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1.
An important step in visual processing is the segregation of objects in a visual scene from one another and from the embedding
background. According to current theories of visual neuroscience, the different features of a particular object are represented
by cells which are spatially distributed across multiple visual areas in the brain. The segregation of an object therefore
requires the unique identification and integration of the pertaining cells which have to be “bound” into one assembly coding
for the object in question. Several authors have suggested that such a binding of cells could be achieved by the selective
synchronization of temporally structured responses of the neurons activated by features of the same stimulus. This concept
has recently gained support by the observation of stimulus-dependent oscillatory activity in the visual system of the cat,
pigeon and monkey. Furthermore, experimental evidence has been found for the formation and segregation of synchronously active
cell assemblies representing different stimuli in the visual field. In this study, we investigate temporally structured activity
in networks with single and multiple feature domains. As a first step, we examine the formation and segregation of cell assemblies
by synchronizing and desynchronizing connections within a single feature module. We then demonstrate that distributed assemblies
can be appropriately bound in a network comprising three modules selective for stimulus disparity, orientation and colour,
respectively. In this context, we address the principal problem of segregating assemblies representing spatially overlapping
stimuli in a distributed architecture. Using synchronizing as well as desynchronizing mechanisms, our simulations demonstrate
that the binding problem can be solved by temporally correlated responses of cells which are distributed across multiple feature
modules.
Received: 25 March 1993/Accepted in revised form: 8 September 1993 相似文献
2.
Zolnai Z Lee PT Li J Chapman MR Newman CS Phillips GN Rayment I Ulrich EL Volkman BF Markley JL 《Journal of structural and functional genomics》2003,4(1):11-23
A computing infrastructure (Sesame) has been designed to manage and link individual steps in complex projects. Sesame is being developed to support a large-scale structural proteomics pilot project. When complete, the system is expected to manage all steps from target selection to data-bank deposition and report writing. We report here on the design criteria of the Sesame system and on results demonstrating successful achievement of the basic goals of its architecture. The Sesame software package, which follows the client/server paradigm, consists of a framework, which supports secure interactions among the three tiers of the system (the client, server, and database tiers), and application modules that carry out specific tasks. The framework utilizes industry standards. The client tier is written in Java2 and can be accessed anywhere through the Internet. All the development on the server tier is also carried out in Java2 so as to accommodate a wide variety of computer platforms. The database tier employs a commercial database management system. Each Sesame application module consists of a simple user interface in the client tier, corresponding objects in the server tier, and relevant data stored in the centralized database. For security, access to stored data is controlled by access privileges. The system facilitates both local and remote collaborations. Because users interact with the system using Java Web Start or through a web browser, access is limited only by the availability of an Internet connection. We describe several Sesame modules that have been developed to the point where they are being utilized routinely to support steps involved in structural and functional proteomics. This software is available to parties interested in using it and assisting to guide its further development.Deceased, 30 August 2000 相似文献
3.
Goral AM Tkaczuk KL Chruszcz M Kagan O Savchenko A Minor W 《Journal of structural and functional genomics》2012,13(1):27-36
Isochorismatase-like hydrolases (IHL) constitute a large family of enzymes divided into five structural families (by SCOP).
IHLs are crucial for siderophore-mediated ferric iron acquisition by cells. Knowledge of the structural characteristics of
these molecules will enhance the understanding of the molecular basis of iron transport, and perhaps resolve which of the
mechanisms previously proposed in the literature is the correct one. We determined the crystal structure of the apo-form of a putative isochorismatase hydrolase OaIHL (PDB code: 3LQY) from the antarctic γ-proteobacterium Oleispira antarctica, and did comparative sequential and structural analysis of its closest homologs. The characteristic features of all analyzed
structures were identified and discussed. We also docked isochorismate to the determined crystal structure by in silico methods,
to highlight the interactions of the active center with the substrate. The putative isochorismate hydrolase OaIHL from O. antarctica possesses the typical catalytic triad for IHL proteins. Its active center resembles those IHLs with a D–K–C catalytic triad,
rather than those variants with a D–K–X triad. OaIHL shares some structural and sequential features with other members of
the IHL superfamily. In silico docking results showed that despite small differences in active site composition, isochorismate
binds to in the structure of OaIHL in a similar mode to its binding in phenazine biosynthesis protein PhzD (PDB code 1NF8). 相似文献
4.
