Bicarbonate assimilation by fresh-water charophytes and higher plants: I. Membrane transport of bicarbonate ions is not proven

Summary

Although it is generally believed thatChara and some fresh-water angiosperms transport bicarbonate ions inwards across their plasma membranes, there has been no direct demonstration of such transport in these plants. The (indirect) arguments for their transporting HCO ⁻₃ are arguments against the inward diffusion of CO₂ at the observed rates. They rest on calculations of the equilibrium concentration of CO₂ or of the maximum rate at which CO₂ might be produced from HCO ⁻₃ at the pH of the medium outside the cells. SinceChara acidifies the medium over about half the cell surface during C assimilation, these calculations have been based on questionable premises.

We propose a model forChara in which the acidification is attributed to active efflux of H⁺, and we calculate that both the equilibrium concentration of CO₂ and its rate of production outside the cell can be high enough to support the observed rates of C assimilation, without postulating transport of the species HCO ⁻₃ or H₂CO₃.

Calculations are presented also for alternative models in which there is membrane transport of HCO ⁻₃ . The first includes symport of H⁺ with HCO ⁻₃ , again dependent on active H⁺ efflux. In the second, there is active electrogenic transport of HCO ⁻₃ . In this case the low pH in the medium outside the cell is caused by the dissociation of H₂CO₃ produced by hydration of CO₂ which leaks from the cell cytoplasm.

All three models are consistent with the observations to date, but the first is more economical of postulates. It can also explain the apparent transport of HCO ⁻₃ by fresh-water angiosperms such asEgeria.