Specializations of the Unit Membrane
The three-layered unit membrane is a structure showing several modifica-tinot only in its thickness but also in the nature and amount of materials associated with its outer and inner surfaces. Indeed, the presence of associated layers is so constant that one is led to wonder whether the unit membrane as commonly defined should not be regarded as merely the central component in a complex and highly variable structure which marks cell boundaries. The variation in thickness first pointed out by Ledbetter (1962) for the plant cell has since been described by Sjöstrand (1963) among others, in the animal cell. In plant cells, for example the plasma membrane and that of the tonoplast measure about 100-120 Å, whereas the limiting membranes of the mitochondria and chloroplasts fall in the 50-70 Å range. This kind of variation is now so well known that it requires neither illustration nor further comment.
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