Ionic Currents across the Plasmalemma of Chara inflata Cells: I. OSMOTIC EFFECTS OF SORBITOL ON K+, CI- AND LEAK CURRENTS

This paper describes experiments designed to investigate theeffects of increases in external osmotic pressure on the electrophysiologicalbehaviour of the plasmalemma in cells of the brackish-wateralga, Chara inflata. The electrical conductance of the plasmalemmaof these cells of, due to the diffusion of ions, consists mainlyof K⁺, Cl^– and leak components. The addition of sorbitolat concentrations in the range 40–280 mol m^–3 tothe external solution bathing the cells, progressively and reversiblydepolarized the cell membrane and increased the total membraneconductance, chiefly due to increases in each of the separateionic conductances. At concentrations higher than about 280mol m^–3 when the cells became plasmolysed, the effectsof sorbitol on the electrical properties of the cell ceasedto be fully reversible. When the membrane electrical potentialdifference is stepped in a negative direction with a voltage-clamp,the resulting inward current has voltage-dependent componentsconsisting of an inactivating K⁺ current, an activating Cl^–current and a constant leak current. The addition of sorbitolto the external solution modified these currents by increasingtheir magnitude, by increasing the half-time of the inactivationof the K⁺ current, and by decreasing the half-time of activationof the Cl^– current. Key words: Chara inflata, osmotic effects, K⁺ and Cl^– currents     

Ionic Currents across the Plasmalemma of Chara inflata Cells: II. EFFECTS OF EXTERNAL Na+, Ca2+ AND Cl- ON K+ AND Cl CURRENTS

The electrical conductance of the plasmalemma of cells of Charainflata, due to the diffusion of ions, consists predominantlyof K⁺, Cl^– and leak components. When the membrane electricalpotential difference is stepped in a negative direction witha voltage-clamp, the resulting inward current has componentsI_K, I_Cl and I_L (leak). During such voltage-clamp steps I_K isinactivated, and Ic activated with voltage-dependent half-times.Increases in the external NaCl concentration reduce the magnitudeof I_K and increase the magnitude of I_c, but reduce the half-timeof inactivation or activation. The NaCl-induced changes in I_kand I_Cl and their kinetics were more pronounced at pH₀ =6.5than at pH₀ =9.5. When the concentration of external CaCl₂ wasincreased, I_k, I_Cl and the half-time of inactivation, (T_1/2),of I_k were all reduced. The half-time of activation of I_Cl wasincreased. The NaCI-induced changes could result from increases in bothexternal ion concentration and osmotic pressure. Previous experimentshave shown that an increase in external osmotic pressure alonealters the properties of the conductances. In this paper weattempt to separate the purely ionic effects from the osmoticones. Key words: Chara inflata, ionic effects, K⁺ and Cl^– currents     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : VIII. QUANTITATIVE STUDIES CONCERNING THE ACIDIC PROPERTIES OF COLLODION AND THEIR CORRELATION WITH MEMBRANE STRUCTURE AND ACTIVITY

