Removal of Ca2+ from the medium results in depolarization of the Chara internodal cell and an increase in membrane conductance (Gm). The increase in conductance is associated with an increase in K+ conductance, as judged by Ca2+ effects on the K+ dependence of clamp current. The voltage dependence of Gm is also affected by Ca2+, as is the time course of the response of clamp current to a step change in voltage. Mg2+ restores the low conductance and the fast response to a voltage change, but not hyperpolarization at neutral pH, suggesting that there is an additional, independent effect on the electrogenic pump. The membrane does not show the normal ability to increase proton conductance at high pH in the absence of Ca2+; this is also restored by Mg2+ as well as by Ca2+.
Charasomes are structured elaborations of plasmalemma of some Charophytes which may be involved in enhanced transport of ions or inorganic carbon across the plasmalemma. They show a highly non-uniform distribution. We report here a novel method of quantifying charasome population density, using scanning electron microscopy of cell fragments removed from the cell wall. Consistent differences in density can occur in neighbouring regions of the membrane. The distribution of densities is not normal, but is skewed toward lower densities, with large areas of membrane showing no charasomes. Contrary to previous published reports, we found no correlation with acid and alkaline banding patterns. Inhibition of photosynthesis by either dark or DCMU (dichlorophenyl dimethyl urea, an inhibitor of photosystem II) results in a complete loss of charasomes in 1-3 weeks.
Charasomes are complex structures found in the plasmalemma of the internodal and branchlet cells of the algal genus Chara. They are lost when internodes are stored in the dark for prolonged periods. Prolonged darkness results in the loss of the ability of internodes to form spatially separated bands of low and high pH. Rates for the uptake of inorganic carbon by the internodal cells are also diminished. The ability to band is re-established 8 h after the reintroduction of a light regime. Rates for the uptake of inorganic carbon begin to increase from 24 h. Charasomes reform over the period of 1-15 days after light, beginning as a simple cytoplasmic evagination and progressing toward a typical complex morphology. The re-establishment of charasomes is correlated with an increase in the uptake of inorganic carbon from bicarbonate-containing solutions.
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