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Chapter 5

Bio1140 Notes Chapter 5.pdf

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Kathleen Gilmour

Chapter 5 Cell Membranes and Signalling 5.1 Overview of Structure of Membranes A membrane consists of proteins in a fluid made up of lipid molecules Fluid mosaic model consist of proteins and lipids that are semi-liquid and mobile lipids form a bilayer around 10nm thick a lipid molecule does not flip flop back and forth between the two layers for the most part proteins move more slowly due to their larger size This model is supported by two major ideas: 1. 1970- David Frye and Micheal A. Edidin combined human and mouse cells and tagged each protein ~ saw that the protons on the surfaces of the membranes began to mix 3. Membrane Asymmetry Freeze-fracture technique block of cells frozen in liquid nitrogen fracture the cell ~ splits bilayer into two inner and outer halves shows that the particles on the two layers differ in size, number, and shape therefore, both sides are distinct and different 5.2 The Lipid Fabric of a Membrane foundation of membrane is made up of lipid molecules; which are composed of mainly phospholipids, but also steroids organisms can adjust membrane fluidity lipid bilayer is formed of phospholipids each phospholipid consists of a head group bound to two long carbon/hydrogen chains called fatty acids head group consists of glycerol linked by phosphate group to one of several alcohols/amino acid groups so in essence, [x] linked by phosphate to glycerol + 2 fatty acids molecules are amphipathic, head group = polar = hydrophilic, fatty acids = nonpolar = hydrophobic when lipids are added to an aqueous solution they assemble into one of three things: micelle lyposome bilayer this is due to its amphipathic nature saturated = straight linear bonds = less liquid membrane due to more packing unsaturated = double bonds in lipids btwn carbons = more fluid membrane due to less packing together requires a lower temperature than for saturated membranes in order to become solid in other words, the more unsaturated the membrane, the lower the temperature may fall before the membrane solidifies and stops functioning for most organisms, membrasne have both types of lipid Low temperature causes membranes to gel this prevents membrane permeability, membrane bound enzymes from functioning, ETC due to the requirement of rapid migrating of molecules within the 2 membranes high temperature membrane leakage due to too permeable/too liquid membrane ions begin to freely diffuse across the membrane ~ leading to disrupted ion balance = cell death Organisms can adjust the fatty composition of their membranes to resist temperature changes Ectotherms (bacteria, archea, protists, plants) can surivive in rly low temps due to adjustments in their membrane structures all fatty acids are initially synthesized as a linear saturated chain desaturase enzymes remove 2 hydrogen bonds each of two adjacent carbons and form a double bond desaturase abundance is regulated by gene transcription by regulating desaturase abundance, organisms regulate membrane fluidity in various temps. Sterols also influence membrane fluidity ex: cholesterol, which is found in animal cells act as buffers as temp goes up, they restrain movement of lipid molecules = less fluid as temp goes down, they disrupt fatty acids from associating by occupying space in between 5.3 Membrane Proteins Membrane proteins can be separated into four groups: 1. Transport - proteins can make hydrophilic channels to allow movements of specific compounds, or use conformation changes to shuttle compounds across 2. Enzymatic activity - such as in the ETC 3. Signal transduction - receptor proteins bind chemicals that trigger changes in the inside of the cell leading to signal transduction 4. Attachment/recognition - components on either side of membrane allow for sites of attachment/cell-to-cell recog. Integral membrane proteins proteins that are imbedded in the phospholipid bilayer transmembrane proteins proteins that transverse the entire lipid bilayer domain that interacts with lipid bilayer consists of non-polar animo acids (about 17-20) that form an alpha helix portions exposed on either side of membrane are composed of polar amino acids Peripheral membrane proteins positioned on the surface of the membrane and do not interact with the hydrophobic core they are held to the membrane surface by noncovalent bonds ~ primarily hydrogen bonds and ionic bonds 5.4 Passive Membrane Transport Passive transport diffusion drives passive transport No ATP expendature the larger the gradient, the faster the rate of diffusion diffusion is driven by entropy simple diffusion molecules move directly across membrane no use of transporter depends on molecular size & lipid solubility small
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