1. The electrochemical behavior ("activity") of collodion membranes depends upon acidic, dissociable groups located in the interstices of the membranes. The active groups can be determined by base exchange measurements. High base exchange capacity is always found with preparations of great "electrochemical activity;" medium and low base exchange capacities occur with electrochemically active as well as with inactive preparations. The observed base exchange capacity is determined by two factors: the inherent acidity of the collodion (its mean equivalent weight) and the submicroscopic micellar structure of the collodion. A comparison of the base exchange capacity of various collodion preparations and their inherent acidities therefore allows certain conclusions to be drawn concerning the relative availability of the micellar surfaces in the different preparations. 2. The inherent acidity of various collodion preparations, their "acid number," was determined by electrometric titration. Collodion in the acidic state, i.e. after exchange of all other cations for H⁺ ions, was titrated in an organic solvent mixture with alcoholic KOH using a quinhydrone electrode. Details of the experimental procedure are given in the paper. The acid numbers, expressed in milliliters of 0.01 N KOH per gram dry collodion, vary from 1.0 for a highly purified collodion preparation of very low electrochemical activity to 3.3 for a highly oxidized sample of very high activity. Acid numbers of about 1.5 (corresponding to an equivalent weight of about 67,000) are found both with inactive commercial and with fairly active oxidized preparations. The base exchange capacity of the same preparations in the fibrous state as measured after 48 hours of exchange time varies from 0.0013 ml. 0.01 N NaOH per gm. dry collodion for the most inactive preparation up to 0.26 ml. 0.01 N NaOH per gm. for the most active preparation. Thus the acid numbers over the whole range investigated differ only in the ratio of 1:3.3, whereas the base exchange values differ in the range of 1:200. 3. In the inactive preparation only one in 770 acid groups is available for base exchange, in the most active collodion one group in 13; values between these extremes are found with commercial and alcohol purified oxidized preparations. 4. The high base exchange capacity of the electrochemically active preparations is not so much due to their higher acid number as to their more open structure. This difference in structure is ascribed to the presence of a small fraction of low molecular weight material which inhibits normal formation and arrangement of the micelles. 5. Short time base exchange experiments with fibrous collodion indicate that the number of acid groups available for the typical electrochemical membrane functions may be estimated to be about 50 to 1000 times less numerous than those found in the 48 hour base exchange experiments. It is estimated that in membranes prepared even from the most active collodion not more than one in 500 acid groups may be available for the typical membrane functions; with the less active preparations this ratio is estimated to be as high as one in 1,000,000 or more.     

Plasmalemma Transport of OH in Chara corallina: III. FURTHER STUDIES ON TRANSPORT SUBSTRATE AND DIRECTIONALITY

The identity of the plasmalemma-transported species that develops the alkaline bands of Chara corallina was investigated. The effect of fusicoccin on the rate of HCO₃⁻ assimilation, and on the time-dependent alkaline band pH buildup following low pH flushing, was found to be small, with no stimulatory effect. Computer simulation of the flushing experiments showed that in the experimental situation the alkaline band transport system was slowed down, rather than speeded up, by low pH flushing. A detailed theoretical examination of the maximum rate of proton production from water showed that measured alkaline band fluxes are too large to be explicable in terms of an H⁺ influx system. The experimental and theoretical results indicate that the plasmalemma transport of OH⁻ ions is responsible for the measured negative external electric potential and alkalinity flux which are associated with the alkaline band phenomenon. Consequently, HCO₃⁻ influx across the characean plasmalemma must be charge-balanced by the efflux of OH⁻ ions.     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : III. THE BASE EXCHANGE PROPERTIES OF COLLODION

1. Theoretical considerations lead to the conclusion that dissociable acidic groups present to a varying extent in different collodion preparations determine the electrochemical behavior of membranes cast from these preparations. It is further reasoned that the base exchange capacity of the collodion surfaces is the true quantitative measure of the abundance of the dissociable groups. 2. The concept of base exchange capacity and the base exchange method are discussed. The conditions which allow a purposeful application of the latter are stated. 3. The base exchange properties of a number of fibrous collodion preparations of different origins and after various types of treatment, having widely varying electrochemical activities, are determined. 4. With the chemical (titration) and physical (electrometric) methods employed, no regular correlation can be found between electrochemical activity and base exchange. The base exchange capacity which is necessary to cause even great electrochemical activity of collodion is extremely small. 5. Measurable to high base exchange capacity always seems to be associated with good or high electrochemical activity; but base exchange capacity too low to be definitely measurable with the available methods may be found with collodion preparations of high as well as with preparations of low electrochemical activity. 6. The bearing of these results upon the problem of the spatial and electrical structure of the collodion membrane is indicated briefly.     

