| Abstract: | The permeability (P) of a lipophilic cation, triphenylmethylphosphonium(TPMP+) which is frequently used as a membrane potential probe,has been measured in Chara australis (Charophyceae). PTPMP+across biological membranes is usually thought to be very highbut this is not the case across the plasmalemma of Chara. Thepermeability of TPMP+ across the plasmalemma was found to betypical of inorganic cations, about 1.0 nm s–1. Estimateswere made of the permeability of lipophilic cations across someother cell membranes, based on previously published work. Thepermeability of TPMP+ across the plasma membranes of the redalga, Griffithsia monilis and the blue-green alga, Anabaenavariabilis was about 2–5 nm s–1. The permeabilityof TPMP+ across the plasma membranes of eukaryotes and prokaryotesappears to be similar. The permeability of lipophilic cationsacross the cristae of isolated mitochondria are exceptionallyhigh, about 170 nm s–1. TPMP+ did not behave as a thiamineanalogue in Chara, unlike in the case of yeast. The means ofentry of TPMP+ into the Chara cell, driven by the electrochemicalgradient across the plasmalemma, has not been identified. Thepresence of a second lipophilic cation probe, DDA+ (dibenzyldimethylammonium),caused a decrease in the uptake flux of TPMP+; this suggeststhat the two lipophilic cations compete for the same site atthe surface of the plasmalemma. Key words: Chara australis, TPMP+, Permeability, Lipophilic cation |
