BIOS E-1a Lecture 05 092111 annotated

BIOS E-1a Lecture 05 092111 annotated - Lecture 5: Membrane...

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Lecture 5: Membrane Structure and Function Reading: Chapter 5 (concepts 5.1 to 5.4) 1
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Biological Membranes • Framework of the membrane is the phospholipid bilayer • Phospholipids are amphipathic molecules – Hydrophobic (water-fearing) region faces in – Hydrophilic (water-loving) region faces out • Membranes also contain proteins and carbohydrates – Relative amount of each vary 2
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Fluid-mosaic model • Membrane is considered a mosaic of lipid, protein, and carbohydrate molecules • Membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane 3
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Cytosol Glycoprotein Glycolipid Carbohydrate Polar Nonpolar Polar Integral membrane protein Phospholipid bilayer Cholesterol (found only in animal cells) Peripheral membrane proteins Cytosolic leaflet Extracellular leaflet Extracellular environment HO Fluid-mosaic model of membrane structure 4
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Proteins bound to membranes • Integral or intrinsic membrane proteins – Transmembrane proteins • One or more regions that are physically embedded in the hydrophobic region of the phospholipid bilayer – Lipid-anchored protein • Covalent attachment of a lipid to an amino acid side chain within a protein • Peripheral membrane or extrinsic proteins – Noncovalently bound to regions of integral membrane proteins that project out from the membrane, or they are bound to the polar head groups of phospholipids 5
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Extracellular environment Cytosol 1 7 2 6 3 4 5 Transmembrane α helix Transmembrane protein Lipid Peripheral membrane protein Lipid- anchored protein Types of membrane proteins 6
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Membranes are semifluid • Fluidity: individual molecules remain in close association yet have the ability to readily move within the membrane • Semifluid: most lipids can rotate freely around their long axes and move laterally within the membrane leaflet • “Flipflop” of lipids from one leaflet to the opposite leaflet does not occur spontaneously – Flippase requires ATP to transport lipids from one leaflet to another 7
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(a) Spontaneous lipid movements (b) Lipid movement via flippase Lateral movement Rotational movement Flip-flop Flippase ATP ADP + P i Movements of lipids in the bilayer 8
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Factors affecting fluidity • Length of fatty acyl tails – Shorter acyl tails are less likely to interact, which makes the membrane more fluid • Presence of double bonds in the acyl tails – Double bond creates a kink in the fatty acyl tail, making it more difficult for neighboring tails to interact and making the bilayer more fluid • Presence of cholesterol – Cholesterol tends to stabilize membranes – Effects depend on temperature 9
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Experiments on lateral transport of proteins 10
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Not all integral membrane proteins can move • Depending on the cell type, 10–70% of membrane proteins may be restricted in their movement • Integral membrane proteins may be bound to components of the cytoskeleton, which restricts the proteins from moving laterally
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BIOS E-1a Lecture 05 092111 annotated - Lecture 5: Membrane...

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