Potassium Transport Across the Membranes of Chara: II.42K FLUXES AND THE ELECTRICAL CURRENT AS A FUNCTION OF MEMBRANE VOLTAGE

Smith, J. R. 1987. Potassium transport across the membranesof Chara. II. ⁴²K fluxes and the electrical current as a functionof membrane voltage.—J. exp. Bot. 38: 752–777. The current required to clamp the trans-membrane voltage ofinternodal cells of Chara australis at different levels wasmeasured simultaneously with either the ⁴²K influx or efflux.Examination of the voltage-dependence of the ratio of the electricalcurrent to the unidirectional tracer fluxes yielded no evidenceof any amplification of the electrical driving force on theK⁺ ions. There was thus no evidence for the interaction of K⁺ions with themselves or any other species during their passageacross the membrane. These measurements allow the determinationof , the fraction of the electrical current carried by K⁺ ions.When the external [K⁺] = 10 mol m^–3, the average valueof was 0?85 for V_m > –125 mV and 07?5 for V_m <–150 mV. When the external [K⁺] = 0?1 mol m^–3, was 0?6 for V_m < –80 mV and 0?1 for V_m > –250mV. It was also found that the conductance associated with K⁺transport was inhibited by hyperpolarization. Key words: Potassium, conductance, flux-ratio     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : I. THE BEHAVIOR AND PROPERTIES OF COMMERCIAL COLLODION

1. The electrochemical behavior of membranes prepared from commercial collodion preparations varies widely, some preparations showing very high, other ones very low electrochemical efficiency ("activity"). 2. The electrochemical activity of a collodion membrane depends entirely upon impurities of an acidic nature contained in the collodion used for casting the membrane. 3. The active acidic impurities are substantially due to partial oxidation which occurs in the manufacturing process. Sulfuric acid compounds; e.g., acid sulfuric acid esters play only a minor rôle, if any. 4. The electrochemical behavior of collodion membranes in solutions of strong electrolytes is decisively dependent upon the acidic groups built permanently into the collodion surfaces. Preferential ion adsorption plays only a minor, if any, rôle.     

HCO(3) Influx across the Plasmalemma of Chara corallina: Physiological and Biophysical Influence of 10 mm K

The effect of 10 mm K⁺ on the HCO₃⁻ influx in Chara corallina has been used to distinguish a Ca²⁺-dependent membrane integrity site from the HCO₃⁻ transport site which is also Ca²⁺-dependent (Lucas and Dainty, Plant Physiology 1977 60: 862-867).     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : XI. THE PREPARATION AND PROPERTIES OF "MEGAPERMSELECTIVE" COLLODION MEMBRANES COMBINING EXTREME IONIC SELECTIVITY WITH HIGH PERMEABILITY

1. The electronegative membranes described in the literature which show a high degree of ionic selectivity (permitting cations to pass and restricting the anions) have serious shortcomings: their absolute permeability is extremely low, much too small for convenient experimentation; their ionic selectivity in most cases is not as perfect as would be desirable, and is moreover adversely affected by prolonged contact with electrolyte solutions. 2. A method has been worked out to prepare membranes substantially free from these defects. Porous collodion membranes were cast on the outside of rotating tubes and then oxidized with 1 M NaOH. By allowing the oxidized porous membranes to dry in air on the tubes membranes of desirable properties are obtained. These membranes are smooth, have a well defined shape, and allow considerable handling without breaking. 3. This new type membrane when tested for ionic selectivity by the measurement of the "characteristic concentration potential," consistently gives potentials of 54 to 55 mv., the maximum thermodynamically possible value (at 25°C.) being 55.1 mv. This high degree of ionic selectivity is not lost on prolonged contact with water, and is only very slowly affected by electrolyte solutions. 4. The absolute permeability of the new type membranes can be varied over a very wide range by changing the time of oxidation. Under optimum conditions membranes can be obtained with a resistance in 0.1 N KCl solution of only 0.5 ohms per 50 cm.² membrane area. The absolute rate of cation exchange through these membranes between solutions of different uni-univalent electrolytes is very high, in one case, e.g. 0.9 m.eq. cations per 4 hours, the anion leak being 0.02 m.eq. Thus, the absolute permeability of the new type membranes is two to four orders of magnitude greater than the permeability of the dried collodion membranes and the oxidized ("activated") dried collodion membranes used heretofore. Because of the characteristic properties of the new type membranes the term "megapermselective" (or "permselective") collodion membranes is proposed for them.     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : XII. THE PREPARATION AND PROPERTIES OF "MEGAPERMSELECTIVE" PROTAMINE COLLODION MEMBRANES COMBINING HIGH IONIC SELECTIVITY WITH HIGH PERMEABILITY