The efflux of Cl at the plasma membrane of Chara wasstudied in relation to two treatments known to affect the flux:that of removal of external Cl and of light. It is shownthat although removal of external Cl results in a rapidreduction in Cl efflux (consistent with a direct effectof external Cl on the transport system) the magnitudeof this reduction in the dark is greater than the measured darkinflux. Therefore, in the dark at least, it is proposed that1:1 exchange diffusion cannot account for the trans-stimulationof efflux by external Cl. Light induces an inhibition of efflux and a concomitant stimulationof Cl influx at 20 °C, but at 10 °C the responsesto light of the two fluxes can be separated temporally. It istherefore suggested that the fluxes are not reciprocally dependenton the same factor which mediates in the light response. Furtherconsiderations show that it is unlikely that the cytoplasmicCl concentration mediates in the light response of eitherflux, but that changes in cytoplasmic pH may do so. 相似文献
5.
Wolf Singer 《Cognitive neurodynamics》2009,3(3):189-196
The cerebral cortex presents itself as a distributed dynamical system with the characteristics of a small world network. The
neuronal correlates of cognitive and executive processes often appear to consist of the coordinated activity of large assemblies
of widely distributed neurons. These features require mechanisms for the selective routing of signals across densely interconnected
networks, the flexible and context dependent binding of neuronal groups into functionally coherent assemblies and the task
and attention dependent integration of subsystems. In order to implement these mechanisms, it is proposed that neuronal responses
should convey two orthogonal messages in parallel. They should indicate (1) the presence of the feature to which they are
tuned and (2) with which other neurons (specific target cells or members of a coherent assembly) they are communicating. The
first message is encoded in the discharge frequency of the neurons (rate code) and it is proposed that the second message
is contained in the precise timing relationships between individual spikes of distributed neurons (temporal code). It is further
proposed that these precise timing relations are established either by the timing of external events (stimulus locking) or
by internal timing mechanisms. The latter are assumed to consist of an oscillatory modulation of neuronal responses in different
frequency bands that cover a broad frequency range from <2 Hz (delta) to >40 Hz (gamma) and ripples. These oscillations limit
the communication of cells to short temporal windows whereby the duration of these windows decreases with oscillation frequency.
Thus, by varying the phase relationship between oscillating groups, networks of functionally cooperating neurons can be flexibly
configurated within hard wired networks. Moreover, by synchronizing the spikes emitted by neuronal populations, the saliency
of their responses can be enhanced due to the coincidence sensitivity of receiving neurons in very much the same way as can
be achieved by increasing the discharge rate. Experimental evidence will be reviewed in support of the coexistence of rate
and temporal codes. Evidence will also be provided that disturbances of temporal coding mechanisms are likely to be one of
the pathophysiological mechanisms in schizophrenia.
This article was part of LNCS 5286 (2008), Maria Marinaro, Silvia Scarpetta, Yoko Yamaguchi (eds.), “Dynamic Brain—from Neural
Spikes to Behaviors, 12th International Summer School on Neural Networks Erice, Italy, December 2007 Revised Lectures” and
summarized some of the putative functions of temporal codes resulting either from the timing of external events (feed forward/bottom
up) or from internal timing mechanisms (top down). For comprehensive reviews of the theoretical prerequisites of synchronization
in these processes see Yamaguchi and Shimizu (1994) and Shimizu et al. (1985). 相似文献
6.
Franzin CM Gong XM Teriete P Marassi FM 《Journal of bioenergetics and biomembranes》2007,39(5-6):379-383
The FXYD membrane proteins constitute a family of conserved auxiliary subunits of the Na,K-ATPase, and have been the focus
of recent attention due to their ability to finely regulate the activity of the enzyme complex in various physiological settings.
In this review we describe the structures of the proteins, as well as their dynamics and their associations with the lipid
bilayer membrane, which we have recently determined by NMR spectroscopy. Although the proteins are relatively small, their
genes contain as many as six to nine small exons, and the coincidence of structured protein segments with their genetic elements
suggests assembly from discrete structural modules through exon shuffling. The three-dimensional structures and backbone dynamics
provide the foundation for understanding their intra-membrane association with the Na,K-ATPase α subunit, and the structure
of FXYD1 suggests a mechanism whereby the phosphorylation of conserved Ser residues, by protein kinases A and C, could induce
a conformational change in the cytoplasmic domain of the protein, to modulate its interaction with the α subunit. 相似文献
7.
