Karel Janáček
Czechoslovak Academy of Sciences
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Ceskoslovenská fysiologie / Ústrední ústav biologický | 1975
A. Kotyk; Karel Janáček
The cell membrane forms a boundary that is essential for life. This boundary supports life by controlling the movement of molecules and substances across it. Some molecules and substances are transported across the cell membrane by processes that require the expenditure of energy. However, many molecules cross the cell membrane by passive processes that do not require an expenditure of energy. In this laboratory we will demonstrate two passive processes, diffusion and osmosis.
Biochimica et Biophysica Acta | 1971
Karel Sigler; Karel Janáček
Abstract The time course of oocyte volume in mannitol hypertonic media is characterized by a biphasic curve; shrinkage of the cells leads to a volume minimum and this is followed by a slower swelling. Behaviour of this type may be explained by a mathematical model in which the inflow of mannitol is followed by osmotically driven uptake of water. The permeability of the oocyte surface towards mannitol appears to increase with increasing the mannitol concentration in the medium. In hypotonic saline the time course of the oocyte volume is again biphasic; from chemical analysis it may be concluded that the initial swelling is followed by a slower shrinkage mostly due to a loss of osmotically active potassium chloride from the cells. This process may be described by a model taking into account the non-zero hydrostatic pressure inside the oocytes.
Zeitschrift für Pflanzenphysiologie | 1972
Renata Rybová; Karel Janáček; Marie Slavíková
Summary Flows of sodium, potassium and chloride ions across the plasmalemma of the alga Hydrodictyon reticulatum were studied and their light dependence examined. Both the inflows and the outflows of the ions were found to be light-dependent in the sense of being considerably reduced in the dark. In the case of potassium the two flows appear to be influenced approximately to the same degree, since the potassium content is not significantly changed in the dark. It seems that a special mechanism is responsible for changes of the content of sodium, the highly significant decrease of which during prolonged dark periods may be shown to be due to a release of sodium bound by intracellular structures. Most of the intracellular sodium is found to be in a non-exchangeable form under normal day-and-night pattern of illumination and part of this sodium fraction is released reversibly during 48 hours in the dark, irreversibly after still more prolonged dark periods. The outflow of chloride ions in the dark seems to be less reduced than the inflow, for a significantly lower content of chloride ions was found in the dark. Coupling of individual flows with photosynthetic processes was examined with specific inhibitors. Chloride flows were found to be practically abolished with DCMU, an inhibitor of the second photosynthetic system; the flows of sodium and potassium, on the other hand, were markedly reduced with CCCP, an inhibitor of photophosphorylation. The most important conclusion drawn from these experiments seems to be that not only the unidirectional flows proceeding against a gradient of electrochemical potential are inhibited by the inhibitors, the flows in the opposite direction and in the direction of simple physico-chemical forces being inhibited in the same sense and often to the same degree. Hence the passive permeability of the plasmalemma of Hydrodictyon reticulatum appears to be very low, the flows of the ions taking place almost exclusively through metabolically-coupled active pumps in both directions. When these mechanisms are impaired by inhibition, both net flows and tracer exchange are almost completely prevented, the plasmalemma becoming virtually impermeable. In addition to that, the sodium transport in Hydrodictyon reticulatum is once more found to be insensitive to ouabain like in some of the algal cells and unlike in those of other algae.
Archive | 1977
A. Kotyk; Karel Janáček
The whole field of water transport in cells and tissues was recently reviewed in an extensive and scholarly manner by House (1974). The content of the present chapter is much more modest being mostly limited to the phenomenological description of water transport across membranes by the methods of steady-state thermodynamics (see p. 172), as developed by Kedem and Katchalsky (1958).
Pflügers Archiv: European Journal of Physiology | 1970
Karel Janáček; Renata Rybová
SummaryIn a frog bladder preparation with the mucosal surface covered with liquid paraffin the content of sodium chloride and of water in the epithelial cells is reduced, while the polarity and the magnitude of the membrane potential across the non-mucosal parts of the membranes are preserved. The transport of ions between the cells and their surroundings appears to be mediated solely by the non-mucosal membranes of the epithelial cells, and for this reason the tissue in this condition was called a nonpolarized preparation. Oxytocin (50 mU/ml) brings about an extrusion of sodium and potassium chlorides and of water from the epithelial cells of the nonpolarized preparation. Since sodium ions leave the cells against their electrochemical potential gradient, a conclusion about a direct stimulation of the sodium pump by oxytocin (possibly due to a reduction of its internal resistance) appears to be warranted.