The technique of Abrams and Sollner for the preparation of electropositive dried protamine collodion membranes has been improved. Porous collodion membranes cast on the outside of rotating tubes are treated for 48 hours with a solution of 2 per cent protamine sulfate buffered at pH 11. After being washed thoroughly the membranes are dried in air for several hours, soaked in water for several hours, and removed from the tubes. Further drying in air but without support shrinks the membranes slightly. The resulting membranes are designated "permselective" or "megapermselective" protamine collodion membranes. These membranes regularly give characteristic concentration potentials of –52 to –53 mv. and (in 0.1 M KCl) resistance of 0.5 to 15 ohms per membrane of 50 cm.² area. This resistance is several orders of magnitude smaller than that of the conventional dyestuff- and alkaloid-impregnated positive membranes. The megapermselective protamine collodion membranes can be kept either dry or in water for prolonged periods without detectable deterioration. They are quite smooth, have a regular shape, and stand considerable handling without breakage. The megapermselective protamine collodion membranes are the electropositive analogues of the electronegative megapermselective collodion membranes described by Carr and Sollner.     

Potassium Transport Across the Membranes of Chara: I. THE RELATIONSHIP BETWEEN RADIOACTIVE TRACER INFLUX AND ELECTRICAL CONDUCTANCE

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassiumtransport across the membrane of Chara. I. The relationshipbetween radioactive tracer influx and electrical conductance.—J.exp. Bot. 38:731–751. The ⁴²K influx () and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’of internodal cells of Chara australis as a function of theexternal [KCl] (K?. In bathing solutions of pH = 5?0, progressively increased from 20?5to 430?60 nmol m^–2 s^–1 and G_m increased from 0?36?0?02to 3?8?0?8 S m^–2 when K? was increased from 0?1 to 10mol m^–3. The resting membrane potential difference (p.d.)was approximately -135 mV for low K? and approached the expectedNernst equilibrium p.d. for K⁺ ions when K? > 1?0 mol m^–3.Measurements of ³⁶Cl influx suggested that the ⁴²K influx waspredominantly electrogenic. The equivalent Goldman permeabilityto K⁺ ions (P_k) was approximately 20–30 nm s^–1 anddid not vary significantly with increasing K?. The equivalentconductance attributable to the electrogenic transport of K⁺ ions was calculated from assuming passive, independent diffusionof K⁺ ions and the ratio was found to be typically close to one. It was also found that themagnitudes of and G_m measuredsimultaneously for each individual cell were also well correlatedfor K? 1?0 mol m^–3, and that the slope of the line ofbest fit was close to one. For each K? it was found that theconductance not attributable to K⁺ translocation and presumablyassociated primarily with the transport of protons or theirequivalents was typically 0?2–0?5 Sm^–2. For K? >1?0 mol m^–3 the results indicated that the transport ofK⁺ ions was essentially independent, i.e. there was no evidencefor flux interactions. The results also indicated that the equivalentconductance derived from the measured ⁴²K influx could usefullyindicate the fraction of the electrical conductance attributableto the translocation of K⁺ ions. Key words: Potassium, conductance, influx     

