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  • 1
    Electronic Resource
    Electronic Resource
    The uptake and metabolism of urea by Chara australis: IV. Symport with sodium—A slip model for the high and low affinity systems (1993)
    Springer
    The journal of membrane biology 136 (1993), S. 263-271 
    Details
    ISSN: 1432-1424
    Keywords: Sodium ; Urea ; Symport ; Cotransport ; Charophytes ; Slip model
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Abstract We have previously investigated the electrogenic influx of urea in Chara, and the urea- and sodium-dependent membrane current. We have shown that there is a sodium-stimulated component of urea influx and a urea-stimulated component of sodium influx, and that these are of the same size. We conclude that the electrogenic inward transport of urea, and of its analogues acetamide and acrylamide, is by sodium symport, with a stoichiometric ratio of 1∶1. The kinetics of the fluxes and currents show two different K M values for sodium in different cells and two different kinds of kinetics for the effect of urea on membrane current, one of which fits the Michaelis-Menten equation, while the other shows a maximum and fits the difference of two Michaelis-Menten terms, suggesting a phenomenon like cis-inhibition. Similarities in kinetic characteristics between the inhibitory site and the electrically silent uptake site (System II) lead us to suggest that the same protein may be responsible for both the low-K M, electrogenic influx of urea (System I) and the high-K M, electrically silent influx by System II. We suggest a “slip” model for urea uptake in Chara.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00233665
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  • 2
    Electronic Resource
    Electronic Resource
    Proton/chloride cotransport in Chara: mechanism of enhanced influx after rapid external acidification (1985)
    Springer
    Planta 163 (1985), S. 411-418 
    Details
    ISSN: 1432-2048
    Keywords: Chara ; Choride influx ; Cotransport ; pH jump ; Proton motive force
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Rapid lowering of the external pH (“pH jump”) enhances Cl− influx in Chara. Experiments were conducted to distinguish between two factors which have previously been proposed to mediate in the response: raised cytoplasmic pH and lowered cytoplasmic Cl− concentration. It is concluded that the latter alternative is more likely because: i) Cl− influx is reduced at high external pH; ii) influx following the pH jump is never greater than that following pretreatment in Cl−-free solution, which reduces cytoplasmic Cl− concentration (“Cl− starvation”); iii) the joint application of pH jump and Cl− starvation does not result in a greater Cl− influx than does Cl− starvation alone; and iv) addition of NH 4 + , which increases cytoplasmic pH, does generate an additional stimulation of Cl− influx following a pH jump. It is suggested that the increased cytoplasmic pH at the end of pretreatment at high external pH decays rapidly during the pH jump, and thus any effect on Cl− influx is so transient as to be undetectable by the methods used. The results are discussed in terms of a reaction kinetic model for 2H+/Cl− cotransport (Sanders, D. and Hansen, U.-P, 1981, J. Member. Biol. 58, 139–153) which describes quantitatively; i) the effects of NH 4 + on Cl− influx in terms involving only a change in cytoplasmic pH; and ii) the combined effects of Cl− starvation and NH 4 + in terms involving only changes in Cl− concentration and cytoplasmic pH. Conversely, the combined effects of Cl− starvation and pH jump cannot be described by the model if the effect of the pH jump is the consequence of increased cytoplasmic pH. The simple interpretation of experiments on whole cells involving manipulation of $$\Delta \bar \mu _{{\text{H}}^ + } $$ (the electrochemical potential difference for protons across the plasma membrane) is questioned in the light of these and previous findings that secondary factors can determine the response of Cl− transport in Chara.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00395151
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    Paper (German National Licenses)
    Fulltext
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