.It is suggested that these effects are not due to changes in photosynthesis or respiration, but reflect the dependence of Cl− transport on the pH gradient across the plasmalemma. Possible ways in which Cl− transport might be linked to H+ or OH− fluxes are discussed. It is proposed that the pH gradient across the plasmalemma is controlled by charge-separating reactions which produce an active H+ efflux, and that Cl− uptake is linked to OH− efflux by ion-exchange reactions.The metabolic implications of this scheme are discussed in detail, with reference to previous work on Characean and other plant cells.
Article
1. Like other photosynthesizing organisms which have been investigated, Ankistrodesmus braunii absorbs more glucose from the surrounding medium in the light than in the dark. 2. When the algae are incubated with glucose and 32P-labelled orthophosphate in short-time-experiments, the TCA-soluble organic phosphate fraction is markedly increased. No such effect is seen when 2-desoxy-glucose is administered to the algae instead of glucose. 3. In pre-labelled algae glucose causes an increase in the TCA-soluble acid-stable organic P-fraction which shows light-dependent saturation kinetics. In such experiments 2-desoxyglucose causes a linear increase in the acid-stable organic P-fraction which shows no light dependence. 4. DCMU and Antimycin A when given together block oxidative as well as light phosphorylation. These compounds do not, however inhibit the increase in the sugar-P-fraction caused by 2-desoxy-glucose in 32P-labelled algae. 5. The increase in the sugar-32P after administration of substrates to the algae is accompanied by a decrease in the fraction of the polyphosphates C and/or D. 6. These results are explained by assuming that an inorganic polyphosphate-glucose-phosphotransferase is active in Ankistrodesmus braunii.
Article
1.1. A method is described of blocking microelectrodes with silver so that they become sensitive to chloride ion concentration.2.2. These electrodes have been used to measure the internal chloride concentration of the H cells and D cells in the brain of the snail.3.3. The H cells (which are hyperpolarized by acetylcholilne) have a lower chlotide concentration, 11·2 ± 0·6 mM, than the D cells (which are depolarized by acetylcholine), 24·7 ± 0·8 mM.4.4. It is suggested that the different actions of acetylcholine on the D and H cell is not simply to bring them to the chloride equilibrium potential, but that instead there are differences in the membranes of these types of cells and that in the H cellas, there is an increased permeability to chloride whilst in the D cell, there is an increased permeability to sodium ions.
Article
Radioisotope fluxes of Na+, K+, Cs+, Rb+, Cl−, Br− and I− over the salt glands of Limonium vulgare have been measured under short-circuit conditions. All these ions are actively transported out of the parenchyma by the gland cells.
Article
The exchange of water between the giant algal cells of Nitella mucronata and the surrounding medium was studied using the method or enhanced proton magnetic relaxation (PMR) in the presence of paramagnetic Mn2+ ions outside the cells (10-25 mM MnCl2). A large value of τa=0.80±0.02 s was obtained for the exchange-time between the intracellular water and the extracellular space. This gave an average diffusional permeability Pd=2.5×10-3 cm s-1. Two components of longitudinal relaxation were determined (T1a and T1b) following soaking of the cells in the solutions of paramagnetic ions, and their relative contributions were found to be X=0.145 and 1-X=0.855, respectively. These two types of water were assigned to cytoplasm (short component T1a) and the vacuole (long component T1b). Taking into account the short component we calculated τp=19.9±2.5 ms as water exchange time through the plasmalemma.
Article
La distribution subcellulaire des ions Na⁺ est déterminée à partir de l'analyse sérielle d'une courbe d'échange isotopique ²⁴Na/²³Na. La mesure simultanée des potentiels électriques permet de montrer que la distribution de Na dans les cellules d'Acer est très inférieure à celle prévue par les conditions d'équilibre thermodynamique; ce qui suggère l'intervention d'une pompe d'efflux de Na dont l'existence est confirmée par l'action du froid et des effecteurs métaboliques. A condition que ses précurseurs protéiques soient préalablement synthétisés, ce système d'expulsion deviendrait opérationnel au-delà d'une certaine dose de Na⁺ interne.
