They can therefore be used for the reconstruction of earlier vegetation- and climate-conditions. Thus, in the plant world Sporopollenin is one of the most resistant substances, and consequently, pollen grains and spores can be preserved under anoxic conditions in lakes and fens for thousands to millions of years. Sporopollenin is chemically unsaturated and is corroded by oxygen, but is otherwise resistant even to strong alkaline substances and organic acids. Although the surface of pollen grains and spores from various species may appear different under the scanning electron microscope (SEM), transmission electron microscopy (TEM) demonstrates that the layers present within their walls are broadly similar.
The walls are usually composed of a N-free polymeric substance related to terpenes, with chemical formulae approximating C 90, H 130-158, O 24-44. Their outer shells, called exines, are composed of sporopollenin, which is often described as nature’s most robust polymeric system. The inner layer of the cell walls of spores and pollen, called the intine, consists mainly of cellulose and pectin, which degrades during fossilization. Their shapes and sizes are genus specific and reproducible, and range from spherical to oblate and prolate. Plant, fungal and bacterial spores and plant pollen provide a unique and extremely large collection of biological particles and microcapsules that range in size from a few microns up to millimetres depending on genus.
Comparatively, nature provides its own variety of microcapsules and particles based on cell walls and membranes, but unlike their synthetic equivalents, they possess discrete size, shape, morphology and uniform (mono) dispersity within each species. Furthermore, it has recently been noted that certain polymeric microcapsules are detrimental to the environment.
Their sizes and stabilities vary depending on material selection, production, and the environment of their use. The shells of synthetic particles and microcapsules are usually composed of lipids or polymers, or materials such as alginates, gelatin and gum Arabic. Microcapsules and functional particles have a wide variety of applications, from paints, coatings, E-inks, thermochromics, time-release pharmaceuticals, to visual indicators, etc.
0 kommentar(er)
