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Bio 1201 Chapter 7 -

Bio 1201 Chapter 7 - - CHAPTER 7 MEMBRANE STRUCTURE AND...

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CHAPTER 7 MEMBRANE STRUCTURE AND FUNCTION Membranes: Basic Structure – 6.2.4 Membrane structural models have evolved as new data is acquired. Earliest models were deduced from indirect evidence We didn’t have the ability to see the membranes so it was harder Biochemical detective work I. Membranes A. Membranes are made of lipid 1. (Evidence) Lipid and lipid soluble material enter quicker than substances, which are insoluble in lipids. B. Phospholipid s are amphipathic molecules that can form membranes 1. It means that it can interact in water environments as well as hydrophobic environments * amphipathic – has a polar as well as a nonpolar portion A condition where a single molecule has both hydrophilic and hydrophobic C. Cell membranes are phospholipid bilayers (see fig 7.2) 1. (Evidence) The phospholipid content of red blood cells is just enough to cover the cell with two layers; the red blood cell has no internal membranes No organelles D. Biological membranes contain proteins 1. (Evidence) Membranes isolated from red blood cells contain proteins in addition to lipids Protein interact with water very well Electron microscope – can see very small things E. Biological membranes are coated with proteins that generally absorb water 1. Biological membranes absorb more water on their surface than artificial phospholipid membranes II. Davson-Danieli model phospholipid (PL) bilayer globular protein coatin Hydrophilic zones = polar head groups + globular proteins Hydrophobic zone = 2 x hydrophobic tails about 8 mm thick 1. Confirmed in 1950's 2. Problems with this model: a. All membranes are not the same b. How do the proteins stay attached? III. Singer and Nicolson: FLUID-MOSAIC MODEL (see fig 7.3) Proteins embedded in the lipid bilayer Hydrophilic zones = polar head groups + hydrophilic portion of proteins Hydrophobic zone = 2 x hydrophobic tails + hydrophobic portion of proteins Confirmed by freeze-fracture and freeze-etch micrographs (see fig 7.4) – freeze the cell membrane and split it so that it reveals only a single layer of phospholipids Some proteins were sitting on the surface and being held there by reactions * Fluid – lateral two-dimensional movements of the lipids and some of the proteins in the plane of the membrane; transverse (flip-flop) across the membrane is rare; side to side motion A. Factors which influence fluidity of membranes Lipid composition and temperature; you can adjust it by having more saturation Double carbon bonds Presence of cholesterol * Mosaic – means that there are proteins interspersed within the lipid bilayer The Plasma Membrane: The Fluid Mosaic Model – 7.1.2 A membrane is a fluid mosaic of lipids, proteins, and carbohydrates.
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