Silica deposition vesicles

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

Studies with diatoms have revealed that silicic acid is pumped from the surrounding marine or freshwater environment into the cell against a concentration gradient through specific, outer membrane-associated transporters that were revealed recently by Hildebrand, Volcani and their colleagues14. Transporters related to those in the outer membrane may also facilitate entry into the membrane-enclosed silica deposition vesicle within the cell15. 

Other than accelerating the formation of spherical nanoparticles of silica from metastable silicic acid solutions, the silaffins appear to have no structure-directing activity. If they are responsible for silica formation in the living diatom, as seems quite likely, control of the higher order architecture of the resulting silica apparently must be determined by the pre-formed shape of the silica deposition vesicle (the envelope within which the silica grows) serving as a complex three-dimensional mold. 

The original mechanism proposed for templated silicification was that proposed for diatoms by Hecky et al. and is still referred to today by workers in the field. It was envisaged that the serine/threonine-enriched protein would form the inner surface of the silicalemma, and as such would present a layer of hydroxyls, facing into the silica deposition vesicle, onto which orthosilicic acid molecules could condense. This initial layer of geometrically constrained orthosilicic acid molecules would promote condensation with other orthosilicic acid molecules. The carbohydrate moiety present in the diatom cell wall was... 

Imaging/Labeling Applications Lysosomes acidic vesicles " silica/silica deposits " ... 

The mostly used methods to monitor LbL deposition on monodisperse PS-latex particles for various substances are SPLS method and microelectrophoresis. Inorganic (magnetite, silica, titania and fluorescent quantum dots) nanoparticles [32-34], lipids [35-37] and proteins (albumin, immunoglobulin and others) [29, 38, 39] were incorporated as building block for shell formation on colloidal particles. In paper [39] the construction of enzyme multilayer films on colloidal particles for biocatalysis was demonstrated. The enzyme multilayers were assembled on submicrometer-sized polystyrene spheres via the alternate adsorption of poly(ethyleneimine) and glucose oxidase. The high surface area bio-multilayer coated particles formed were subsequently utilized in enzymatic catalysis. The step-wise coating of different lipids alternated with polyelectrolytes was performed by adsorption of preformed vesicles onto... 

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