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  • 1
    Electronic Resource
    Electronic Resource
    Variation of Permeability with Porosity in Sandstone Diagenesis Interpreted with a Fractal Pore Space Model (2000)
    Springer
    Pure and applied geophysics 157 (2000), S. 603-619 
    Details
    ISSN: 1420-9136
    Keywords: Key Words: Permeability, sandstone, fractals, diagenesis.
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract —Permeability is one of the key rock properties for the management of hydrocarbon and geothermal reservoirs as well as for aquifers. The fundamental equation for estimating permeability is the Kozeny-Carman equation. It is based on a capillary bundle model and relates permeability to porosity, tortuosity and an effective hydraulic pore radius which is defined by this equation. Whereas in clean sands the effective pore radius can be replaced by the specific surface or by the grain radius in a simple way, the resulting equations for permeability cannot be applied to consolidated rocks. Based on a fractal model for porous media, equations were therefore developed which adjust the measure of the specific surface and of the grain radius to the resolution length appropriate for the hydraulic process. These equations are calibrated by a large data set for permeability, formation factor, and porosity determined on sedimentary rocks. This fractal model yields tortuosity and effective pore radius as functions of porosity as well as a general permeability-porosity relationship, the coefficients of which are characteristic for different rock types. It can be applied to interpret the diagenetic evolution of the pore space of sedimentary rocks due to mechanical and chemical compaction with respect to porosity and permeability.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/PL00001110
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  • 2
    Electronic Resource
    Electronic Resource
    Born to lose. I. Measures of tissue loss and regeneration by the brittlestar Microphiopholis gracillima (Echinodermata: Ophiuroidea) (1994)
    Springer
    Marine biology 118 (1994), S. 451-462 
    Details
    ISSN: 1432-1793
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract To measure amounts of tissue lost in natural populations of the burrowing amphiurid ophiuroid Microphiopholis gracillima (Stimpson), individuals were collected from subtidal mud flats in North Inlet, South Carolina, USA, at monthly intervals between February 1985–February 1987 and December 1989–November 1990. Between 20 and 70% of all individuals were regenerating the disc, and 85% of the 2045 arms examined had regeneration scars; 〉50% had one scar and some arms had up to 4 scars. Fewer individuals were regenerating discs in warmer months, but there was no seasonality in arm-loss frequency. To quantify rates of arm regeneration in the field, individuals which had 1, 2, or 3 arms removed were placed in mud-filled cores in the field in late July and November 1988 and in March and May 1989, and recovered after periods of about one month. Another set of cores was held in a running seawater laboratory during the May 1989 experiment. No regeneration occurred during the cooler times of year (November and March), and rates of regeneration were slower in May (total: 0.13 mg/d; tissue: 0.03 mg/d) than July (total: 0.17 mg/d; tissue: 0.05 mg/d). These rates indicate complete replacement of lost tissue in 100 to 120 d during the growth season. Within experiments, per arm regeneration rates were similar regardless of the number of arms removed. This finding is complicated by small sample size, high variability and low statistical power, but in general individuals which lost 2 or 3 arms regenerated proportionally more tissue than individuals which lost 1 arm. Individuals held in the laboratory regenerated the same amount of tissue but 30% less skeleton than individuals in the field. Sublethal tissue loss is common in this population, and M. gracillima is capable of regenerating at least 50% (each arm=17% of total body weight x 3) of its standing crop in a single growing season. Burrowing brittlestars probably constitute a significant renewable energy source for higher trophic levels in areas where they occur in dense populations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00350302
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  • 3
    Electronic Resource
    Electronic Resource
    Effect of calcium on the membrane potential ofAmphiuma red cells (1976)
    Springer
