1,720,969 research outputs found
Relazioni flusso-forza non lineari in un modello di epitelio con trasporto attivo di soluto
No full textL'esperienza mostra che le relazioni fra i flussi di materia attraverso membrane singole ed epiteli e le forze generalizzate che li sostengono sono non lineari. Un ragionevole modello di epitelio è costituito da un sistema di più membrane semplici in serie. Tale modello consente di spiegare l'accoppiamento fra il trasporto "attivo" (a sua volta accoppiato a reazioni chimiche) di soluto e il trasporto d'acqua. Lo studio teorico del modello e la simulazione al calcolatore di tale sistema hanno mostrato che: 1) la non linearità del flusso volumetrico si osserva solo in presenza di soluto, e quindi di trasporto attivo. L'equazione si riduce allora alla legge lineare di Darcy; 2) solo sistemi asimmetrici hanno comportamento non lineare, in quanto consentono l'accumulo di soluto nei compartimenti interni; 3) i coefficienti che correlano i flussi alle loro forze traenti sono funzione del flusso stesso e delle variabili operative; 4) il comportamento non lineare di membrane semplici si spiega con il fatto che gli strati limite di fluido non mescolato si comportano come membrane non selettive
A simulation of mass transport across series arrays of membranes with chemical-reaction-coupled solute flow
No full textThis paper presents the results of a simulation of water and solute transport across a series array of several membranes, using a model which allows the treatment of arrays of n membranes with “active” (chemical reaction coupled) solute transport. Several peculiar aspects of the system are demonstrated, showing that its behaviour is consistent with experimental results and that an asymmetry is required, to have a volume flow driven by the active solute transport. Such a coupling, experimentally demonstrated both in synthetic and biological membranes, is well described by the thermodynamics of irreversible processes, as treated in the present paper
Flussi di acqua e soluti attraverso n membrane in serie, con trasporto attivo
No full textTrasporto di acqua e soluti attraverso n membrane in serie in presenza di trasporto "attivo" di soluto. L'esperienza mostra che le relazioni fra i flussi di materia attraverso membrane, e le forze generalizzate che li sostengono sono non lineari. Ciò è vero sia per il trasporto attraverso singole membrane biologiche sia per quello attraverso barriere complesse, come gli epiteli, che si possono ritenere costituiti da più membrane semplici in serie. Il modello più semplice, ma poco realistico, costituito da due sole membrane, già consente di spiegare l'accoppiamento fra il trasporto "attivo" (accoppiato a reazioni chimiche) di soluto e il trasporto d'acqua (1-2). La generalizzazione a un sistema costituito da n membrane in serie, ma puramente passivo (3-4) viene ora estesa con l'introduzione del trasporto attivo. Lo studio teorico del modello e la simulazione al calcolatore di tale sistema hanno mostrato che: 1. la non linearità del flusso volumetrico si osserva solo in presenza di soluto e l'equazione si riduce alla legge lineare di Darcy per il solvente puro, 2. la non linearità richiede comunque l'asimmetria del sistema ed è dovuta all'accumulo di soluto nei compartimenti interni, 3. i coefficienti non costanti che correlano i flussi alle loro forze traenti sono funzione, oltre che del flusso stesso, anche delle variabili operative, 4. il comportamento non lineare di membrane semplici può venire spiegato con il fatto che gli strati limite di fluido non mescolato si comportano come membrane non selettive. (1) C. S. Patlack, D. A. Goldstein, J. F. Hoffman: J. Theor. Biol. 5, 426-442 (1963). (2) G. Monticelli, F. C. Celentano: Bull. Math. Biol. 45, 1073-1096 (1983). (3) F. C. Celentano, G. Monticelli: Atti VI Congresso SIBPA, Camogli, 1983, pp 6062. (4) F. C. Celentano, G. Monticelli in V. Capasso, E. Grosso, S. L. Paveri Fontana: Mathematics in Biology and Medicine, Springer Berlino, 1985, pp 293-299
A simulation of mass transport across series arrays of membranes with chemical reaction-coupled solute flow
No full textBoth in biology and technology, membranes can be seldom treated as single, thin and linearly behaving barriers. Mass transport across thick membranes, as first suggested by Patlak, Goldstein and Hoffman (1), can be described integrating across the membrane thickness the local, linear, practical equations by Kedem and Katchalsky (2), which have been recently shown to derive directly fom the local energy dissipation function of the membrane (3). When the heat flow associated to chemical reactions or to a temperature gradient can be neglected, the presence of a solute "active" transport can also be accounted for (1,4). In the present paper we extend to n membranes, by means of a recursive procedure, the previous treatment of a series array of few membranes (1, 4) and derive some parameters characterizing the transport properties of the complex barrier. This task has been performed analytically, with the help of the symbolic computation program REDUCE, obtaining a non-linear correlation between the flows and their driving forces, depending on the volume flow and on the solute concentration of the transported solution. The nonlinearity appears to be a consequence of the solute accumulation in the inner compartments of the array. The classical linear law by Darcy is a limiting case of our volume flow equation when only pure solvent is transported. Around the volume and solute flow equations we have written a program in Pascal allowing the simulation of a series array of up to 10 membranes and unstirred layers, assimilated to non-selective membranes. The results of the simulation are in agreement with experimental data obtained using complex biological barriers like epithelia. 1. C.S. Patlak, D.A. Goldstein, J.F. Goldstein: J. Teor. Biol. 5, 426-442 (1963); 2. O. Kedem, A. Katchalsky: Biochim. Biophys. Acta 27, 229-246 (1958); 3. F. Celentano, G. Monticelli: Local Practical Equations for Heat and Mass Transport Driven by Temperature Gradients, Proc. Europe-Japan Congr. Membranes and Membrane Proc., Stresa, June 18-22 1984, in press; 4. G. Monticelli, F. Celentano: Bull. Math. Biol. 45, 1073-1096 (1983)
Simulation of mass transport in series arrays of membranes with “active” solute flow
No full textAbstractThe behaviour of the volume and solute flows across a series array of several membranes is investigated by simulation, using a model based on the linear thermodynamics of irreversible processes. Locally linear phenomenological equations are integrated along the direction of the flows, giving non-linear flow-force relationships whose shape is in good agreement with the observations. The simulation also shows that an asymmetry of the array is required, in order to obtain both the non-linearity of the flows and the coupling between the water and solute transport, when the former is coupled to a chemical reaction (metabolism coupled ‘active transport)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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