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Membrane transport
2019.10.01
Dr. Ildiko Pecsi
Dept. of Biophysics, University of Pécs
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The cellular organization
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Biological membranes / cell membranes
Major functions of the cytoplasmic membrane
Semipermeable membrane- transport of different molecules
Anchor soluble proteins- transport, signaling, bioenergetics
Energy storage- generation of proton motive force
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Biological membranes / cell membranes
Key functions:1. Barrier, semi-permeable lipid bilayer
2. Specific functions – integral or peripheralmembrane proteins
3. Regulation of the transport of molecules- aktive, passive, facilitated difussion, endo-exocytosis
4.Detection of signal molecules and transmission
5. Cell-cell communication
6. Cell movement
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Biological membranes / cell membranes
Structure Phospholipid
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Red blood cell
10 6 lipid molecule/ µm2
6-8
nm
Singer and Nicholson fluid mosaic model from1972
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Constituents of membrane lipid bilayers
Amphiphilic molecules
Phospholipids (phosphoglycerides)• 3-carbon backbone of glycerol• 2 long- chain fatty acid esterified to C1 and C2 of the glycerol• The main constituent• Synthetized in the ER Sphingolipids• sphingosine+ fatty acid=ceramid• ceramid+phosphate+choline=sphingomyelins;• Synthetized in the Golgi • More abundant in the plasma membrane then in
the membranes inside cells
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Constituents of membrane lipid bilayers
Glycolipids
• 3 types of glycolipids:• Sphingolipids• Glycerol glycolipids with sugar chains attached to the hydroxyl
on C3of diglycerides• Glycosylphosphatidylinositols (GPI)They are on the outer membrane surface and not incorporatedinto membrane bilayers.
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Sterols
CholesterolThe rigid four ring structure is apolar so it inserts into the core of bilayers• regulation the fluidity of the membrane• temperature• short and/or unsaturated bonds- influence the physical properties ofmembranes
Lipid raft in the outer leaflet of the plasma membrane enriched in cholesterol and sphingolipids.
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Lipid % composition in the hepatocyte rat liver cells of the different cellcompartments
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Mobility within the membrane
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Membranes with barrier functions
The lipid bilayer of cell membranes prevents the passage of most polar molecules
Large uncharged polar molecules and ions cant diffuse throughthe membrane and require transport
In general the smaller the molecule and the more hydrophobic or non polar it is, the more rapidly it will diffuse across the membrane bilayer
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Membrane proteins main functions
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Transport across membrane may occur eitheractively or passively
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Some functions of membrane proteins
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Potassium Channels
Integral proteins with a hydrophilic inner pore viawhich potassium ions may be transported
The channel is comprised of four transmembranesubunits, while the inner pore contains a selectivity filterat its narrowest region that restricts passage ofalternative ions
Potassium channels are typically voltage-gated andcycle between an opened and closed conformationdepending on the transmembrane voltage
Transport proteins
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Uniport mechanism
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Symport and antiport mechanisms
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The sodium-potassium pump
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Active transport
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Example: Membrane potential
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Glycocalyx (cell coat)