    The journal of membrane biology 26 (1976), S. 51-70 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary An increase in extracellular Ca concentration causes the membrane of giant red cells of the salamander,Amphiuma means, to undergo a marked, transient hyperpolarization. This hyperpolarization is caused by an increase in K permeability of the membrane as judged from the K sensitivity of the membrane potential and from the rate of K loss under influence of raised extracellular Ca concentration. At constant external pH, the induction of hyperpolarization by increased extracellular Ca has a relatively well-defined threshold concentration. Furthermore the phenomenon is of an “all or none” type with most of the cells having membrane potential values either in the normal range (about −15 mV) or in the range −40 to −70 mV. Shortly after suspension in Ringer's with 15mM Ca, most if not all of the individual cells are hyperpolarized. Upon continued exposure (5–20 min) to the higher Ca concentration the membrane potential returns to the normal value in a fashion compatible with an “all or none” response. The observed Ca effect is sensitive to the pH of the suspending medium. At pH 6.2 the response is absent whereas the hyperpolarization is markedly stronger at pH 8.2 than at pH 7.2. It is argued that a reliable transport number for K under influence of Ca cannot be estimated from the slope of membrane potentialvs. log (extracellular K concentration). This is probably related to the fact that the membrane potentials of the cells in the population do not stay constant in time. The above phenomenon is compared with the Ca-induced K permeability in poisoned human red cells or red cell ghosts. It is important to note that the cells employed in the present study are neither poisoned nor mechanically disrupted. This study emphasizes that the role of Ca in regulating cell membrane permeability to K seems to be a general feature.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01868866
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  • 4
    Electronic Resource
    Electronic Resource
    The membrane potential of Ehrlich ascites tumor cells microelectrode measurements and their critical evaluation (1971)
    Springer
    The journal of membrane biology 6 (1971), S. 269-288 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Intracellular potentials were measured, using a piezoelectric electromechanical transducer to impale Ehrlich ascites tumor cells with capillary microelectrodes. In sodium Ringer's, the potential immediately after the penetration was −24±7 mV, and decayed to a stable value of about −8 mV within a few msec. The peak potentials disappeared in potassium Ringer's and reappeared immediately after resuspension in sodium. Ringer's, whereas the stable potentials were only slightly influenced by the change of medium. The peak potential is in good agreement with the Nernst potential for chloride. This is also the case when cell sodium and potassium have been changed by addition of ouabain. It is concluded that the peak potentials represent the membrane potential of the unperturbed cell, and that chloride is in electrochemical equilibrium across the cell membrane. The membrane potential of about −11 mV previously reported corresponds to the stable potential in this study, and is considered as a junction potential between damaged cells and their environment. Similar potential differences were recorded between a homogenate of cells and Ringer's. The apparent membrane resistance of Ehrlich cells was about 70 Ωcm2. This is two orders of magnitude less than the value calculated from36Cl fluxes, and may, in part, represent a leak in the cell membrane. For comparison, the influence of an eventual leak on measurements in red cells and mitochondria is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02116574
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  • 5
    Electronic Resource
    Electronic Resource
    Calcium-related hyperpolarization of theAmphiuma red cell membrane following micropuncture (1974)
    Springer
    The journal of membrane biology 18 (1974), S. 125-144 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary The erythrocyte ofAmphiuma means was chosen as a model for elucidation of membrane properties of red cells because the large size of this cell permitted direct measurements of plasma membrane potential. In the 30-sec period following micropuncture and withdrawal of the electrode the plasma membrane reseals and hyperpolarizes to a value of about −50 mV. The hyperpolarization is followed by a gradual return to the unperturbed potential of −15 mV. The magnitude of the hyperpolarization is strongly reduced by an increase in extracellular K concentration and is therefore related to an increase in relative K permeability. The transference number for K is calculated to have a maximal value of about 0.6. However, it is not yet clear whether the hyperpolarization can be solely attributed to a rise in K permeability, or whether there is a concomitant decline in Cl permeability as well. The magnitude of the hyperpolarization is unaffected by the presence of either ouabain or oligomycin. Extracellular Ca is prerequisite to the observed hyperpolarization and presumably acts by permitting the membrane to seal, and having entered the cell during the leak period, causes an increase in the relative K permeability. This permeability change resembles the Ca-induced rise in K permeability seen in metabolically depleted human red cells and red cell ghosts. An important difference, however, is that theAmphiuma red cells used in the present study are neither poisoned nor metabolically depleted so that the Ca effect is not prevented by the presence of cellular ATP as seems to be the case in human erythrocytes. The transient nature of the hyperpolarization may be related to the active transport of Ca out of the cell.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01870107
