Rote of Ca2+ in the formation of an excitable membrane on the surface of a protoplasmic droplet of nitella

Dynamic property of membrane formation in a protoplasmic droplet of Nitella

A theory is presented to explain the dynamic characteristics of an electric potential and the resistance of a surface membrane during the formation of a protoplasmic droplet isolated from Nitella. Basic equations are coupled ones for describing ion concentrations near the surface of the droplet, active and passive ion fluxes on the surface, and kinetics of membrane-constituting molecules diffusing from the inside of the protoplasm. The present results give a good explanation of the observed kinetics of electric properties throughout the formative process of surface membranes after the ion concentrations are replaced by lower ones. The results can also explain well the observed data on the steady state. Oscillatory changes in the membrane potential induced by ions strongly adsorbed on the surface membrane are discussed in relation to growth and regeneration phenomena in biological systems such as bean roots and Acetabularia.

Relation of growth process to spatial patterns of electric potential and enzyme activity in bean roots

The electric spatial pattern and invertase activity distribution in growing roots of azuki bean (Phaseolus chrysanthos) have been studied. The electric potential near the surface along the root showed a banding pattern with a spatial period of about 2 cm. It was found that the enzyme activity has a peak around 3-7 mm from the root tip, in good agreement with the position of the first peak of the electric potential, which is located a little behind the elongation zone. An inhomogeneous distribution of ATP content was also detected along the root. Experiments on the electric isolation of the elongation zone from the mature zone and acidification treatment showed that H+ is transported from the mature-side to elongation-side regions, causing tip elongation through an acid-growth mechanism. Both acidification and electric disturbance on growing roots affected growth significantly. Simultaneous measurements of electric potential and enzyme activity clearly showed a good correlation between these two quantities and growth speed. From an analogy with the Characean banding, the spatio-temporal organization via the cell membrane in electric potential and enzyme activity can be regarded as a dissipative structure arising far from equilibrium. These experimental results can be interpreted with a new mechanism that the dissipative structure is formed spontaneously along the whole root, accompanied by energy metabolism, to make H+ flow into the root tip.

Self-sustained oscillations of electric potential in a model membrane

A model membrane constructed from a Millipore filter, whose pores are filled with dioleyl phosphate molecules, exhibits a self-oscillation of the electric potential with a period of about a few seconds in the presence of a salt-concentration difference, pressure difference and/or electric current across the filter. In this paper, the effects of chemicals such as KCl, CaCl2, pH and sucrose on the self-oscillation are investigated experimentally. These chemical substances are shown to alter the characteristic properties as the frequency of oscillation. Theoretical consideration of electrochemical interaction between these substances and DOPH molecules gives a fairly good explanation of the observed results.