Nitrate Transport into Chara corallina Cells using as an Analogue for Nitrate: III. INTERACTIONS BETWEEN CHLORIDE AND NITRATE TRANSPORT PROCESSES

The effects of Cl^– and pretreatment on ³⁶Cl^–influx and influx into Characorallina cells were examined. Both treatments reduced ³⁶Cl^–influx into C. corallina cells in the acid pH range (4.5–7.0). pretreatment stimulated influx into C. corallina cells, but Cl^– pretreatment hadno effect. There was a direct inhibitory effect of CI on influx into C. corallina cells, but no apparenteffect on net NO uptake. The time course of ³⁶Cl accumulation into cytoplasmic and vacuolarcompartments during incubation of the cells with showed that significant radioactivity appeared in the vacuolarsap after 30 min. There was a linear increase in the percentageof total ³⁶CI which crossed the tonoplast (_c-v). There was noeffect of Cl^– or pretreatment on accumulation of radioactivity in the vacuole. Thin layer chromatography ofthe vacuolar sap showed that after 2 h only one component waspresent which had an R_F which was similar to ³⁶CI^–. Therate of accumulation of ³⁶C1^– in the vacuole could beused to estimate rates of reduction. Key words: Chloride, Chlorate, Chara, Nitrate     

THE STRUCTURE OF THE COLLODION MEMBRANE AND ITS ELECTRICAL BEHAVIOR : V. THE INFLUENCE OF THE THICKNESS OF DRIED COLLODION MEMBRANES UPON THEIR ELECTROMOTIVE BEHAVIOR

1. Experiments were carried out to decide whether or not the electromotive properties of dried collodion membranes depend upon their thickness. 2. A number of dried collodion membranes of varying thickness, 3–160 µ, were prepared from collodion preparations of different electrochemical activity. The characteristic concentration potentials across them were measured and the means of these values determined for each thickness. 3. The characteristic concentration potentials across dried collodion membranes are a function of their thickness. The thinnest membranes yield in all cases the lowest concentration potentials; increasingly thicker membranes give increasingly higher potential values, until a constant value is reached which is characteristic of the particular collodion preparation used. With electrochemically active collodion, characteristic concentration potentials approaching the thermodynamically possible maximum are obtained with membranes of only 10 µ thickness, thinner membranes giving appreciably lower values. With two rather inactive commercial collodion preparations the characteristic concentration potential increases from about 30 mv. for membranes 3 µ thick to about 42 mv. for 20 µ membranes; still thicker membranes do not show a significant increase in the potential values. With a highly purified collodion preparation the constant maximum value was found to be about 32 mv., 4 µ thick membranes giving only about 22 mv. 4. These results do not support the homogeneous phase theory as applied to the dried collodion membrane. They are readily compatible with the micellar-structural theory. Several special possible cases of the latter as applied to the dried collodion membrane are discussed.     

Potassium Transport Across the Membranes of Chara: III. EFFECTS OF pH, INHIBITORS AND ILLUMINATION

Smith, J. R., Walker, N. A. and Smith, F. A. 1987. Potassiumtransport across the membranes of Chara. III. Effects of pH,inhibitors and illumination.—J. exp. Bot. 38: 778–787. The effects of several treatments, normally used to inhibitelectrogenic proton transport, upon the potassium permeability(P_k) of the membranes of Chara were examined by means of simultaneousmeasurements of the ⁴²K influx (ⁱⁿ_K) and the membrane electricalconductance (G_m). ⁱⁿ_K, P_K and G_mwere found to be substantiallyunaffected when the external pH (pH?) was varied over the range5?0 to 85. However, when pH? was increased to 11 it was foundthat, although G_m increased considerably, both P_k and ⁱⁿ_K decreasedtypically by an order of magnitude. When cells were placed intotal darkness, P_K decreased substantially only after one dayhad elapsed. For the particular experimental conditions used,the inhibitors DES, NaN₃, and La₃₊ were found to alter P_K, whereasDCCD left P_K substantially unaffected. These results suggestthat care must be taken with some common procedures used toexamine the electrical properties of the electrogenic protonpump. Key words: Potassium, pH, illumination, inhibitors     