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  • 6
    Electronic Resource
    Electronic Resource
    Chloride conductance of the Amphiuma red cell membrane (1978)
    Springer
    The journal of membrane biology 39 (1978), S. 27-48 
    Details
    ISSN: 1432-1424
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Chemistry and Pharmacology
    Notes: Summary Like most other red cells, the giant erythrocytes ofAmphiuma means possess a system for rapid exchange of chloride across the membrane. Also, there are indications that the net transport of chloride in these cells is slow. The size ofAmphiuma erythrocytes allows direct measurements of membrane potential with microelectrodes. The present work exploits the possibility that such measurements can be used to give a quantitative estimate of the chloride conductance (G Cl) of the Amphiuma red cell membrane. The membrane potential was measured as a function of extracellular chloride concentration (5–120mM), using an impermeant anion (Para-amino-hippurate) as a substitute. Furthermore, the effect of different pH values (6.0–7.2) was studied. For each extracellular chloride concentration the membrane potential was determined at a pH at which hydroxyl, hydrogen, and bicarbonate ions were in electrochemical equilibrium. From these membrane potentials and the corresponding chloride concentrations in the medium (at constant intracellular ion concentrations), theG Cl of the membrane was calculated to be 3.9×10−7 {ie27-1} cm−2. This value is some six orders of magnitude smaller than that calculated from the rate of tracer exchange under equilibrium conditions. The experimental strategy used gives the value for a “partial transference number” which takes into account only ions which arenot in electrochemical equilibrium. Whereas this approach gives a value forG Cl, it does not permit calculation of the overall membrane conductance. From the calculated value ofG Cl it is possible to estimate that the maximal value of the combined conductances of hydroxyl (or proton) and bicarbonate ions is 0.6×10−7 {ie27-2} cm−2. The large discrepancy between the rate of exchange of chloride and its conductance is in agreement with measurements on human and sheep red cells employing the ionophore valinomycin to increase the potassium conductance of the membrane. The results in the present study were, however, obtained without valinomycin and an accompanying assumption of a constant field in the membrane. Therefore, the present measurements give independent support to the above mentioned conclusions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01872753
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  • 7
    Electronic Resource
    Electronic Resource
    Mediterranean types of β-thalassemia in the German population (1990)
    Springer
    Human genetics 〈Berlin〉 85 (1990), S. 135-137 
    Details
    ISSN: 1432-1203
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine
    Notes: Summary Forty β-thalassemia genes from unrelated German heterozygotes with no known foreign ancestry were examined using the oligonucleotide technique and DNA restriction analysis, with the aim of determining the contribution of Mediterranean β-thalassemia mutations to the prevalence of this trait in the German population. Of the 40 β-thalassemia genes, 26 were identified as Mediterranean types (20 β39 nonsense, 3 IVS2 nt 110, 2 IVS2 nt1, 1 IVS1 ntl G→A). The geographic distribution of the birthplaces of the probands' grandparents revealed no difference in the proportion of Mediterranean and unidentified β-thalassemia genes in the west and the north of Germany.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00276342
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  • 8
    Electronic Resource
    Electronic Resource
    Stoffwechselprodukte von Mikroorganismen (1973)
    Springer
    Archives of microbiology 88 (1973), S. 25-35 
    Details
    ISSN: 1432-072X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Zusammenfassung Ein zellfreier Extrakt aus Streptomyces rimosus katalysiert die Synthese von TDP-Mycarose aus TDP-d-Glucose und S-Adenosyl-l-methionin. Die Reaktion benötigt NADPH. Das Reaktionsprodukt enthält einen weiteren methylierten TDP-Zucker unbekannter Struktur. Die Reaktion verläuft über TDP-4-Keto-6-desoxy-d-glucose als Zwischenprodukt.
