Turgescence

Résumé

La turgescence est l'état d'une cellule vivante dilatée par l'eau qui y est entrée, et qui s'accumule dans ses vacuoles ou ses vésicules. Mécanisme L'eau étant devenue abondante dans la vacuole, la pression exercée de l'intérieur de la cellule vers le milieu s'exerce sur la paroi primaire. Ce phénomène est appelé pression de turgescence. L'entrée d'eau est elle-même causée par le phénomène de l'osmose, selon lequel deux milieux de concentrations différentes et séparés par une membrane semi-perméable (laissant passer l'eau mais non les substances dissoutes) opèrent un échange : l'eau du milieu le moins concentré (milieu ou solution hypotonique) traverse la paroi ce qui permet de rétablir l’équilibre des concentrations, ce flux traduisant la pression osmotique. Lorsque l'eau entre dans la cellule végétale, la vacuole se remplit et grossit et ses membranes se tendent. On parle alors de turgescence des cellules. L'eau des cellules exerce une pression sur leur paroi et donn

Source officielle

https://fr.wikipedia.org/wiki/Turgescence

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Non-linear transfer function identification of pressure probes using Siren Disks

Fatma Ceyhun Sahin, Jürg Alexander Schiffmann, Karim Magdi Zakaria Abdelrahman Shalash

This article presents a Siren Disk proof of concept for the dynamic excitation of pressure probes, and a method to reconstruct distorted signals due to pneumatic channels. Constraints in sensor installation require placing a pressure transducer distant from the measurement point. The transducer is usually connected through a pneumatic channel – creating a probe, which alter its dynamic response. The Siren Disk is used for the identification of transfer functions of different pressure probe geometries. The device is capable of producing pressure signals up to 10 kHz and 3.5 bara (peak-to-peak = 2.5 bars). The transfer function is obtained through the comparison of the probe signal to a flush mounted reference transducer that is subjected to the same pressure signal. The response of the probes was shown to be highly non-linear. Hence, a multi-dimensional transfer function is developed for the system identification of the probes. The function is based on the Fourier series, and consists of a set of sub transfer functions describing the average gain and phase lag for the offset and the harmonics. The approach is well suited to capture the non-linear frequency response of complex sensor installations. Experiments show that the flat response of transducers is jeopardized by the introduction of the low pass filter behavior from the pneumatic channels. The probe’s signal was significantly distorted compared to the reference signal. The inverse transfer function is used to reconstruct the probe’s signal in the time domain. Good agreement is found between the reconstructed and the reference signals even at excitation frequencies beyond the probe’s resonant frequency. Hence, highlighting a wide range of validity for the proposed method.

2018

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Time-lapse high-resolution microscopy to study the morphogenesis of microorganisms

Mélanie Thérèse Marie Hannebelle

Mycobacterium tuberculosis, the etiological agent for the tuberculosis disease, is a bacterial pathogen thought to infect about a quarter of the global human population. It is the first cause of death among infectious diseases, and is most prevalent in low-income populations. Much remains unknown about how these bacteria grow, divide, and manage to persist in the host even during month-long antibiotic treatments. Bacteria are typically at the edge of the spatial resolution that conventional optical microscopy techniques can offer, and so new tools are required to study bacterial substructures or surface morphologies with nanometer resolution. We developed a platform for automated optical microscopy and atomic force microscopy (AFM) in order to study the morphogenesis of mycobacteria. What is the growth dynamics of mycobacteria? Opposite models coexist in the literature for the pole growth dynamics of mycobacteria: the unipolar model and the bipolar model. We show that the growth dynamics of mycobacteria follows a new end take off NETO model. There is a surprising similarity between the NETO growth dynamics of mycobacteria and of fission yeast, which hints at shared mechanisms for pole growth in evolutionarily distant pole growing organisms. How do pole-growing organisms maintain their shape through insertion of new cell wall material at the tip? To explore further pole growth morphogenesis, we developed a method for imaging with AFM the pole of a growing rod-shaped microorganism. We observed the formation of a stiff fibril mesh on the pole of fission yeast cells, and showed that this mesh is stretched as the cell grows and insert new cell wall material at the tip. How do mycobacteria divide into two sibling cells? Another essential aspect of morphogenesis of rod-shaped cell is the division of a mother cell into two sibling cells. Using AFM, we measured the relative contributions of mechanical forces and molecular mechanisms driving the division process in mycobacteria. We showed that there is a concentration of mechanical stress at the future division site, using a combination of COMSOL finite element modeling and AFM measurements while controlling the cell turgor pressure. The accumulation of mechanical stress, in combination of enzymatic activity, ultimately leads to mechanical fracture and separation of the sibling bacteria. Conventional optical microscopy techniques tend to have low resolution and high speed, while conventional AFMs have comparatively high resolution and low speed. We designed and built a 3D-printed structured illumination (SIM) module, the openSIM, as an add-on for fluorescence microscopes. It provides structured illumination to obtain SIM images with higher resolution. With the aim of improving the imaging speed of AFM, we implemented photothermal actuation in a tip-scanning AFM head, and quantified the design constraints for high-speed photothermal off-resonance tapping imaging. AFM is most often used to scan and acquire high-resolution images of a sample. Its nanometer precision make it a unique tool for other applications. We combined an AFM head with a volcano-shaped probe for electrogram measurements, and demonstrated simultaneous recording of extracellular field potentials and contraction of the cell. This new tool opens the way for future studies exploring the intriguing links between mechanobiology and electrophysiology, with single-cell resolution.

EPFL2020

Chargement

, ,

Even if models are able to predict more and more accurately pollutant discharge in streams, surface water sampling remains a very common practice to monitor substance concentrations and loads in streams and to calibrate models. However, as this method is temporally and spatially punctual, monitoring a whole catchment requires multiple sampling sites with time-distributed samples. Instruments are expensive, and sample collection, on-site interventions and maintenance are costly and time-consuming, in particular if the experimental site is remote. Another issue is the estimation of the discharge loads of a pollutant, especially for non-chemostatic compounds; their hydrograph-related chemical dynamics may be miss-evaluated when a rapid storm occurs using a time-paced sampling strategy with large sampling intervals. Many manufacturers provide discharge gauges (pressure probes or ultra-sonic sensors) or other instruments (rain gauge, chemical probes, etc.) that can be coupled with automatic water samplers in order to program an event-paced sampling. However, automatic samplers usually provide limited programming options that may not meet the needs of the experimenter of a specific catchment. The concept presented here proposes to use a simple microcontroller board in order to determine the timing of the samples by sending electrical pulses to a conventional automatic sampler with input capability. The flow level is measured by a low-cost ultrasonic sensor and sent to the microcontroller, which will process the signal according to user and site-custom parameters. For example, a simple power-law recession model can be apply to approximate the duration of the recession period given the maximal discharge rate measured for a storm. The sample intervals can thereafter be set in order to distribute all the bottles available over the total recession duration. The microcontroller sends a pulse (grab sample query) to the sampler at every sample time calculated by the program. A GSM/GPRS shield coupled with the microcontroller allows the data upload (flow rates, sample time, state, number of bottles available, etc.) on a web-server and can inform the experimenter by SMS about the beginning of a sampling event, a water level alert or a technical issue. This technique provides a low-cost and very flexible solution for designing advanced sampling schemes.

2013

Chargement

Time-lapse high-resolution microscopy to study the morphogenesis of microorganisms

Mélanie Thérèse Marie Hannebelle

EPFL2020

, ,

2013