Potassium Transport Across the Membranes of Chara: IV. INTERACTIONS WITH OTHER CATIONS

Smith, J. R. and Kerr, R. J. 1987. Potassium transport acrossthe membranes of Chara. IV. Interactions with other cations.—J.exp. Bot. 38: 788–799. The ⁴²K influx () and the membrane electrical conductance (G_m were measured simultaneously forintemodal cells of Chara australis bathed in solutions containingdifferent concentrations of various cationic species. It wasfound that the potassium permeability (P_k,) of the membranewas reduced significantly when the bathing [CaSO₄ exceeded 01mol m^–1. Concentrations of tetra-ethylammonium ions (TEA)exceeding 0?3 mol m^–3 were found to reduce significantlyboth and , but even high concentrations (10 mol m^–3)usually did not reduce the fluxes by more than a factor of 3.Na⁺ ions were found to be capable of reducing P_K by a factorof 5?6 to a value of 4 nm s^–1. This appeared to be aminimum value for P_K which was not reduced even if several inhibitorycations were present simultaneously. This suggests that possiblyonly one of two different modes of K⁺ transport can be inhibitedby cations. The possible geometry of the inhibitable K⁺ channelis briefly discussed and the implications of the presence ofNa⁺ and Ca²⁺ ions in many common bathing solutions are considered. Key words: Potassium, calcium, tetraethylammonium, inhibition     

THE CONVERSION OF FAT TO CARBOHYDRATE IN THE GERMINATING CASTOR BEAN : III. THE CHEMICAL ANALYSIS,AND CORRELATION WITH RESPIRATORY EXCHANGE

1. Analyses for fat (ether extract), protein (N x 6.25), sugar including glucose, crude fiber, and ash have been made on all stages of the germinating castor bean up to 250 mm. length of hypocotyl and root system. 2. There is a continual decrease in the amount of fat present in the whole germinating seedling, and a continual increase in the amount of sugar up to about 40 per cent (dry weight) at a hypocotyl length of 80 to 140 mm., after which it decreases as crude fiber (cellulose) increases. The most rapid decrease in fat content coincides roughly with the most rapid increase of sugar. 3. The carbon balance between fat loss and carbohydrate (including fiber) gain is not at all close, except at the very beginning of growth. An undetermined residue occurs, which increases steadily along with the total carbohydrate and accounts for more and more of the carbon. 4. The protein content which in the ungerminated bean is about 26 per cent, at first falls a little and then rather steadily increases to reach nearly 35 per cent (dry) at the last stages studied. The most plausible explanation of this is the occurrence of more and more volatile substance which is lost in drying. 5. The ash increases irregularly but in the end shows about the same ratio of increase as the protein. 6. Respiration studies on several lots of these beans at different stages of germination exhibited the same low respiratory quotients as reported in Paper I. Comparing their composition at the end of the respiration period with that of corresponding stages when the period began, the chemical change can be compared with the respiratory exchange. 7. A trial balance of all the carbon changes including the respiratory carbon and protein carbon is not very satisfactory, because of our ignorance of the undetermined residue. 8. The respiratory quotient found can be accounted for quite satisfactorily on the assumption that two out of six molecules of ricinoleic acid are converted to cane sugar, one to cellulose, and three are oxidized. 9. The oxygen needed to produce the quantities of known carbohydrate found, added to that used for combustion, and the total subtracted from the observed loss from the respired air yields in two experiments a quantity which, combined with the excess carbon, suggests that the undetermined substance may be an oxidation product of pentose.     