    Notes: Summary A cell-free extract from mycelium of Streptomyces rimosus producing the antibiotic tylosin, catalyses the formation of TDP-mycarose from TDP-d-glucose and S-adenosyl-l-methionine. The reaction requires NADPH. The product contains a second methylated TDP-sugar with a presently unknown structure. TDP-4-Keto-6-deoxy-d-glucose is an intermediate in the reaction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00408838
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  • 9
    Electronic Resource
    Electronic Resource
    Stoffwechselprodukte von Mikroorganismen (1972)
    Springer
    Archives of microbiology 85 (1972), S. 233-238 
    Details
    ISSN: 1432-072X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Zusammenfassung Die Biosynthese der Makrotetrolide Monactin, Dinactin und Trinactin wurde in Kulturen von Streptomyces griseoflavus untersucht. Markierungsversuche mit radioaktivem Acetat, Propionat und l-Methionin sowie Abbau der Antibiotica und homologen Untereinheiten zeigen, daß die Kohlenstoffatome 8–10 der Homononactinsäure einer Propionateinheit entstammen, die wahrscheinlich als Startereinheit fungiert.
    Notes: Summary The biosynthesis of the macrotetrolides monactin, dinactin, and trinactin, which differ from nonactin by substitution of one to three homononactinic acid units for nonactinic acid, was studied in cultures of Streptomyces griseoflavus. Labelling experiments with radioactive acetate, propionate, and l-methionine, and degradation of the antibiotics and homologous subunits, showed that the carbon atoms 8 to 10 arise from a propionate residue which probably acts as the starter unit of the carbon chain.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00408848
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  • 10
    Electronic Resource
    Electronic Resource
    Stoffwechselprodukte von Mikroorganismen (1972)
    Springer
    Archives of microbiology 85 (1972), S. 239-248 
    Details
    ISSN: 1432-072X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Description / Table of Contents: Zusammenfassung Nonactinsäure und Homononactinsäure, die bisher nicht als freie Stoffwechselprodukte bekannt waren, wurden aus Kulturlösungen von Streptomyces griseus isoliert. Beide Hydroxysäuren wurden als Methylester gereinigt und chromatographisch und spektroskopisch identifiziert. Die optischen Drehwerte zeigen, daß die Nonactinsäure überwiegend als (-)-Isomer, Homononactinsäure dagegen praktisch einheitlich als (+)-Isomer vorliegt. In produzierenden Kulturen konnten die freien Nonactinsäuren bereits vor Beginn der Makrotetrolidsynthese nachgewiesen werden. (+,-)-Nonactinsäure-8-3H wurde synthetisiert und mit Kulturen von Streptomyces griseus und Streptomyces griseoflavus inkubiert; 6 bis 52% der markierten Verbindung wurden in die Makrotetrolide eingebaut. Die Spezifität des Einbaus wurde durch chemischen Abbau bewiesen. Die Ergebnisse zeigen, daß die chiralen Hydroxysäuren direkte Vorstufen der Makrotetrolide sind.
    Notes: Summary Nonacting and homononactinic acid which were not previously known as free metabolic products, were isolated from culture broths of a macrotetrolide producing strain of Streptomyces griseus. Both hydroxy acids were purified and identified as their methyl esters by chromatographic and spectroscopic methods. The optical rotation values indicated that the free nonactinic acid is mainly the (-)-isomer whereas homononactinic acid appears to be pure (+)-isomer. In producing cultures the nonactinic acids could be detected prior to the beginning of macrotetrolide synthesis. (+,-)-Nonactinic acid-8-3H was synthesized and incubated with cultures of Streptomyces griseus and Streptomyces griseoflavus (the latter producing mainly monactin and dinactin); between 6 and 52% of the labeled acid were incorporated into the macrotetrolides. Chemical degradation showed that no randomization of tritium had occurred. The results suggest that the chiral hydroxy acids are direct biosynthetic precursors of the macrotetrolides.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00408849
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