membraneless, adj.
/mem"brayn/, n.
1. Anat. a thin, pliable sheet or layer of animal or vegetable tissue, serving to line an organ, connect parts, etc.
2. Cell Biol. the thin, limiting covering of a cell or cell part.
[1375-1425; late ME; ME membraan parchment < L membrana. See MEMBER, -AN]

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In biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment.

The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles. Biological membranes have a dual function: separation of vital but incompatible metabolic processes conducted in the organelles; and passage of nutrients, wastes, and metabolic products between organelles and between the cell and the outside environment. Membranes consist largely of a double layer of lipids in which are embedded large proteins, many of which transport ions and water-soluble molecules across the membrane. See also cytoplasm, eukaryote.

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 in biology, the thin layer that forms the outer boundary of a living cell or of an internal cell compartment. The outer boundary is the plasma membrane, and the compartments enclosed by internal membranes are called organelles (organelle). Biological membranes have three primary functions: (1) they keep toxic substances out of the cell; (2) they contain receptors and channels that allow specific molecules, such as ions, nutrients, wastes, and metabolic products, that mediate cellular and extracellular activities to pass between organelles and between the cell and the outside environment; and (3) they separate vital but incompatible metabolic processes conducted within organelles.

      Membranes consist largely of a lipid bilayer, which is a double layer of phospholipid, cholesterol, and glycolipid molecules that contains chains of fatty acids (fatty acid) and determines whether a membrane is formed into long flat sheets or round vesicles. Lipids give cell membranes a fluid character, with a consistency approaching that of a light oil. The fatty-acid chains allow many small, fat-soluble molecules, such as oxygen, to permeate the membrane, but they repel large, water-soluble molecules, such as sugar, and electrically charged ions, such as calcium.

 Embedded in the lipid bilayer are large proteins (protein), many of which transport ions and water-soluble molecules across the membrane. Some proteins in the plasma membrane form open pores, called membrane channels, which allow the free diffusion of ions into and out of the cell. Others bind to specific molecules on one side of a membrane and transport the molecules to the other side. Sometimes one protein simultaneously transports two types of molecules in opposite directions. Most plasma membranes are about 50 percent protein by weight, while the membranes of some metabolically active organelles are 75 percent protein. Attached to proteins on the outside of the plasma membrane are long carbohydrate molecules.

      Many cellular functions, including the uptake and conversion of nutrients, synthesis of new molecules, production of energy, and regulation of metabolic sequences, take place in the membranous organelles. The nucleus, containing the genetic material of the cell, is surrounded by a double membrane with large pores that permit the exchange of materials between the nucleus and cytoplasm. The outer nuclear membrane is an extension of the membrane of the endoplasmic reticulum, which synthesizes the lipids for all cell membranes. Proteins are synthesized by ribosomes (ribosome) that are either attached to the endoplasmic reticulum or suspended freely in the cell contents. The mitochondria (mitochondrion), the oxidizing and energy-storing units of the cell, have an outer membrane readily permeable to many substances, and a less-permeable inner membrane studded with transport proteins and energy-producing enzymes.

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Universalium. 2010.

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Look at other dictionaries:

  • membrane — [ mɑ̃bran ] n. f. • v. 1370; lat. membrana, de membrum « membre » 1 ♦ Anat. Mince couche de tissu qui enveloppe un organe, qui tapisse une cavité ou un conduit naturel (⇒ muqueuse). Fine membrane. ⇒ pellicule. Membranes muqueuses, séreuses,… …   Encyclopédie Universelle

  • Membrane — Mem brane, n. [F., fr. L. membrana the skin that covers the separate members of the body, fr. L. membrum. See {Member}.] (Anat.) A thin layer or fold of tissue, usually supported by a fibrous network, serving to cover or line some part or organ,… …   The Collaborative International Dictionary of English

  • Membrane — Sf dünnes Blättchen, feines Häutchen per. Wortschatz fach. (14. Jh.), spmhd. membrāne Entlehnung. Ist entlehnt aus l. membrāna (eigentlich das die inneren Teile des tierischen Körpers bedeckende Häutchen , auch: Pergament ), zu l. membrum n.… …   Etymologisches Wörterbuch der deutschen sprache

  • membrane — Membrane. s. f. Partie mince, deliée & nerveuse du corps de l animal, servant d enveloppe aux autres parties. Les membranes du corps. les fibres d une membrane. la membrane qui enveloppe le muscle. piquer la membrane, les membranes des muscles.… …   Dictionnaire de l'Académie française

  • membrané — membrané, ée (man bra né, née) adj. Terme de botanique. Qui est aplati comme une membrane …   Dictionnaire de la Langue Française d'Émile Littré

  • membrane — (n.) early 15c., thin layer of skin or tissue, a term in anatomy, from L. membrana a skin, membrane; parchment (skin prepared for writing), from membrum limb, member of the body (see MEMBER (Cf. member)). The etymological sense is that which… …   Etymology dictionary

  • membrane — [mem′brān΄] n. [L membrana, membrane, fine skin, parchment < membrum,MEMBER] 1. a thin, soft, pliable sheet or layer, especially of animal or vegetable tissue, serving as a covering or lining, as for an organ or cell 2. any thin, flexible… …   English World dictionary

  • Membrane — steht für: Thidrekssaga drittes der Pergament Handschriften einer umfangreiche Sagenkompilation des 13. Jahrhunderts in altnordischer Sprache, Membran eine Trennschicht, eine dünne Haut …   Deutsch Wikipedia

  • membrane — membrane. См. мембрана. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • Membrane — vgl. Membran …   Das Wörterbuch medizinischer Fachausdrücke

  • membrane — [n] covering layer film, lamina, leaf, mucosa, sheath, sheet; concept 484 …   New thesaurus

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