Function in the Flow-Differentiation of Membranes
Membrane differentiation within the cell 's endomembrane system may be defined as the progressive change in the appearance, composition, organization, and/or functional specialization observed within a single membrane or among different but ontogenetically related membranes or systems of membranes. Local differentiation is visualized by various cytochemical procedures or by measurements of membrane thickness or distributions of intramembranous particles measured from electron micrographs. Functional specialization of different but ontogenetically related membranes is reflected in various biochemical and physical parameters determined from analyses of isolated and purified cell fractions or from cytochemistry. Differentiation occurs along established export pathways and traffic routes within the cell and, by definition, proceeds from precursor-like to product-like. In the sense of secretion or plasma membrane biogenesis, differentiation proceeds from endoplasmic reticulum like to plasma membrane-like. While observations providing evidence for membrane differentiation are based primarily on static analyses (morphological observations, analyses of isolated cell fractions), they do provide an important conceptual framework within which dynamic events may occur.
For membrane differentiation to contribute significantly as a major mechanism of membrane biogenesis whereby one kind of precursor membrane is converted into another kind of product membrane, e.g., derivation of Golgi apparatus from endoplasmic reticulum or of plasma membrane from Golgi apparatus, transfer of membrane from one compartment to another must accompany, precede or follow the process of differentiation. This dynamic component has been termed membrane flow. Without a flow mechanism to account for the transport of membrane constituents from one region of the cell to another, processes of membrane differentiation could account for the origin of new membrane types only in situations where there was no intermixing of membrane compartments. Working in concert, the combined processes of flow-differentiation offer a mechanism of membrane biogenesis consistent with both ultrastructural and biochemical information. With the availability of highly purified cell fractions and various correlative investigative approaches, the hypothesis has been verified experimentally in several test systems. It is perhaps the most universal function of the Golgi apparatus when a wide range of species and cell types is compared; more universal even than secretion. Products elaborated by the Golgi apparatus for secretion are packaged in membrane-enclosed transport vesicles.
KeywordsEndoplasmic Reticulum Golgi Apparatus Nuclear Envelope Secretory Vesicle Membrane Differentiation
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