, Volume 172, Issue 1, pp 53-59

First online:

Implications for cytoplasmic pH, protonmotice force, and amino-acid transport across the plasmalemma of Riccia fluitans

  • Eva JohannesAffiliated withBotanisches Institut I, Justus Liebig Universität
  • , Hubert FelleAffiliated withBotanisches Institut I, Justus Liebig Universität

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By means of pH-sensitive microelectrodes, cytoplasmic pH has been monitored continuously during amino-acid transport across the plasmalemma of Riccia fluitans rhizoid cells under various experimental conditions. (i) Contrary to the general assumption that import of amino acids (or hexoses) together with protons should lead to cytoplasmic acidification, an alkalinization of 0.1–0.3 pHc units was found for all amino acids tested. Similar alkalinizations were recorded in the presence of hexoses and methylamine. No alkalinization occurred when the substrates were added in the depolarized state or in the presence of cyanide, where the electrogenic H+-pump is inhibited. (ii) After acidification of the cytoplasm by means of various concentrations of acetic acid, amino-acid transport is massively altered, although the protonmotive force remained essentially constant. It is suggested that H+-cotransport is energetically interconnected with the proton-export pump which is stimulated by the amino-acid-induced depolarization, thus causing proton depletion of the cytoplasm. It is concluded that, in order to investigate H+-dependent cotransport processes, the cytoplasmic pH must be measured and be under continuous experimental control; secondly, neither ΔpH nor the protonmotive force across a membrane are reliable quantities for analysing a proton-dependent process.

Key words

Amino acid transport Cytoplasmic pH Protonmotive force Riccia