Properties of Membranes from the Halophyte Suaeda maritima: II. DISTRIBUTION AND PROPERTIES OF ENZYMES IN ISOLATED MEMBRANE FRACTIONS

Membrane fractions from the shoots of Suaeda maritima have beenseparated by sucrose gradient centrifugation. Certain stainingand enzymatic activities were studied with the aim of identifyingspecific membrane types, particularly the plasma membrane. Chloroplastand mitochondrial membranes were largely distributed in thedenser regions of the gradient ( 1.16). A plasma membrane fractionwas not easily identifiable and the problems of separating thesemembranes from plant cells are discussed.     

Chilling-Susceptibility of the Blue-Green Alga Anacystis nidulans: III. LIPID PHASE OF CYTOPLASMIC MEMBRANE

The lipid phase of cytoplasmic membrane was studied by freeze-fracture electron microscopy in the chilling-susceptible blue-green alga, Anacystis nidulans. At growth temperatures, intramembrane particles were distributed at random in the fracture faces of cytoplasmic membrane, whereas, at chilling temperatures, the fracture faces were composed of particle-free and particle-containing regions. These findings indicate that lipids of the cytoplasmic membrane were in the liquid-crystalline state at the growth temperatures and in the phase-separation state at the chilling temperatures. Temperatures for the onset of phase separation were 5 and 16°C in cells grown at 28 and 38°C, respectively.     

MEMBRANE SYNTHESIS IN BACILLUS SUBTILIS : III. The Morphological Localization of the Sites of Membrane Synthesis

The results of several lines of investigation indicate that membrane growth in Bacillus subtilis does not occur at one or a small number of discrete zones. No indications of large regions of membrane conservation were observed. Kinetic labeling experiments of mesosomal and plasma membrane lipids indicate that the mesosomal lipids are not precursors of the plasma membrane lipids. Density shift experiments, in which the changes in buoyant density of membranes were studied after growth in deuterated media, showed no indication of large zones of conservation during membrane growth. Radioautography of thin sections of cells pulse labeled with tritiated glycerol showed no indication of specific zones of lipid synthesis. The consequences of these results for models of cell growth and division are discussed.     

The Movement of Ions to the Xylem Exudate of Maize Roots: III. THE LOCATION OF THE ELECTRICAL AND ELECTROCHEMICAL POTENTIAL DIFFERENCES BETWEEN THE EXUDATE AND THE MEDIUM

Trans-root and membrane potentials have been measured simultaneouslyin the same maize root by using microelctrodes inserted in theexuding sap, external bathing solution, and a vacuole of anepidermal cell. On rapidly increasing the KCl concentrationof the external solution, the membrane and trans-root potantialsfell simultaneously. This initial rapid phase of depolarizationwas complete within 20s of changing the external solution whenthe membrane potential had reached a new stable value. However,the trans-root potential continued to fall slowly and this phaseof depolarization lasted for about 25 min. Then followed a riseto a stable value at 1.5–2.0 h. This secondary rise wasrelatively small compared with the initial fall. The major part(approx. 80 per cent) of the depolarization of the trans-rootpotential occured during the initial rapid phase. These results indicate that the major component of the trans-rootpotential resides at the plasmalemma of the epidermal cellswith a smaller contribution from the cells underlying the epidermis.The rise in the trans-root potential after 25 min suggestedthat this back potential was associated with the plasmalemmaof the xylem parenchyma. From knowledge of the elelctrical propertiesof these cells this back potential could be calculated and trans-rootpotentials accurately predicted from values of the membranepotentials of root cells. It is concluded that in maize roots, ion movement to the xylemvessels is mainly symplasmic, that the outer boundery of thesymplasm is the plasmalemma of the epidermal cells and thatthe inner boundary is the plasmalemma of the xylem parenchyma.This hypothesis has enabled trans-root electrochemical potentialdifferences to be predicted accurately from vacuolar values.