AMTMANN ANNA; KLIEBER HANS-GEORG; GRADMANN DIETRICH 《Journal of experimental botany》1992,43(7):875-885
Neutral carrierbased Ca2+ selective microelectrodeshave been examined for application in concentrated multiionsolutions. Calculations with data from the literature and ourcalibration series with Ca2+ EGTA buffers (a convenientalgorithm for theircalculation is given) provide the physicochemicalconditions for determination of submicromolar concentrationsof free Ca2+ in the cytoplasm (with about 400 mM K+ and 70 mMNa+) of the marine alga Acetabularia acetabulum. The experimentalresults give a cytoplasmic concentration of 560 nM free Ca2+corresponding to 140 nM activity. Recordings of cytoplasmicCa2+ uponremoval and re-addition of external (10 mM) Ca2+ showsteadystate changes by about 50 nM (following the directionof external Ca2+) which are preceded by transient over shoots.These kinetics are better described by damped oscillations ofa feedback control system than by two superimposed exponentials.Using the maximum rate of decrease of cytoplasmic Ca2+ uponremovalof external Ca2+, a unidirectional Ca2+ efflux of 0.3µmol m2 s1 is determined which is consideredto mark the steadystate turnover of Ca2+ at the plasmalemma.This high rate and the high electrochemical driving force forCa2+ (about 580 mV)across the plasmalemma at a restingvoltage of about 170 mV, point to a powerful Ca2+ transportsystem which cannot sufficiently be fuelled by ATPhydrolysisbut requires additional energy Key words: Acetabularia, Ca2+selective microelectrode, cytoplasmic free calcium, EGTAbuffer, homeostasis, plasmalemma 相似文献
8.
Takeda Junko; Senda Mitsugi; Ozeki Yoshihiro; Komamine Atsushi 《Plant & cell physiology》1988,29(5):817-824
Membrane potentials of cultured carrot cells in culture mediumwere about 40 mV and did not change with addition ofsalts of addition (or depletion) of 2,4-dichlorophenoxyaceticacid (2,4-D). When the measurement was performed in the testmedium (containing low concentration of salts), the values werewidely distributed (from 60 to 110mV) and changedlargely with external concentration of K+ but not Mg2+ nor Ca2+.When the cells were fractionated by Ficoll density gradientcentrifugation, the membrane potential of the cells of higherdensity (> 14% Ficoll) was about 150 mV in the testmedium and did not change during embryogenesis with depletionof 2,4-D. The membrane potential of cells of lower density (bandingbetween 6 10% Ficoll) was less negative ( 60 to 110 mV) in the test medium. When these cells were transferredand cultured in medium containing zeatin but lacking 2,4-D,the membrane potential was shifted negatively by about 15 mVprior to anthocyanin synthesis. When 2,4-D was added to anthocyanin-synthesizingcells in the medium containing zeatin, a transient hyperpolarizationand subsequent depolarization of the membrane were observedprior to the inhibition of anthocyanin synthesis. (Received October 22, 1987; Accepted April 20, 1988) 相似文献
9.
Suyama Mikita; Ogiwara Atsushi; Nishioka Takaaki; Oda Jun'ichi 《Bioinformatics (Oxford, England)》1993,9(1):9-15
Recently we have constructed a databasethe EnzymeReactionDatabasewhich links a chemical structure to amino acidsequences of enzymes that recognize the chemical structure astheir ligand. The total number of enzymes registered in thedatabase is 1103 with 6668 NBRFPIR entry codes and 1756chemical compounds. The chemical structures and chemical namesfor 842 compounds are registered in the ChemicalStructureDatabase on the MACCS system. For each enzyme, the sequenceswere divided into clusters, and multiply aligned in each clusterto extract a conserved sequence. A total of 158 781 fiveresiduelongfragments were constructed from 433 conserved sequences andcompared among different clusters of different enzymes. Oneof these motifs shared by different enzymes SGGLD.The motif was conserved in both argininosuccinate synthase (EC6.3.4.5
[EC]
) and asparagine synthase (glutaminehydrolysing)(EC 6.3.5.4
[EC]
). This result showed that the database was usefulfor the analysis of the relationship between chemical structuresand amino acid sequence motifs. 相似文献
10.
