1,721,023 research outputs found
Measurements of electrical parameters of frog skin "in situ" as a function of environmental parameters
No full textElectrical parameters of the abdominal skin of the pithed frog (Rana esculenta) can be measured by means of a couple of double coaxial electrodes /1,2,3/. One of the double electrodes is inserted into the ventral lymphatic sac of the frog, between skin and muscles, and the second is placed in front of this, on the outer surface of the skin. The inner electrode of the coaxial pair measures the skin potential difference (pd), while the outer delivers a countercurrent, for modifying the pd. By this method, pd, short circuit current and skin DC resistance have been determined as a function of temperature (5 to 40 °C) and of pH (4 to 9) on the same living animal. The behaviour of the substrate bears both qualitative and quantitative similarities with the isolated skin in the lower temperature range, but no transport maximum exists around 27 °C /4/. The pH dependence of electrical parameters is also quite different than in the isolated substrate /5/. 1. G. Torelli, F. Celentano, G. Cortili, G. Guella: Boll. Soc. It. Biol. Sper. 44, 501 (1967); 2. F. Celentano, G. Cortili, G. Guella, G. Torelli: Boll. Soc. lt. Biol. Sper. 44, 504 (1967); 3. M. Bianchi, G. Torelli, F. Celentano, G. Cortili: Boll. Soc. It. Biol. Sper. 45, 385 (1968); 4. G.A. Poster: Biochim. Biophys. Acta 211, 487 (1970); 5. E. Schoffeniels: Arch. Int. Physiol. Biochim. 53, 513 (1955)
Coefficienti fenomenologici per il trasporto di acqua e soluti in sistemi a n membrane in serie
No full textLe relazioni flusso–forza nelle membrane biologiche risultano sperimentalmente non lineari. Per descrivere tale comportamento è stato proposto (1) di impiegare le equazioni lineari "pratiche" di Kedem e Katchalsky scritte in forma locale e integrate attraverso lo spessore di un sistema di due membrane in serie. Si ottengono equazioni fenomenologiche non lineari per il trasporto di acqua e soluti, che richiedono la ridefinizione come derivate, anziché rapporti semplici, dei coefficienti di permeabilità idraulica, di flusso osmotico e di riflessione. I risultati già ottenuti per due membrane (2) sono stati ora estesi a un sistema di n membrane, nel quale sono assimilabili formalmente a una membrana non selettiva anche gli strati limite. Per la membrana i-esima si scrive la concentrazione nel compartimento di destra i+1 in funzione di quella nel compartimento di sinistra i, del flusso volumetrico Jv e di quello di soluto Js. Mediante sostituzione ricorsiva delle concentrazioni si ottiene la concentrazione nel compartimento estremo di destra n+1 in funzione di quella nel compartimento estremo di sinistra 1. Di qui si ottiene Js in funzione di Jv, di C = C(n+1) - C(1) e di C(1) stessa. Si segue un procedimento analogo per le pressioni nei vari compartimenti e si ottiene la relazione che fornisce Jv. Da quest’ultima si ricava che: 1) il coefficiente di filtrazione Lp non è solo l’inverso della somma degli inversi dei singoli coefficienti Lpi relativi alle singole membrane ma contiene pure un termine additivo non lineare in C(1); 2) anche il coefficiente di flusso osmotico Lpd contiene un termine in C(1); 3) il rapporto -Lpd/Lp è indipendente da C(1) e, come nella teoria classica lineare, fornisce un coefficiente di riflessione per il sistema che risulta pari alla media dei coefficienti di riflessione delle n membrane pesati sull’inverso dei rispettivi coefficienti di permeabilità; 4) il medesimo risultato si ottiene facendo il limite per Jv tendente a zero del rapporto tra le differenze di pressione osmotica e idrostatica, confermando che si tratta effettivamente di un coefficiente di riflessione; 5) la legge di Darcy risulta una legge limite valida solo per il solvente puro, C(1) = 0. (1) C. S. Patlak, D. A. Goldstein, J. F. Hoffman: J. Theor. Biol. 5, 426-442 (1963) (2) G. Monticelli, F. Celentano: Further Properties of the Two-Membrane Model, Bull. Math. Biol, in stampa (1983)
