Nitella membranes

Equation 3.6, the Nernst equation, is an equilibrium statement showing how the internal and the external activities of ionic species / are related to the electrical potential difference across a membrane (Fig. 3-2). At equilibrium, a 10-fold difference in the activity of a monovalent ion across some membrane is energetically equivalent to and can balance a 59-mV difference in electrical potential (at 25°C). Hence, a relatively small electrical potential difference can energetically balance a large difference in activity or concentration across a membrane. For instance, if the external activity were 1% of the internal activity (aj/ax- = 0.01), the Nernst potential would be -118 mV for K+ and +118 mV for Cl- (Fig. 3-2). For some calculations, y° /y) is set equal to 1 (a less stringent assumption than setting both y° and yj equal to 1). Under this condition, a°fct- in Equation 3.6 becomes the ratio of the concentrations, c°/cj (a = yff Eq. 2.5). Such a substitution may be justified when the ionic strengths on the two sides of a membrane are approximately the same, but it can lead to errors when the outside solution is much more dilute than the internal one, as occurs for Chara or Nitella in pond water. 

As a specific example, we will use the Goldman equation to evaluate the membrane potential across the plasma membrane of Nitella translucens. The concentrations of K+, Na+, and Cl- in the external bathing solution and in its... 

Figure 3-13. For the steady-state condition in the light, the three active fluxes across the plasma membrane of the large internodal cells of Nitella translucens are balanced by net passive K+ and Cl-effluxes and a net passive Na+ influx (see Fig. 3-7).

By Equation 3.27a, the minimum energy required for actively transporting or pumping Na+ out across the plasma membrane of the Nitella cell is... 

Clearly, from Fig. 1, the solubility of a solute in an organic solvent correlates very well with the permeability of the Nitella membrane for that solute. But it is also clear that the correlation is only partial. Thus, of two solutes with the same partition coefficient the one with smaller molecular weight would seem to permeate faster. Solute size as well as hpid solubility are both important determinants of permeation rate. The particular solvent chosen, olive oil, seems however to be a very good model for the ability of the membrane barrier to discriminate between the various permeants, since the overall increase in permeability as the structure of the permeant is varied correlates closely with the increase in partition coefficient. Were the two parameters to be strictly linked all the data would fall on the line of unit slope in the figure, the line of identity. Later we shall see cases where the data do not support such a close similarity between certain membranes and model solvents. 

Fig. 1. Permeability of Nitella cell membranes to non-electrolytes in relation to the olive oil solubility of these solutes. On the ordinate the logarithm of the permeability (in units of 10 cm/sec) on the abscissa, the logarithm of the olive oil/water partition coefficient of the permeant. Data measured at 20°C. Crosses molecular weight up to 50 open circles molecular weights from 60 to 119 filled circles molecular weights above 120. Data taken from Collander [3]. The straight line is of unit slope.

Fig. 3. Calculated relative intramembrane diffusion coefficients across the Nitella cell membrane as a function of molecular weight of the permeant. Ordinate logarithm of the ratio of the permeability coefficient (in 10 cm/s) to the olive oil/water partition coefficient for the permeants of Fig. 1. Abscissa logarithm of their molecular weights. The solid straight line is the Unear regression of log(P/K) on log M with slope of — 1.22. The dashed lines are at a distance of one standard deviation away from the regression line.

U. Ueda, T. and Kobatake, Y. Hydrophobicity of biosurfaces as shown by chemoreceptive thresholds in Tetrahvmena, Physarum and Nitella. J. Membrane Biol., 1977, 3, 351-368. 

T. Takenaka, I. Inoue, Y. Ishima, and H. Horie, Excitability of Surface Membrane of Protoplasmic Drop Produced from Protoplasm in Nitella, Proc. Jpn. Acad. 41, 554-557 (1971). 

Working on Nitella clavata, Kitasato put forward the hypothesis that a large passive influx of protons had to be correlated to an electrogenic efflux of protons. Therefore, he introduced Ch+ and Ch+, respectively, in the numerator and denominator of Equation (2), both terms being affected by a permeability factor Ph+ of high value. He assumed further that the electrogenic pump works as a current source, i.e., as though it has infinite impedance, and he described the membrane potential by the equation... 

Figure 12 displays the response of the membrane potential to inward current pulses of different magnitudes. At low current density a purely passive response is observed. At 0.25 A and above, a spontaneous increase in voltage occurs which is followed by a slight voltage decrease in which large fluctuations can be observed. These results are similar to those observed on intact cells of Chara corallina , Nitella and on... 

The electrical potential difference across the membrane can be measured by placing appropriate electrodes on its two sides. To measure the potential across a cell membrane one electrode must be within the protoplast and preferably we should be able to ascertain whether it is in the vacuole or in the cytoplasm (since in one case it will be separated from the electrode in the external solution by tono-plast and plasmalemma and in the other only by the plasmalemma). To undertake such measurements very fine microelectrodes are needed electrodes have been used, the glass tips of which have a diameter of ca. 1 micron (/tm). Even with such electrodes large vacuolated cells rich in cytoplasm are needed, such large cells are also required to obtain adequate samples of vacuolar sap and of cytoplasm for accurate analysis of all their major ions, and for measurements of their ion fluxes. To meet these requirements experimenters have turned to the giant multinucleate cells of certain algae, notably species of Nitella, Hydrodictyon and Chora. 

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