Biochimica et Biophysica Acta | 1962
Karel Janáček
Abstract The effect of some thiol-group-blocking agents in the solution bathing the outside of isolated frog skin was investigated by a simple method permitting of measuring two separate membrane potentials in the skin and of membrane d.c. resistances to individual ions. It was found that, while higher concentrations of these agents bring about a drop in the membrane potentials and permeability changes, similarly to mercuric chloride, lower concentrations, after a short period of incubation of the skin, cause considerable hyperpolarization of the outer membrane. The hyperpolarization is explained as a result of increased permselectivity of the outer membrane in Ussings model of frog skin. The conditions of the reaction involved are described and a crude model of the outer membrane is suggested.
Biochimica et Biophysica Acta | 1971
Karel Sigler; Karel Janáček
Abstract The intracellular potential of the oocytes of Rana temporaria and Rana esculenta was found to be reduced when the osmolarity of the external medium was increased by various concentrations of mannitol. A model is described in which the permeability coefficients for Na + and K + were calculated from the rates of isotopic exchange by Goldman-Hodgkin-Katz approximations and the phenomenon may be explained by a dependence of the two coefficients and of the intracellular concentrations of the two ions on osmolarity. The state of Na + , K + and Cl − inside the cells as well as the nature and magnitude of the active transport processes across the surface of the oocyte are discussed.
Archive | 1970
A. Kotyk; Karel Janáček
The experimental approaches applied in volume flow measurements (the theoretical implications of these are described in section 4.2.) are summarized in the following scheme: 13.1. Volume flow between cell and its surroundings 13.1.1. Determination of volume flow from cell weight changes 13.1.2. Estimation of cell volume a) direct measurement of cell diameter or thickness b) measurement of the volume of cells packed by centrifugation c) optical methods 13.2. Transcellular volume flow and flow across cell layers 13.2.1. Determination based on the change of concentration of an impermeant substance 13.2.2. Determination based on weighing 13.2.3. Determination based on direct volume measurement a) apparatuses with calibrated vessels and capillaries b) automatic maintenance of volume in one compartment
Journal of Plant Physiology | 1993
Ludmila Nešpͧrková; Jana Lazarová; Karel Janáček; Renata Rybová
Summary The artificial electron acceptors, potassium ferricyanide and tetrahiafulvalene cation radical, are shown to inhibit uptakes of chloride, nitrate and sulphate anions in the green fresh-water alga Hydrodictyon reticulatum . The same is true about the phosphate anion in an alkaline medium, where its divalent form prevails. The unidirectional efflux of these anions was not inhibited. TTF + , the more potent of the two inhibitors, appears to be partly located in the plasma membrane. An electron-anion antiport (a redox pump) is suggested as the mechanism of the active uptake of most of the physiologically important anions in the alga. Notable exceptions to this rule are represented by bicarbonate, the carbon source for photosynthesis at air levels of CO 2 , and by phosphate (probably univalent) in a medium buffered to pH6.
Biologia Plantarum | 1988
Renata Rybová; Ludmila Nešpůrková; Karel Janáček
Sulphate ion is accumulated in the cells of the algaHydrodidyon reticulatum to a concentration approximately 30 mmol 1-1. The rate of SO42- uptake is increased markedly in the light, however, the effect is observable only after the cells have been kept in the light for about one hour. Uptake of sulphate is strongly inhibited by phosphorylation uncouplers and also by DIDS. The two-phase sulphate uptake kinetics, reported earlier was confirmed in experiments in which competition with phosphate anion was tested. Phosphate competes with sulphate only in the range of higher substrate concentrations (from 0.2 mmol 1-1) and does not affect the system which works at low substrate concentrations. Efflux of SO42- is very slow; the reason of the flux hindrance is not yet known.