Phenomenological description of selectivity in actively transporting membranes
No full textA phenomenological description of active and passive flows of solute and solvent across a biological membrane can be made explicitly considering the dependence of matter flows upon the rate of metabolic reactions /1/, or introducing a generalized chemical potential including a term accounting for active transport /2/, or making the hypothesis that solute flow can be splitten in two superimposed and thermodynamically couplet active and passive components. With the two latter approaches, by means of a transformation of flows and forces at constant temperature and in absence of electric field, two systems of three interacting flows, sustained by three different forces, can be obtained. The two systems lead to equivalent descriptions of volumetric flow and allow the determination of the reflection coefficient for solute passive transport /3/. The relationship between reflection coefficient and apparent reflection coefficient /4/ is also obtained. 1. A. Katchalsky, P. F. Curran. Nonequilbrium Thermodynamics in Biophysics, Cambridge Mass. (1965); 2. J. M. Diamond. J. Physiol. 161, 503 (1962); 3. F. Celentano, G. Monticelli, G. Torelli. Proc. Ist. Europ. Biophys. Congr. 3, 309 (1971); 4. C. J. Bentzel, M. Davies, W. N. Scott, M. Zatzman, A. K. Solomon. J. Gen. Physiol. 51, 517 (1968
Un confronto tra alcune descrizioni termodinamiche del trasporto attivo
No full textLa descrizione fenomenologica del trasporto attivo di materia attraverso membrane biologiche è stata affrontata da diversi Autori con i metodi della termodinamica dei fenomeni irreversibili lineari. In tale trattazione un importante problema preliminare è costituito dalla determinazione della forza che sostiene tale trasporto anche contro un gradiente di potenziale elettrochimico. Kedem ha proposto l’accoppiamento del trasporto attivo con una reazione metabolica, la cui affinità è la forza direttamente responsabile del trasporto stesso. Diamond ha proposto invece una generalizzazione del potenziale chimico del soluto trasportato, aggiungendovi un nuovo termine, detto potenziale di trasporto attivo. Quest’ultima proposta, pur superata dagli sviluppi della precedente, presenta comunque qualche caratteristica interessante. Un’altra possibilità consiste nel considerare le componenti attiva e passiva del flusso di soluto come due flussi indipendenti ed interagenti tra di loro. Scopo della presente comunicazione è l’impostazione di un primo confronto fra le tre proposte succitate, limitatamente alla descrizione della selettività di una membrana
A comparison between linear and non linear thermodynamic models of membrane water and solute transport
No full textThe linear thermodynamic description of membrane transport, although successfully employed for small flows, has some known limitations. Patlak, Goldstein and Hoffman suggested that the linear practical equations by Kedem and Katchalsky may be written as a function of the local forces driving-water and solute flows and integrated across the membrane thickness.
By such a procedure the resulting solute flow equation, non linearly related to the driving solute concentration difference is formally identical to the one obtained by Bresler and Wendt from the Hertzian continuity equation.
