31) - Membranes Boundary and barrier that controls...

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Membranes Boundary and barrier that controls environment of cell Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability , allowing some molecules to penetrate and others not Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it. Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
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Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Fig 7-4 two layers break apart with low temperatures: extracellular layer and cytoplasmic layer The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip-flop transversely across the membrane Some cells will mix membranes when mixed, hybrid cell, and allows for protein work As temperatures cool, membranes switch from a fluid state to a solid state The temp at which a membrane solidifies depends on the types of lipids Membranes rich in unsat fatty acids are more fluid than those rich in sat fatty acids Membranes must be fluid to work properly they are usually about as fluid as salad oil. Sat fat membranes more viscous and less space The steroid cholesterol has diff effects on membrane fluidity at diff temp At warm temp (such as 37 degrees C) cholesterol restrains move’t of phospholipids At cool temp, it maintains fluidity by preventing tight packing Membrane Proteins and Their Functions A membrane is a collage of diff proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific fxns Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices C-terminus and N-terminus can be either way depending on protein
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Fxn as portals, proton transporter Specific in what they do b/c of 3-D structure, H+ may go thru but not Na+ Six major functions of membrane proteins: o Transport o Enzymatic act o Signal transduction o Cell-cell recog. o Intercellular joining o Attach’t to the cytoskeleton and extracellular matrix (ECM) Fig. 7-9 The Role of Membrane Carbs in Cell-Cell Recog Cells recog each other by binding to surface molec, often carbs, on the plasma membrane Membrane carbs may be covalently bonded to lipids (forming
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