Cell Biology

... from active transport to vesicles

cell membranes

Cell membranes provide adjustable barriers between the cell and the extracellular environment (ESF) or adjacent cells in eukaryotes. The membranes of cellular organelles provide functional compartmentalization from the cytosol.

The cell itself is surrounded by the plasma membrane, and specific functional membranes form intracellular organelles (endoplasmic reticulum, Golgi apparatus) or isolate the contents of cellular organelles (chloroplasts, endosomes, exosomes, lysosomes, mitochondria, peroxisomes) from the cytoplasm. art - cell membrane translucent : art - cell membrane opaque :

Left -click to enlarge image: Schematic three dimensional cross section of a cell membrane. There are two major components of this dynamic, fluid, structure: lipids and proteins. The lipid bilayer provides the basic structure within which proteins are free to diffuse. Sugar moieties can be present as part of either proteins (glycoproteins) or of lipids (glycolipids). Cholesterol intercalates between lipid molecules and affects membrane fluidity/stability.

Cell membranes are variably constituted of carbohydrates (adhesion and address loci), phospolipid bilayers (hydrophobic barriers), and proteins, which control permeability and cellular signalling. Peripheral membrane proteins are confined to the surfaces of membranes while integral membrane proteins are embedded in the membrane and may pass through the lipid bilayer one or more times.

Specialized membrane proteins function in cell adhesion (junctions) and as energy transducers, enzymes, ion channels, pumps, and receptors for neurotransmitters and hormones. Cell junctions utilize proteins that anchor cells together (desmosomes), that occlude passage of water between cells (tight junctions), and that permit direct communication between cells (gap junctions).

Lipid rafts are mobile areas within cell membranes that are more rigid than the rest of the bilayer by virtue of their enrichment in different lipids, cholesterol, and proteins. []fm, fm2[]Several types of lipid raft have been postulated: inside rafts (PIP2 rich and caveolae) and outside rafts (GEM) constituted of the three postulated compositions: caveolae (with caveolin-1), glycosphingolipid enriched membranes (GEM), and polyphosphoinositol rich rafts. Caveolin-1 is a 21kDa cholesterol binding, integral membrane protein. Muscle-expressed caveolin-3, which is involved in some types of muscular dystrophy, forms muscle-type caveolae.

Glycosphingolipids, and other lipids with long, straight acyl chains are preferentially incorporated into the rafts such that fatty-acid chains tend to be extended and thus more tightly packed, creating higher order domains. It is believed that rafts exist in a separate ordered phase that floats within the regular sea of poorly ordered lipids.

It is most likely that lipid rafts are involved in cellular signaling. Many actin binding proteins (ABP) bind to, and are regulated by polyphosphoinositides. These ABPs include gelsolin, which is a Ca2+, pH and polyphosphoinositide-regulated actin capping/severing protein that partitions neural membranes into biochemically isolated rafts. []fluorescence micrograph gelsolin[] GEMs also appear to link to the actin cytoskeleton through ABPs, in particular ERM proteins through EBP50, which binds members of the ERM proteins through the ERM C-terminus.

Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows plasma and organellar membranes, a lipid raft, and exocytosis; and, Interpretation: Inner Life of the Cell Џ

• A • adhesion • C • cell membranescellular signal transductioncentrioleschemotaxischloroplastciliacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R •receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

diagram . desmosome : tem_desmosome : art - tight junction zonula adherens desmosome gap : diagram . tight junction : diagram . gap junctionball-stick - globular proteins in phospholipid bilayer : ball-stick - carrier proteins : ball-stick - marker protein : ball-stick - marker proteins : ball-stick - receptor proteins : ball-stick - ion channel proteins: animation - carrier proteins : animation - receptor protein : animation - cholesterol


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. . . developing since 10/06/06