| 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) |
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| 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) |
| membrane potential | <physiology> More correctly, transmembrane potential difference: the electrical potential difference across a plasma membrane. See: resting potential, action potential. (18 Nov 1997) |
| membrane potentials | Ratio of inside versus outside concentration of potassium, sodium, chloride and other ions in diffusible tissues or cells. Also called transmembrane and resting potentials, they are measured by recording electrophysiologic responses in voltage-dependent ionic channels of (e.g.) nerve, muscle and blood cells as well as artificial membranes. (12 Dec 1998) |
| membrane protein | <protein> A protein with regions permanently attached to a membrane (peripheral membrane protein) or inserted into a membrane integral membrane protein). Insertion into a membrane implies hydrophobic domains in the protein. All transport proteins are integral membrane proteins. (18 Nov 1997) |
| membrane proteins | Proteins which are found in membranes including cellular and intracellular membranes. They consist of two types, peripheral and integral proteins. They include most membrane-associated enzymes, antigenic proteins, transport proteins, and drug, hormone, and lectin receptors. (12 Dec 1998) |
| membrane recycling | <cell biology> The process whereby membrane is internalised, fuses with an internal membranous compartment and is then re incorporated into the plasma membrane. In cells that are actively secreting by an exocrine method (in which secretory granules fuse with the plasma membrane), it is obviously essential to have some way of reducing the area of the plasma membrane. The membrane can then be used to form new secretory vesicles. The converse is true for phagocytic cells. (18 Nov 1997) |
| membrane transport | <cell biology> The transfer of a substance from one side of a plasma membrane to the other, in a specific direction and at a rate faster than diffusion alone. See: active transport. (18 Nov 1997) |
| membrane-type 3 matrix metalloproteinase | <enzyme> Sm3 is a soluble form of mt3-mmp, probably an alternatively sliced variant. Registry number: EC 3.4.24.- Synonym: mt3-mmp, sm3-mmp (26 Jun 1999) |
| membrane-type 4 matrix metalloproteinase | <enzyme> Cloned from breast carcinoma. Registry number: EC 3.4.24.- Synonym: mt4-mmp, mmp-17 gene product, mmp-17 (26 Jun 1999) |
| membrane-type matrix metalloproteinase | <enzyme> Activates gelatinase a; isolated from a human placenta cdna gene library; contains a transmembrane domain; do not use for any other numbered matrix metalloproteinases; genbank d26512 Registry number: EC 3.4.24.- Synonym: mt-mmp, mmp-x1 protein, matrix metalloproteinase, membrane-type, mmp14 gene product, mmp-14 gene product, mt1-mmp, matrix metalloproteinase 14, mt2-mmp, mmp15 gene product, mmp16 gene product (26 Jun 1999) |
| membrane vesicle | <cell biology> Closed unilamellar shells formed from membranes either in physiological transport processes or else when membranes are mechanically disrupted. They form spontaneously when membrane is broken because the free ends of a lipid bilayer are highly unstable. (18 Nov 1997) |