Spatial variability of soil nitrate nitrogen after potatoes and its change during winter 总被引:4,自引:0,他引:4
Knowledge of the frequency distribution and spatial structure of the soil NO3-N is required to develop an efficient sampling strategy. A 1 ha polder field was sampled after the harvest of potatoes in
October 1987, and in February and April 1988, without being fertilitzed since March 1987. These data sets were examined by
a classical statistical as well as a spatial structure analysis. The October and February data sets were found to be lognormally
distributed, the April data showed a normal frequency distribution. All three data sets had a spatial structure, although
the October data were anisotropic and needed removal of a trend. The spatial variability of soil NO3−N decreased, became isotropic and evolved towards a larger range of spatial dependence during the winter. Knowledge of this
structure permitted to krige or cokrige the data. The number of samples required to estimate the mean NO3−N content with an acceptable precision was found to be 39, 43 and 17 in October, February and April respectively. 相似文献
11.
Phytoplankton size and relations between phytoplankton and microzooplankton(ciliates and heterotrophic dinoflagellates) biomass are analysedin 12 globally distributed areas. In view of the results, ahypothesis is posed where blooming species are those able toescape control by microzooplankton through a combination ofpredation avoidance mechanisms (e.g. larger size, colonies,spines, and toxic compounds) at the beginning of the bloom.Factors that help to enhance subsequent bloom development includepositive feedback from the poor nutritional status of the phototrophicprey which adversely affects predation, inter-microzooplanktongrazing and topdown grazing by mesozooplankton on microzooplankton.Blooming conditions are interpreted as physical or chemicalperturbations disrupting the predatorprey controls thatnormally operate at the level of the microbial loop, openingloopholes into which some phytoplankton speciespopulations can explode. 相似文献
12.
Structural RNA modules, sets of ordered non-Watson-Crick base pairs embedded between Watson-Crick pairs, have central roles as architectural organizers and sites of ligand binding in RNA molecules, and are recurrently observed in RNA families throughout the phylogeny. Here we describe a computational tool, RNA three-dimensional (3D) modules detection, or RMDetect, for identifying known 3D structural modules in single and multiple RNA sequences in the absence of any other information. Currently, four modules can be searched for: G-bulge loop, kink-turn, C-loop and tandem-GA loop. In control test sequences we found all of the known modules with a false discovery rate of 0.23. Scanning through 1,444 publicly available alignments, we identified 21 yet unreported modules and 141 known modules. RMDetect can be used to refine RNA 2D structure, assemble RNA 3D models, and search and annotate structured RNAs in genomic data. 相似文献
13.
J. Scott Turner 《Swarm Intelligence》2011,5(1):19-43
Eusociality has evolved independently at least twice among the insects: among the Hymenoptera (ants and bees), and earlier
among the Isoptera (termites). Studies of swarm intelligence, and by inference, swarm cognition, have focused largely on the
bees and ants, while the termites have been relatively neglected. Yet, termites are among the world’s premier animal architects,
and this betokens a sophisticated swarm intelligence capability. In this article, I review new findings on the workings of
the mound of Macrotermes which clarify how these remarkable structures work, and how they come to be built. Swarm cognition in these termites is in
the form of “extended” cognition, whereby the swarm’s cognitive abilities arise both from interaction amongst the individual
agents within a swarm, and from the interaction of the swarm with the environment, mediated by the mound’s dynamic architecture.
The latter provides large scale “cognitive maps” which enable termite swarms to assess the functional state of their structure
and to guide repair efforts where necessary. The crucial role of the built environment in termite swarm cognition also points
to certain “swarm cognitive disorders”, where swarms can be pushed into anomalous activities by manipulating crucial structural
and functional attributes of the termite system of “extended cognition.” 相似文献
14.
