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Chapter 5

Membrans and Transport - Textbook Chapter 5.docx

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Biology 1202B
Richard Gardiner

Lecture 2: Chapter 5 Membranes and Transport An overview of the Structure of Membranes - One of the key evolution of life was the development of the cell or plasma membrane o Selectively permeable barrier o Allows for the uptake of nutrients and elimination of waste products o Maintains a protective environment for metabolic process to occur o Example Nuclear envelope, which defines the hallmark of the eukaryotic cell, the nucleus. Fluid Mosaic Model of Membranes - Membranes are not rigid with molecules locked into place but rather consist of fluid lipid molecules in which proteins are embedded and float freely - Double later, bilayer, - 10 nanometers thick - bilayer vibrates, flex back and forth, spin around there long axis move side ways and exchange places within the same bilayer half - exchanging places within a layer occurs millions of times a second - wide assortment of different types of proteins each with a specific function - proteins involved in transport and attachment - the proteins move more slowely in the fluid environemtn of the membrane - small number of membrane protein anchor cytoskeleton filament to the membrane and thus do not move - glycolipids and glycoproteins – carbohydrate groups - proportion of lipid to protein varies considerable depending on the type of membrenae o inner mitochondrial membranes – large percent protein o plasma membrane – equal amount o myeline membrane – large percent lipid - Membrane asymmetry: one half of the membrane is different then the other half, different functions performed on each hald o External and internal side o Hormones and growth factors bind to receptor proteins that are found only on the external surface of plasma memreane The fluid mosaic model is supported by two major pieces of experimental evidence 1. Membranes are fluid a. David Frye and Michael A Edidin – mouse and human proteins grown in skin culture and the two proteins mixed and fused membranes. 2. Membrane asymmetry a. Freeze fracture technique in combination with electron microscopy b. Break open membrane and use electron microscope to see the membrane were it clearly depicts the two different sides The Lipid Fabric of a membrane - The lipid molecule is the foundation of all biological membranes - Keep membranes in a fluid state is important to membrane function - Many organism can adjust the type of lipids in the membrane such that membranes do not become too stiff (viscous) or to fluid (liquid) Phospholipids are the dominant lipids in the Membrane - phospholipids: two fatty acid tails, liked to one of several types of alcohols or amino acids by a phosphate group o they are amphipathic  contains a region of hydrophobic and a region of hydrophilic  fatty acid chains are hydrophobic (nonpolar)  phosphate contain head of hydrophilic (polar) o when added to aqueous solution , phospholipids associate with each other and assemble into a bilayer Membrane Fluidity - Influenced by two major factors o The composition of the lipid molecules that make up the membrane  Saturated hydrocarbons – straight shape, therefore pack more tightly  Unsaturated hydrocarbons – less straight, therefore pack more loosely o The temperature  Temperature drops too low phospholipids becomes tightly packed and form a highly viscous semisolid gel  The more UNSATURATED hydrocarbons the lower the gelling temperature is  The more space will exist between neighboring lipids and the more fluid the resulting membrane will be Organisms can adjust Fatty Acid Composition - Exposure to low temperature may result in membrane viscosity increasing to the point where normal membrane permeability is inhibited o Causing enzymes and other proteins in the membrane to stop functioning o If membrane solidifies, electron transport ceases to operate - At high temperatures membranes may become too fluid, results in membrane leakage, (ions such as K+, Na+, Ca2+ begin to diffuse too often) this results in a irreversible disruption of cellular ion balance that can rapidly lead to cell death - Prokaryotes, protists and plants can survive at far below the temperature that animal membranes would solidify, o Due to the fact that they can increase the relative proportion of unsaturated fatty acids in there membranes o Unsaturated fatty acid are produced during fatty acid synthesis through the action of a group of enzymes called Desaturases  There are a wide range of these enzymes, each one adding a double bond at a specific point  Transcript abundance of desaturase gene increases as the temperature is lowered  Important to realize that high temperature and low temperature are relative terms - Sterols also influence membrane fluidity, the best example of a sterol is cholesterol o Found in animal membranes but not plants - Acts as a membrane buffer - At high temperatures they help restrain the movement of lipid molecules , thus reducing the fluidity of the membrane - Lower temperatures, disrupt fatty acids from associating by occupying space between lipid molecules, thus slowing the transition to the non fluid gel state Membrane proteins - four major functional categories o Note that all these function may exist in a single membrane and that one protein or protein complex may serve more that one of there functions 1. Transport a. Proteins may provide a hydrophilic channel that allows movement of a specific compound b. A membrane protein may change its shape and by doing so shuttle specific molecules from one side of a membrane to the other 2. Enzymatic Activity a. Enzymes are membrane proteins 3. Signal transduction a. Receptor proteins, bind to chemical hormones b. On binding these receptors trigger changes on the inside surface of the membrane that lead to transduction of the signal through the cell 4. Attachment/recognition a. Proteins exposed to both Internal and external membranes surfaces act as attachment point for a range of cytoskeleton elements, b. Cell – cell recognition  all proteins are either a integral or peripheral membrane protein Integral membrane proteins - membrane proteins that are embedding in the phospholipids bilayer are called integral membrane proteins - one region that interacts with the hydrophobic core of the membrane - most are transmembrane proteins, thus span the entire width of the membrane o exposed to aqueous environments on both sides of the membrane - To interact with hydrophobic core, composed of regions of predominantly non polar amino acids that are usually coiled into alpha helices o About 17 to 20 amino acids in length  length to span the membrane once o Usually span the membrane multiple times o Thus usually linked together by portion of the protein that conists mainly of polar amino acids because these regions are exposed to the aqueous environment on either side of the membrane Peripheral Membrane Proteins - surface of the membrane - do not interact with the hydrophobic core of the membrane - held to the membrane by non- covalent bonds o hydrogen bonds or ionic bonds o usually by interacting with the exposed portions of integ
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