Plasma Membrane (Structure) PDF

Cell membrane (structure) Content of the lesson 1. General structure 2. Molecular composition 3. Plasma membrane (PM) asymmetry 4. Fluid mosaic model (Singer & Nicholson, 1970) 5. Mobility of the PM molecules 6. PM fluidity  1.General structure Trilamellar Structure...

Cell membrane (structure) Content of the lesson 1. General structure 2. Molecular composition 3. Plasma membrane (PM) asymmetry 4. Fluid mosaic model (Singer & Nicholson, 1970) 5. Mobility of the PM molecules 6. PM fluidity  1.General structure Trilamellar Structure 3 bands (leaflets) Fig.1. Microscopic structure of the plasma membrane.  2.Molecular composition Phospholipids Lipids (49%) Sphingolipids Steroïds Intrinsic proteins Proteins (43%) Extrinsic proteins Glycolipids Carbohydrates(8%) Cell coat Glycoproteins =Glycocalyx  2.Molecular composition (Phospholipids) Glycerol Phosphoric acid Fatty acids Fig. 2. Chemical structure of a Phospholipid 2.Molecular composition(Phospholipids) Fig.3. General structure of a Phospholipid. 2.Molecular composition (Sphingolipids) Shpingosine: the basic molecule 18 C 2.Molecular composition (Sphingolipids) Shpingosine + fatty acid= Ceramide Ester bond 2.Molecular composition (Sphingolipids) Ceramide + X= Sphingolipid Ceramide + sugar= Glycosphingolipid Ceramide + Choline= Sphingomyelin  2.Molecular composition (Steroids) The most important is cholesterol for animal cells and ergosterol for plant cells; Cholesterol has one polar group (the alcohol function), and one apolar group; It stabilizes membranes by intercalating between phospholipid molecules, thus preventing excessive fluidity.  2.Molecular composition (Steroids) Apolar group Polar group 2.Molecular composition (Proteins) GPI Fatty acid Fig.4. Types of membrane proteins.  2.Molecular composition (Carbohydrates) They combine with lipids and proteins on the extracellular side to form glycolipids and glycoproteins. Form Glycocalyx (monosaccharides glucose, glucosamine, galactose, galactosamine, mannose and sialic acid). The role: (i)protect the cell from mechanical and chemical aggression, e.g. proteolytic degradation, (ii)play a role in cell identification (MHC, blood groups, etc.), (iii) cell adhesion.  3.PM asymmetry The arrangement of the various membrane molecules gives the plasma membrane a certain asymmetry resulting from:  The different composition of lipid molecules in the 2 layers;  The presence of oligosaccharides, glycoproteins and glycolipids directed towards the extracellular surface, forming the glycocalyx;  The arrangement, number and nature of the proteins anchored in the two layers;  Cytoskeleton binding to PM always on the cytosolic side.  4.Fluid mosaic model (Singer &t Nicholson, 1970)  5.Mobility of PM molecules Lipids (4 types of mouvements) Proteins (mobile) Carbohydrates (immobiles) 5.Mobility of PM molecules 5.Mobility of PM molecules  6.Fluidity The fluid state corresponds to molecular agitation without the need of energy; this fluidity increases when:  Hydrocarbon chains are shorter and richer in double bonds, because double bonds weaken interactions between neighboring chains.  Temperatures are high.  Cholesterol content is low.