Co-transport-induced instability of membrane voltage in tip-growing cells

A salient feature of stationary patterns in tip-growing cells is the key role played by the symports and antiports, membrane proteins that translocate two ionic species at the same time. It is shown that these co-transporters destabilize generically the membrane voltage if the two translocated ions diffuse differently and carry a charge of opposite (same) sign for symports (antiports). Orders of magnitude obtained for the time and lengthscale are in agreement with experiments. A weakly nonlinear analysis characterizes the bifurcation

Ca2+ influx and phosphoinositide signalling are essential for the establishment and maintenance of cell polarity in monospores from the red alga Porphyra yezoensis

The asymmetrical distribution of F-actin directed by cell polarity has been observed during the migration of monospores from the red alga Porphyra yezoensis. The significance of Ca2+ influx and phosphoinositide signalling during the formation of cell polarity in migrating monospores was analysed pharmacologically. The results indicate that the inhibition of the establishment of cell polarity, as judged by the ability of F-actin to localize asymmetrically, cell wall synthesis, and development into germlings, occurred when monospores were treated with inhibitors of the Ca2+ permeable channel, phospholipase C (PLC), diacylglycerol kinase, and inositol-1,4,5-trisphosphate receptor. Moreover, it was also found that light triggered the establishment of cell polarity via photosynthetic activity but not its direction, indicating that the Ca2+ influx and PLC activation required for the establishment of cell polarity are light dependent. By contrast, inhibition of phospholipase D (PLD) prevented the migration of monospores but not the asymmetrical localization of F-actin. Taken together, these findings suggest that there is functional diversity between the PLC and PLD signalling systems in terms of the formation of cell polarity; the former being critical for the light-dependent establishment of cell polarity and the latter playing a role in the maintenance of established cell polarity

Many roads to symmetry breaking: Molecular mechanisms and theoretical models of yeast cell polarity

Mathematical modeling has been instrumental in identifying common principles of cell polarity across diverse systems. These principles include positive feedback loops that are required to destabilize a spatially uniform state of the cell. The conserved small G-protein Cdc42 is a master regulator of eukaryotic cellular polarization. Here we discuss recent developments in studies of Cdc42 polarization in budding and fission yeasts and demonstrate that models describing symmetry-breaking polarization can be classified into six minimal classes based on the structure of positive feedback loops that activate and localize Cdc42. Owing to their generic system-independent nature, these model classes are also likely to be relevant for the G-protein–based symmetry-breaking systems of higher eukaryotes. We review experimental evidence pro et contra different theoretically plausible models and conclude that several parallel and non–mutually exclusive mechanisms are likely involved in cellular polarization of yeasts. This potential redundancy needs to be taken into consideration when interpreting the results of recent cell-rewiring studies

En skole for alle? : ungdom og gjennomføringen av videregående skole

Det store antallet ungdommer som ikke fullfører videregående skole er bekymringsfullt. Jeg arbeider selv i den videregående skolen, og har derfor et nært kjennskap til denne problematikken. Tall fra Statistisk sentralbyrå (SSB 2006) viser at nærmere to av fem (36%) elever på yrkesforberedende ikke fullfører videregående skole. For hele den videregående skolen under ett, slutter èn av fire (24%). Hensikten eller formålet har vært å finne ut om det finnes noen beskyttelses, - eller risikofaktorer som påvirker et være eller ikke være i den videregående skolen. Problemstillingen er formulert slik: Hvilke faktorer opplever ungdommene selv er viktige for om de blir værende i den videregående skolen? Jeg ønsket at min undersøkelse i størst mulig grad skulle styres av det ungdommene/elevene selv mener er viktige temaer. Ut ifra ønsket om å ta elevenes perspektiv, har valget av metode falt på narrativ tilnærming. Narrativ tilnærming forutsetter at noen forteller en historie til en tilhører som oppfatter og tolker historien på sin måte (Lundby 1998). Narrativer hører til i den hermeneutiske vitenskapstradisjon ( Kokkersvold 2005). Hermeneutikken kan defineres som læren om tolkning, og den er det vitenskaplige fundamentet i kvalitativ forskning (Dalen 2004). Jeg avgrenser min undersøkelse ved å få historiene til to ungdommer. Den ene ungdommen har sluttet på videregående skole, mens den andre har vært svært nære ved å slutte. Hovedkilden til narrativer er intervju, og det er den ustrukturerte eller åpne samtalen jeg har benyttet i min narrative tilnærming. Den teoretiske forankringen har tatt utgangspunkt i hva faglitteraturen presenterer som viktig når det gjelder trivsel og mistrivsel i skolen. Dette er et stort fagfelt, men jeg har valgt å sette søkelyset på to hovedemner: betydningen av mestring og lærerrollen. Ved analyse av narrativene pekte det seg ut tre hovedlinjer jeg fulgte i tolknings og drøftningsarbeidet. Dette var: lærerens betydning, fagvansker/ mestring og selvverd. Jeg konkluderer med at rent metodisk var narrativ tilnærming godt egnet til mitt formål om å ta elevperspektivet. Kort oppsummert kan jeg med utgangspunkt i mine informanters skolehistorier konkludere slik: Beskyttelsesfaktorer for å bli værende i den videregående skolen: Godt og nært forhold til lærerne, mestre skolefaglige utfordringer og praktisk relatert opplæring. Risikofaktorer for ikke å bli værende i den videregående skolen: Oppleve opplæringen som teoritung, for lite praktisk relatert opplæring, skolelei allerede i ungdomsskolen og fagvansker som fører til manglende mestringsopplevelser

Bijdragen tot de geschiedenis en de folklore der geneeskunde.

OPLADEN-RUG0

Pattern formation of transcellular ionic currents in plant cells

Stationary and non-stationary spatio-temporal pattern formations are a common feature of biological cells and tissues. The propagation of an action potential along an excitable cell is a well-known example of non-stationary spatio-temporal pattern. Stationary patterns of ion concentration and electrical potential occur in plant cells and tissues. They are correlated to steady ionic currents through the cell membrane. For instance, in chara, a freshwater plant, periodic spatial variations of pH appear along the cylindrical internodal and whorl excitable cells (length from few millimetres to 15 cm) breaking the translation symmetry. The characteristic wavelength and time vary respectively from 1 mm to 2 cm and from 5 min to 60 min. This spatial pH pattern occurs upon activation of the plasma membrane H+-ATPase. In the absence of any external cue, the first pH band appears at a random position suggesting that the initial symmetric state is unstable. The cable model is widely used to understand spatio-temporal pattern formation in excitable cells. The success of this model rests on a typical feature of excitable cells: a negative differential conductance in part of the current-voltage characteristic of their membrane. It cannot explain stationary pattern formation of transcellular ionic current. We have used an electrodiffusive model coupled with cellular electric activity. We report here an unexpected property of the resulting model: a phase space domain that gives rise to stationary spatio-temporal patterns in the presence of a positive total differential membrane conductance. The model can explain stationary spatio-temporal pattern formation in both excitable and non-excitable cells. Our results indicate that these patterns occur if the H+-ATPase is coupled either to voltage-dependent ion channels or to a symport

Planar Lipid Bilayers for Electrophysiology of Membrane-Active Peptides

info:eu-repo/semantics/publishe