Regulation of Sulphate Transport in a Tropical Legume, Macroptilium atropurpureum, cv. Siratro 总被引:2,自引:0,他引:2
CLARKSON DAVID T.; SMITH FRANK W.; BERG PETER J. VANDEN 《Journal of experimental botany》1983,34(11):1463-1483
When young plants of Macroptilium atropurpureum, cv. Siratrowere deprived of external sulphate (-S plants) growth of shootsand roots continued at rates comparable to those in plants wellsupplied with sulphate (control) for 3 d and 5 d respectively.Dilution of internal sulphur therefore took place and redistributionof sulphur occurred between inorganic and organic forms andbetween roots and younger leaves. Even when S-deficiency limitedgrowth, plants contained 16% of their total sulphur as sulphate,but most of this was retained in old leaves and redistributedslowly to growing zones. The capacity for sulphate uptake increased in roots of Splants very soon after they were deprived of external sulphate;within 24 h the absorption from 0.25 mol m3 SO42was more than five times that of control roots. Maximum increasedcapacity was reached after 23 d stress when the Vmaxof system 1 was 1948 nmol h1g1root fr. wt. inS plants and 337 nmol h1g1root fr. wt.in controls. The Kmfor system 1 did not change significantlywith S-stress being between 58 µM in both setsof plants. Absorption of L-cysteine was not stimulated by S-stress. There was a close, positive relationship between plant growthrate and the rate at which sulphate uptake capacity was enhancedby withholding sulphate from culture solutions. When S plants were replaced in sulphate-containing solutiontheir capacity for SO42 declined to the control levelwithin 24 h. Very marked repression of capacity was also foundwhen S plants were treated with L-cysteine, but therewas no immediate effect with methionine. Roots of this species appear to have a very active system fordegrading L-cysteine to sulphate, 30% of the label in 35S-cysteineabsorbed by roots was recovered in 35SO42 after 20 minor 2 h incubation. By contrast, roots had a very weak abilityto reduce sulphate. When part of the root system was in solution lacking sulphatethere was enhanced uptake of sulphate by other parts which themselveswere amply supplied with sulphate. This is seen as an exampleof compensatory absorption. The response to S-stress is specific and there were no positiveinteractions between S-stress and the absorption of phosphate,or P-stress and the uptake of sulphate. The results are discussed in relation to the close control ofsulphate uptake by internal sulphate concentration, redistributionof forms of sulphur during stress and mobility of sulphate inthe phloem. Key words: Kinetics, Amino-S, Sulpholipid, Repression;, Deficiency 相似文献
15.
Tikhodeev ON 《Zhurnal obshche? biologii》2005,66(1):13-27
The present paper is devoted to the evolutionary role of genetic modules shuffing. The mechanisms capable to produce new molecular functions and significant complications of ontogenesis are reviewed. Two-step model of macroevolution is proposed. This model comprises: (1) Arising of a new combination of genetic modules. This step does not result in formation of a new taxon but makes necessary ground for that process. (2) Precise structure completing of the new combination of modules and corresponding genome optimization by use of various mechanisms including point mutations. This step concerns many genes and finally leads to formation of a new taxon. It is shown that arising of new combinations of genetic modules might work out as molecular basis for progressive evolution, while alternative structural completing of the same combination might result in adaptive radiation. 相似文献
16.
Borate absorption in excised sugarcane leaves 总被引:1,自引:0,他引:1
Borate absorption in sugarcane consists of a rapid and reversibleinflux into the mesophyll cells of the leaf which is completedwithin 20 rains. (Phase I), followed by a slower and irreversibleaccumulatory phase (II). Phase II uptake represents the summationof 3 absorption mechanisms, each dependent upon the externalconcentration. Highly specific mechanisms 1 and 2 transportborate across the initial barrier into the cells, reaction 3carries the borate across the vacuolar membrane. Calcium isshown to be essential for maximum rates of borate absorption.All 3 reactions are inhibited by OH through a combinationof competitive inhibition and irreversible disruption of cellularfunction or structure. Temperature changes over the range of1040 profoundly affect Vmaz and Km1, but have no effecton Km2 and Km3. Reactions 1 and 2 are unaffected by 50 mtl Cl,SO or H2PO4, whereas each of these anionscompetes with H2BO3 for site 3. Specific metabolic inhibitorswere used to delineate a linkage of mechanisms 1 and 2 to respiratoryelectron transport. Mechanism 3 is coupled to oxidative phosphorylation.
1Published with the approval of the Director of the Hawaii AgriculturalExperiment Station as Technical Paper No. 954. 相似文献
17.
Munns, R. 1985. Na+, K+ and Cl in xylem sap flowing toshoots of NaCl-treated barley.J. exp. Bot. 36: 10321042. Na+, Cl and K+ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 510 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na+]x, was only 4.0 mol m3 at external [NaCl] of25150 mol m3, and increased to 7.0 mol m3at 200 mol m3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25150 mol m3 externalNaCl and indicates little change even at 200 mol m-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na+ or Cl. The fluxes of Na+, Cl and K increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K+ increased even when [K+]x wasgreater than external [K+], indicating that membrane transportprocesses modify the K+ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na+, K+, Cl fluxes, salinity, barley 相似文献
18.
