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

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Unformatted text preview: 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 • 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 • 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....
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This note was uploaded on 06/26/2011 for the course BIOL 2107H taught by Professor Darvill during the Fall '10 term at UGA.

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

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