Article
Itisshownthattheinflux oftracer toa plant cell mustbemeasured forless thanone third ofthehalf-time forexchange ofcytoplasmic tracer, andanysubsequent wash mustbenegligibly brief, iftheinitial influx measured overa finite period istobea good estimate oftheplasmalemma influx. Complioations duetolackofknowledge ofthecyto- plasmic exchange rateconstant andtoextracellular contents makeitdifficult tomakesuch an estimate frominflux measurements alone.The useofinflux measurements isfurther discussed. Thisnoteconsists ofaquantitative consideration oftheideathattheinflux oftracer toplant cells overashort period isameasure oftheplasmalemma influx. Thesimplest description ofa plantcellisin termsofa smallcytoplasmic compartment outside andinseries witha largevacuolar compartment. Atornearasteady state (i.e. whenchemical fluxes andconcentrations areeffectively constant with time), themovementofa labeled isotope fromone compartment toanother willbeproportional tothe difference inspecific activity. Themovementof tracers willtherefore follow first orderkinetics. Inthisparticular model, tracer movements willbe described byrelatively simple equations, whichwill relate tracer kinetic datatochemical fluxes and compartment contents (2,6, 7,9).Thismodelhas beenshowntobeafirst approximation totheactual situation inthegiant algal coenocyte, Nitella trans- lucens (6,7) andanattempt hasbeenmadetofind ifthisisalsotrueforexcised carrot tissue (2). Thefollowing considerations arebasedonthis simple model.Theplasmalemma influx andefflux- willbe called M.CandMC.;thetonoplast influx andefflux, M00andM00;andthecytoplasmic content, QO.. Tracer fluxes will hecalled M*.,, etc. Thenettracer influx tothecell isinitially equal toM0,s,.sowill betaken as1,andomitted fromthe following equations. Subsequently thespecific ac- tivity inthecytoplasm willrise: M. SC = (1 e-kct)
Article
THE small, leaf-shaped marine slug Elysia viridis (Ophisto-branchia, Sacoglossa) owes its green colour to borrowed pigments. Unusual, although not unique in the animal kingdom, it is part of a `chloroplast symbiosis' and is photosynthetically active. The chloroplasts, which are found in the cells of the highly branched digestive channel (hepatic diverticula), have been identified as originating from the siphonous green alga Codium fragile, on which Elysia feeds1. Removed from its food source but kept in the light, Elysia can live for months by means of photosynthesis. In the dark, however, even when offered food, it soon dies2. As part of the relationship the animal cells affect the morphological integrity and function of the symbiotic chloroplasts, and I report here that the cells themselves are influenced by the presense of the chloroplasts. The ion balance of Elysia is regulated by light through the mediation of the chloroplasts.
Article
Measurements of ion exchange were made between isolated cell walls of Ohara australis and an external solution. Comparison between intact cells and cell walls showed that nearly all the easily exchangeable cations are located in the cell wall. The wall is hown to consist of "water free space" (W.F.S.) and "Donnan free space" (D.F.S.); the concentration of in diffusible anions in the D.F.S. is about O· 6 equivjl. This finding is contrary to past suggestions that the D.F.S. is in the cytoplasm of plant cells.
Article
Three intracellular compartments for potassium exchange have been observed in intact cells of the giant-celled alga, Nitella axillaris. These compartments have been compared with the exchange properties of isolated subcellular structures. The smallest and fastest compartment (apparent half-time, 23 seconds) appears to involve passive absorption on the cell wall. The next largest (apparent half-time, 5 hours) may represent exchange with the cytoplasmic layer through the plasma membrane, the chloroplasts being in rapid equilibrium with the surrounding cytoplasm. The largest and slowest compartment (apparent half-time, 40 days) has been identified with the central vacuole. The vacuolar membrane and the plasma membrane have similar properties with respect to K permeability. Thus, the experimental data from the whole cell can be accounted for by a structural model of the compartments. Cyanide in concentrations up to 10(-3)M causes no net loss of K. The fastest compartment in Nitella and in higher plants is compared, and the ecological significance of the slow rate of potassium transport in Nitella is discussed.
Article
This chapter summarizes the concepts of structure as elucidated by the light microscope, followed by the contributions of electron microscopy to those aspects of cytoplasmic ultrastructure that were either controversial or unknown before the advent of new techniques. Submicroscopic structures known through optical microscopy present in the protoplasm are also described. The earliest applications of the electron microscope in biology were directed toward particles such as viruses, which did not require the complicated techniques necessary to preserve the living animal cell. The protoplasm consists of a hyaline ground substance, the cytoplasm, claimed to be “optically empty” under dark-field illumination, and susceptible to reversible gel or sol formation. It contains certain living constituents, such as the nucleus in which chromosomes, nucleoli, chondriosomes, and plastids are localized. The cytoplasm is limited by a thin lipoproteinic membrane, the ectoplasmic pellicle or cell membrane, whose properties of permeability are essential for the life and functioning of the cell. The electron microscope has first and foremost facilitated the identification of the diverse constituents of plant protoplasm and has aided in the definition of their ultrastructure.