RESUMEN
We describe a technique for probing the elastic properties of biological membranes by using an atomic force microscope (AFM) tip to press the biological material into a groove in a solid surface. A simple model is developed to relate the applied force and observed depression distance to the elastic modulus of the material. A measurement on the proteinaceous sheath of the archaebacterium Methanospirillum hungatei GP1 gave a Young's modulus of 2 x 10(10) to 4 x 10(10) N/m2. The measurements suggested that the maximum sustainable tension in the sheath was 3.5 to 5 N/m. This finding implied a maximum possible internal pressure for the bacterium of between 300 and 400 atm. Since the cell membrane and S-layer (wall) which surround each cell should be freely permeable to methane and since we demonstrate that the sheath undergoes creep (expansion) with pressure increase, it is possible that the sheath acts as a pressure regulator by stretching, allowing the gas to escape only after a certain pressure is reached. This creep would increase the permeability of the sheath to diffusible substances.
Asunto(s)
Archaea/fisiología , Metano/metabolismo , Membrana Celular/fisiología , Elasticidad , Modelos BiológicosRESUMEN
Methanospirillum hungatei GP1 possesses paracrystalline cell envelope components including end plugs and a sheath formed from stacked hoops. Both negative-stain transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) distinguished the 2.8-nm repeat on the outer surface of the sheath, while negative-stain TEM alone demonstrated this repeat around the outer circumference of individual hoops. Thin sections revealed a wave-like outer sheath surface, while STM showed the presence of deep grooves that precisely defined the hoop-to-hoop boundaries at the waveform nodes. Atomic force microscopy of sheath tubes containing entrapped end plugs emphasized the end plug structure, suggesting that the sheath was malleable enough to collapse over the end plugs and deform to mimic the shape of the underlying structure. High-resolution atomic force microscopy has revised the former idea of end plug structure so that we believe each plug consists of at least four discs, each of which is approximately 3.5 nm thick. PT shadow TEM and STM both demonstrated the 14-nm hexagonal, particulate surface of an end plug, and STM showed the constituent particles to be lobed structures with numerous smaller projections, presumably corresponding to the molecular folding of the particle.
Asunto(s)
Membrana Celular/ultraestructura , Pared Celular/ultraestructura , Euryarchaeota/ultraestructura , Microscopía Electrónica/métodos , Microscopía de Túnel de Rastreo/métodos , Coloración Negativa , Técnica Histológica de SombreadoRESUMEN
In this series of 41 cases of acute fractures of the femur treated with the Brooker-Wills nail, 20 technical problems with insertion or removal of the nail occurred in 19 of the total. Final outcomes were determined in 33 cases followed for 1 year or more. Technical difficulties had a deleterious effect on the outcome in three of these 33 cases. Mechanical failure seems to be a significant problem with the Brooker-Wills system in this and other series. We found that bending of the proximal nail during the insertion, breaking of the tip of the distal fixator inserter, and detachment of the blades from the fixator shaft had occurred. Although satisfactory results were obtained in this series, comparable to other "start-up" series, it is our opinion that the advantages of the Brooker-Wills nail over other interlocking nails are outweighed by the disadvantages of difficult insertion and removal.