| membrane attack complex | <immunology> A term originally used to refer to the heat labile factor in serum that causes immune cytolysis, the lysis of antibody coated cells and now referring to the entire functionally related system comprising at least 20 distinct serum proteins that is the effector not only of immune cytolysis but also of other biologic functions. Complement activation occurs by two different sequences, the classic and alternative pathways. The proteins of the classic pathway are termed components of complement and are designated by the symbols C1 through C9. C1 is a calcium dependent complex of three distinct proteins C1q, C1r and C1s. The proteins of the alternative pathway (collectively referred to as the properdin system) and complement regulatory proteins are known by semisystematic or trivial names. Fragments resulting from proteolytic cleavage of complement proteins are designated with lower case letter suffixes, for example, C3a. Inactivated fragments may be designated with the suffix i, for example C3bi. Activated components or complexes with biological activity are designated by a bar over the symbol for example C1 or C4b, 2a. The classic pathway is activated by the binding of C1 to classic pathway activators, primarily antigen-antibody complexes containing IgM, IgG1, IgG3, C1q binds to a single IgM molecule or two adjacent IgG molecules. The alternative pathway can be activated by IgA immune complexes and also by nonimmunologic materials including bacterial endotoxins, microbial polysaccharides and cell walls. Activation of the classic pathway triggers an enzymatic cascade involving C1, C4, C2 and C3, activation of the alternative pathway triggers a cascade involving C3 and factors B, D and P. Both result in the cleavage of C5 and the formation of the membrane attack complex. Complement activation also results in the formation of many biologically active complement fragments that act as anaphylatoxins, opsonins or chemotactic factors. (05 Jan 1998) |
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| membrane bone | A bone that develops embryologically within a membrane of vascularised primitive mesenchymal tissue without prior formation of cartilage. (05 Mar 2000) |
| membrane-bound proton-translocating PPi synthase | <enzyme> From rhodospirillum rubrum; functions as an alternative coupling factor; n,n'-dicyclohexylcarbodiimide-sensitive; catalyses the phosphorylation of pi to ppi, the hydrolysis of ppi and the concomitant translocation of proton across the plasma membrane Registry number: EC 3.6.1.- Synonym: h(+)-ppi synthase (26 Jun 1999) |
| membrane capacitance | The electrical capacitance of a membrane. Plasma membranes are excellent insulators and dielectrics: capacitance is the measure of the quantity of charge that must be moved across unit area of the membrane to produce unit change in membrane potential and is measured in Farads. most plasma membranes have a capacitance around 1 microfarad cmexp 2. (18 Nov 1997) |
| membrane-coating granule | A membrane-bound granule, 100 to 500 nm in diameter, located in the upper layers of the stratum spinosum of certain stratified squamous epithelia. Synonym: lamellar granule, membrane-coating granule, Odland body. (05 Mar 2000) |
| membrane depolarisation | The process or act of neutralising polarity, depriving of polarity, or the result of such action; reduction to an unpolarised condition. <physiology> The reversal of the resting potential in excitable cell membranes when stimulated i.e., the tendency of the cell membrane potential to become positive with respect to the potential outside the cell. A positive shift in a cells resting potential (that is normally negative), thus making it numerically smaller and less polarized, for example 90mV to 50mV. <optics> Depolarisation of light, a change in the plane of polarization of rays, especially by a crystalline medium, such that the light which had been extinguished by the analyser reappears as if the polarization had been anulled. The word is inappropriate, as the ray does not return to the unpolarised condition. Origin: Cf. F. Depolarisation. (27 Oct 1998) |
| membrane dipeptidase | <enzyme> Renal dipeptidase which metabolises thienamycin and related carbapenem antibiotic Registry number: EC 3.4.13.19 Synonym: dehydropeptidase-i, dehydropeptidase I, microsomal dipeptidase (26 Jun 1999) |
| membrane enzyme | <enzyme> An enzyme present or embedded in a biomembrane. (05 Mar 2000) |
| membrane expansion theory | That adsorption of anaesthetics into membranes so alters membrane volume and/or configuration that membrane function is affected in such a way as to produce anaesthesia. (05 Mar 2000) |
| membrane fluidity | Biological membranes are viscous 2 dimensional fluids within their physiological temperature range. (18 Nov 1997) |
| membrane fracture | Method of specimen preparation for the electron microscope in which rapidly frozen tissue is cracked so as to produce a fracture plane through the specimen. The surface of the fracture plane is then shadowed by heavy metal vapour, strengthened by a carbon film and the underlying specimen is digested away, leaving a replica that can be picked up on a grid and examined in the transmission electron microscope. The great advantage of the method is that the fracture plane tends to pass along the centre of lipid bilayers and it is therefore possible to get en face views of membranes that reveal the pattern of Integral membrane proteins. The E face is the outer lamella of the plasma membrane viewed as if from within the cell, the P face the inner lamella viewed from outside the cell. Fracture planes also often pass along lines of weakness such as the interface between cytoplasm and membrane, so that outer and inner membrane surfaces can be viewed. Further information about the structure can be revealed by freeze etching. Extremely rapid freezing followed by deep etching has allowed the structure of the cytoplasm to be studied without the artefacts that might be introduced by fixation. (18 Nov 1997) |
| membrane fusion | The adherence of cell membranes, intracellular membranes, or artifical membrane models of either to each other or to viruses, parasites, or interstitial particles through a variety of chemical and physical processes. (12 Dec 1998) |
| membrane glycoproteins | Glycoproteins found on the membrane or surface of cells. (12 Dec 1998) |
| membrane lipids | Lipids, predominantly phospholipids, cholesterol and small amounts of glycolipids found in membranes including cellular and intracellular membranes. These lipids may be arranged in bilayers in the membranes with integral proteins between the layers and peripheral proteins attached to the outside. Membrane lipids are required for active transport, several enzymatic activities and membrane formation. (12 Dec 1998) |
| membrane of tympanum | <anatomy> The eardrum. (13 Nov 1997) |