Effect of Temperature and External pH on the Cytoplasmic pH ofChara corallina

Abstract
Cytoplasmic pH (pHc) in Chara corallina was measured (from [14C]stribution) as a function of external pH (pH0)and temperature. With pH0 near 7, pHc at 25°C is 7.80; pHcincreases by 0.005 pH units °C−1 temperature decrease, i.e. pHc at 5 °C is 7.90. With pH° near 5.5, the increase in pHc with decreasing temperature is 0.015 units °C−1 between 25 and 15°C, but 0.005 units °C−1 between 15 and 5°C. This implies a more precise regulation of pHc with variations in pHo at 5 or 15 °C compared with 25 °C. The observed dp Hc/dT is generally smaller than the −0.017 units °C−1 needed to maintain a constant H+/OH−1, or a constant fractional ionization of histidine in protein, with variation in temperature. It is closer to that needed to maintain the fractional ionization of phosphorylated compounds or of CO2−HCO3 The value of dpHc/dT has important implications for several regulatory aspects of cell metabolism. These include (all as a function of temperature) the rates of enzyme reactions, the Δμ¯H+ at the plasmalemma (and hence the energy available for cotransport processes), and the mechanism for pHc regulation by the control of bidirectional H+ fluxes at the plasmalemma.

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