ADI Unit 5 Plasma Membrane PDF - Biology Past Paper

9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane General aspects Every cell (eukaryotic or prokaryotic) has a plasma membrane that First studies on plasma allows:...

9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane General aspects Every cell (eukaryotic or prokaryotic) has a plasma membrane that First studies on plasma allows: membrane composition and structure ‐ To maintain the composition of the cell ‐ To regulate the passage of material into and out of the cell (1925) In eukaryotes, there are other membranes in several organelles (1935) All of them have a common structure: a lipid bilayer with proteins and carbohydrates Thickness: 7 to 10 nm ‐ Is it possible to see plasma membrane at the LM? ‐ Is it possible to see plasma membrane at the EM? Membranes and their cellular functions J.B. (1972) Finean, R. Coleman and R.H. Michell Cell Biology and Histology Cell Biology and Histology Unit 5: Plasma membrane Unit 5: Plasma membrane Membrane structure Membrane composition At EM: Trilaminar structure carbohydrate Chemically cell membrane is composed of three components namely: Lipids 3 types: 1. Phospholipids 2. Glycolipids 3. Cholesterol Proteins Carbohydrates Cell Biology and Histology Cell Biology and Histology  9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane 1. Phospholipids (PL) 1. Phospholipids (PL) Amphipathic (or amphiphilic) molecules: ‐ Polar head (hydrophilic) ‐ 2 nonpolar tails (hydrophobic) In the membrane there are 4 PL with different polar heads phosphatidylethanolamine phosphatidylserine phosphatidylcholine sphingomyelin Cell Biology and Histology Cell Biology and Histology Unit 5: Plasma membrane Unit 5: Plasma membrane Arrangements of amphiphilic molecules in an aqueous environment Arrangements of amphiphilic molecules in an aqueous environment Liposome Artificial lipid bilayer Cell Biology and Histology Cell Biology and Histology  9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane 2. Glycolipids Glycocalix Amphipathic Contain sugar residues Cell Biology and Histology Cell Biology and Histology Unit 5: Plasma membrane Unit 5: Plasma membrane 3. Cholesterol Proteins Integral proteins: Incorporated within the lipid bilayer More hydrophobic regions More interactions with lipids Harder extractable Peripheral proteins: Less hydrophobic regions More hydrophylic regions exposed to water on either side of the membrane Less interaction with lipids More easily extractable Most of these proteins are not bound rigidly in place and are able to move laterally Cell Biology and Histology Cell Biology and Histology  9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane Proteins Fluid mosaic model (1972) 1‐3 4‐8 Mosaic: The different components are like the pieces of a mosaic Transmembrane Exposed to water on either side of the membrane Fluidity: The membrane is a bidimensional fluid. Its fluidity depends on the Incorporated within the lipid bilayer movement of the components, particularly, the lipids Cell Biology and Histology Cell Biology and Histology Unit 5: Plasma membrane Unit 5: Plasma membrane Asymmetry of the membrane The mobility of phospholipids and proteins in the membrane Phosphatidylcholine Sphingomyelin Phosphatidylethanolamine Phosphatidylserine ‐ Asymmetrical distribution of the 4 PL in the two monolayers ‐ Sugar residues only in the external hemilayer ‐ Asymmetrical distribution of proteins Cell Biology and Histology Cell Biology and Histology  9/5/2019 Unit 5: Plasma membrane Unit 5: Plasma membrane Specimen Lipid rafts Specimen Specialized membrane patches with higher concentrations of cholesterol and Liquid N2 saturated fatty acids which reduce lipid fluidity. In these domains proteins are usually less mobile (for instance, proteins involved Cold knife in the transduction of signals from outside the cell). Freeze fracture or Cold knife cryofracture Specimen Extracellular Cytoplasm P face E face Metal shadowing Cell Biology and Histology Cell Biology and Histology