The structural organization of biological systems is one of nature’s most fascinating aspects, but its origin and functional role is not yet fully understood. For instance, basic adaptational mechanisms like genetic mutation and Hebbian adaptation seem to be generic and invariant across many species and are, on their own, fairly well investigated and understood. However, it is the organism’s structure – the representations these mechanisms act upon – that bears the complex functional effects of these mechanisms. While typical technical approaches to system design require detailed problem models and suffer from the need to explicitly take care of all possible cases, the organization of biological systems seems to induce inherent adaptability, flexibility and robustness. In this discussion paper we address the concept of structured variability, particularly the role of system structure as implementing a certain representation on which basic variational mechanisms act on. The functional adaptability (or search distribution) depends crucially on this representation. 相似文献
19.
Ricinus communis L. (castor bean) plants were grown in the absence(control) and in the presence of 100molm3NaCl with areciprocal split-root system, in which K+ was supplied to oneand NO3 to the other part of the root system. In theseplants shoot and, to a lesser extent, total root growth wereinhibited compared to plants with non-split roots. Without andwith NaCl, growth of roots receiving NO3 but noK+ (minusK/plus N-roots) was substantially more vigorous thanunder the reverse conditions (plus K/minus N-roots1).100mol m3 NaCl inhibited growth of minus K/plus N-roots1to the same extent as that of non-split roots, indicating thatexternally supplied K+ was not required for root growth undersaline conditions. In growth media without added K+ the rootdepleted the external low K + levels resulting from chemicalsdown to a minimum value Cmln (1.0 to 1.4 mmol m3); inthe presence of 100 mol m3 NaCl, Cmin, was higher (1018mmol m3) and resulted from an initial net loss of K +.Cmin, was pH-dependent The distribution of K+, Na+ and Mg2+along the root was measured. In meristematic root tissues, K+ concentrations were scarcely affected by external K+ or byNaCl, where Na + concentrations were low, but somewhat elevatedat low external K+ and/or high NaCl. In differentiated, vacuolatedtissues K + concentrations were low and Na+ concentrations high,if K + was not supplied externally and/or NaCl was present.The longitudinal distribution of ions within the root was usedto estimate cytoplasmic and vacuolar ion concentrations. Thesedata showed a narrow homoeostasis of cytoplasmic K+ concentrations(100140 mol m3) independent of external K + supplyeven in the presence of 100 mol m 3 NaCl. CytoplasmicNa + concentrations were maintained at remarkably low levels.Hence, external K+ concentrations above Cmin, were not requiredfor maintaining K/Na selectivity, i.e. for controlling Na+ entry.The results are discussed with regard to mechanisms of K/Naselectivity and to the importance of phloem import of K+ forsalt tolerance of roots and for cytoplasmic K+ homoeostasis. Key words: Ricinus communis, nitrate, potassium, root (split-root), salt tolerance, phloem transport 相似文献
20.
Chloride Transport in Chara corallina and the Electrochemical Potential Difference for Hydrogen Ions 总被引:6,自引:0,他引:6
The pH of the cytoplasm of Chara corallina cells has been measuredwith the weak acid 5,5-dimethyloxazolidine-2,4-dione (DM0).Over an external pH range 4·59·5 the resultsfit the regression equation pHcytoplasm=6·28+0·22pHout. Using measured values of the electric potential difference acrossthe plasmalemma we have calculated the electrochemical potentialdifference across this membrane for H+ and Cl. Thesedata are used to test the hypothesis that the inward transportof Cl is coupled to the inthix of H+ or, which comesto the same thing, efflux of OH. One-for-one couplingwill not give net Cl uptake from solutions with pH greaterthan about 7·2, unless the cytoplasmic Cl concentrationis lower than 10 mM, or the pH just outside the membrane islower than that in the bulk solution. It is shown that net Cluptake proceeds from solutions with pH up to 9. The alternative possibility is that Cl transport is broughtabout by co-transport of two H+ for each Cl; this isnot ruled out by the results reported. Such a mechanism mightbe detectable by its electrogenic effect: although such effectshave not been detected, it is shown that they would be smallunder most conditions. Other possible mechanisms are discussed. 相似文献