![]() ![]() The membrane of specialized cells, such as neurons, is involved in propagation of nervous signals (see chapter 6) towards other neurons in the brain or muscle and glandular cells. Being placed at the exterior of a cell, the membrane is also the first target of physical, chemical, and biological agents such as thermal and mechanical stress, toxins, hormones, viruses, microbes, etc. For example, macrophages, involved in the immune defence system, are able to engulf and destroy microbes and other foreign particles, this complex cellular process being called phagocytosis (see chapter 4). The membrane is involved also in intake ( endocytosis) and secretion ( exocytosis) of large particles. It plays an important role in the economy of the cell, exerting a selective control on the entire traffic of ions, water, and molecules. The plasma membrane undergoes continual changes both in its molecular composition and its structure (i.e., spatial distribution of its components), although during the entire lifespan of the cell its global architecture remains the same. The cell membrane (or plasma membrane) is a thin closed sheet that fulfils a double role: (a) morphological – delimitates the cell from its external microenvironment and confines all of its subcellular organelles (b) functional – regulates the exchange of substance between internal and external media, maintains actively the ionic asymmetry between its sides, and intermediates internalization or externalization of physical and chemical signals important for cell functions.
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