The same procedure may be used for studying series arrays of membranes, which are necessary for the coupling of water flow to a chemical reaction driven solute flow. It can be shown that, in particular flow conditions, the results coincide with those obtained by means of linear thermodynamic network models or with a different system structure. The behaviour of the inner compartment concentration is analogous to the one obtained by a non linear thermodynamic network model. The locally linear model thus appears to be an extension of other linear models
Further properties of the two-membrane model
No full textThe properties of nonlinear equations describing the solute and solvent transport across a simplified Patlak-Goldstein-Hoffman model (two membranes in series without unstirred layers) are investigated both analytically and numerically. The analysis shows that the principal coefficients measured in transport experiments in the presence of active transport are dependent on the experimental conditions. These “apparent” system parameters are extensions of the corresponding parameters determined both in passive systems and in the linear Kedem-Katchalsky theory. Moreover, they are related to the local phenomenological coefficients of the single membranes of the array. Several relationships between measurable quantities and the local system parameters are indicated, allowing the planning of experiments aimed at the measurement of the latter. Data in the literature have been used to check the proposed volume flow equation
Trasporto di acqua e soluti attraverso membrane in serie
No full textSono state ottenute le equazioni non lineari che legano alle rispettive driving forces il trasporto di acqua e di un soluto non elettrolita attraverso (2÷5) membrane in serie. Tali equazioni vengono ottenute integrando lungo gli spessori delle membrane le equazioni lineari pratiche di Kedem e Katchalsky, scritte in forma locale e tenendo conto della possibilità di un trasporto accoppiato a reazioni chimiche che avvengono in una delle membrane. Viene discussa la validità dell'ipotesi di Patlak-Goldstein-Hoffman sull'esistenza di equazioni localmente lineari e vengono presentati un algoritmo per la costruzione delle equazioni per più membrane in serie e una procedura sperimentale per la determinazione dei cinque parametri che caratterizzano il sisterna a due membrane
A theoretical model for non linear osmosis in biological membranes
No full textSince the first studies on the osmotic properties of the cell membrane it is known that the water osmotic flow is often characterized by a non linear dependence upon its driving force. To date such a behaviour has been observed in a number of biological membranes, i.e. leucocytes, the alga Chara, erythrocytes, intestine, urinary bladder, the alga Nitella and gall bladder. Three main hypothesis have been proposed to explain such a non linearity. First the existence of unstirred layers between the membrane and the bulk solution may lower the effective osmotic gradient. This hypothesis accounts for part of the non linearity observed in Chara. Subsequently it has been shown that, assuming the validity of local linear phenomenological equations, a non linear flow – force relationship can be obtained by integration along the membrane thickness. Such a treatment, devised for obtaining the force – flow relationship for an asymmetrical series array of membranes, contains the former hypothesis as a particular case. In effect un unstirred layer may be considered as a series membrane with zero selectivity but with a definite permeability. Finally various Authors suggested that the water flow itself, or some concentration dependent solute – membrane interaction modify the membrane structure and thus the resistance to water flow. At present it seems likely that all these phenomena contribute to the observed non linearity in complex membranes. In effect it has been proved that the outer and inner unstirred layers influence the measured permeability and that the volume of the intercellular spaces is related to the functional state of various epithelia. The very fact that the unstirred layers contribute to the non linearity suggests that indeed an asymmetrical series array of membranes, as in epithelia, may be in part responsible of the observed non linear behaviour, but it seems experimentally difficult to obtain a direct measurement of this contribution to the overall non linearity. For these reasons we conducted a further study of the two membrane model introduced by Curran following the method proposed by Patlak, Goldstein and Hoffman, with the aim of individuating a) the relationships between the parameters characterizing the single membranes and the overall properties of the array, b) the parameters who influence more effectively the non linear behaviour and c) the flow dependence of the inner compartment characteristics which may contribute to the observed structural changes
Non linear force-flow relationship in series arrays of membranes
No full textA system composed by two membranes in series has been studied. One of them consists of two kinds of parallel elements: the first accounting for solute active transport which does not interact with solvent or solute passive transport which take place in the other element. The linear phenomenological equations relating volume and solute flows across each membrane to the driving forces have been integrated at steady state. Volume flow across the composite membrane is not linearly related to the driving forces. This expression accounts for non linear volume flow in kidney and gall bladder
Considerations on different thermodynamic models for mass transport across membranes
No full textThe linear Kedem-Katchalsky phenomenological and practical equations are examined in order to show their different information content and the usefulness of the latter as a starting point for a non-linear treatment. A simplified analytical treatment of a series array of membranes, following the Patlak-Goldstein-Hoffman method, is subsequently compared with the Kedem-Katchalsky treatment and with the pseudo-electrical network approach of Mikulecky, showing that the analytical and network treatments give the same results in several instances. A comparison with the models of epithelial transport proposed by Weinstein and Stephenson indicates that the series array model includes, as a particular case, the results obtained by means of the standing gradient osmotic flow model
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