Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Chapter 11: Membrane Structure• lipid components• membrane proteinsChapter 12: Membrane Transport• permeability• transport across the membrane• membrane voltage• stress-activated ion channels• voltage-gated channels• the action potential• generation of the resting membrane potential(3rd ed: p. 363-372, 379-384, 387-416)4th ed. pages: 359-368 376-380383-409The basis of all biological membranes is the lipid bilayer, which forms spontaneously when membrane lipids are placed in water.• phospholipids: polar (hydrophilic) head group and non-polar (hydrophobic) hydrocarbon tails (fatty acids)The cell membranePhospholipids and membrane formationThe fluidity of the cell membraneIn the fluid mosaic biological membrane, lipids and proteins diffuse laterally within the lipid layer.Hybrid cells illustrate the fluidity of the cell membraneCholesterol• polar head group: -OH• nonpolar groups: steroid ring structure and hydrocarbon tailCholesterol:• will not form a lipid bilayer by itself• inserts into pre-formed lipid bilayers• function: modulates membrane fluidity- at high temperatures, cholesterol restrains themovement of fatty acids (maintaining stability)- at low temperatures, cholesterol prevents tightpacking of fatty acids (maintaining fluidity)Lipid components of the cell membraneLipid components of the cell membraneSome lipid components are distributed asymmetrically in the plasma membrane bilayer• glycolipids are always in the outer layer of the membrane• charged phospholipids (phosphatidylinositol andphosphatidyserine) are always in the inner layer of themembraneLipid components of the cell membraneGlycolipids: cell identity markersCharged phospholipids• transporters (Examples: carrier proteins and ion channels)• anchors for extracellular matrix and/or cytoskeletal proteins(Examples: integrins)• surface receptors (Example: hormone receptors)• enzymic proteins (Example: tyrosine kinases)Protein components of the cell membraneProtein components of the cell membraneGlycoproteins & proteoglycans1) Protective carbohydrate layer (along with glycolipids)2) Cell identity markersPermeability of the lipid membraneBiological membranesare selectively permeable, regulating the cell’s molecular traffic.Passive Transport - Simple diffusion:• diffusion of gases, small hydrophobic molecules and smallpolar molecules across the lipid bilayer (no chargedmolecules!)• molecules cross the cell membrane freely• net movement is down the concentration gradient • no transporter proteins are required > no energy is requiredPermeability of the lipid membraneOsmosis: simple diffusion of waterPassive transport - Facilitated diffusion:• diffusion of molecules that can not move freely across thelipid bilayer: large polar molecules, charged moleculesand ions• depends on transporters - Carrier proteins - Channel proteins• net movement is down the concentration gradient > no energy is requiredPermeability of the lipid membraneFacilitated diffusion through carrier proteins and ion channelsActive transport:• the transport of molecules that can not cross the membranefreely (large polar molecules, charged molecules, andions) against their concentration gradient• depends on special types of carrier proteins - Pumps> energy is required (in the form of ATP)Permeability of the lipid membranePermeability of the lipid membranepumpPermeability of the lipid membraneTypes of membrane proteins involved in facilitated diffusionand active transport:• channels• gated channels• carriers• pumps(energy-dependenttransporters) Facilitated diffusion Active transportChannelsChannel - pore that allows passage of ions or small molecules• no energy is required• no conformation changes occurCarriers are proteins that transport molecules down theirconcentration gradients by a conformation change. no energy is requiredThe glucose transporterCarriersElectrochemical gradientNa+K+Categories of gated ion channels= Stress-gated• sound waves are collected by the auditory canal andconcentrated at the tympanic membrane (the eardrum)• these waves are relayed as vibrations through three auditoryossicles (malleus, incus, and stapes) in the middle ear• these vibrations are received by the cochlea through theoval window (the interface between the air in the middleear and the fluid inside the cochlea)• the sound waves then travel as pressure waves through theperilymph fluid inside the vestibular duct, then returnthrough the tympanic duct, and are finally dissipatedthrough the round windowThe physiology of hearingMiddle and inner earUncoiledThe cochleaThe physiology of hearingThe neurotransmission of the hearing impulse begins with theopening of mechanically-gated potassium channels.• sound waves cause the basilar membrane to vibrate, pushingthe stereocilia up against the tectorial membrane• when the bundles of stereocilia of the hair cells tilt, thelinking filaments are stretched, opening gated ion channels• the opened channels allow K+ ions to pass into the cell, causing an ion current and initiating the hearing sensationStereocilia on apical surface of a hair cellStress-activated ion channelsElectric impulses travel along axon to the nerve terminal.• the propagation of electric impulses is caused by ion flowacross the axon membrane by the alternating openingand closing of ion channels (Na+ and K+)The neuron• established by the charge difference between the two sides of the membrane• Vm measured in millivolts (mV)Voltage-gated ion channelsrespond to the membrane potentialMembrane potential – voltage across the membrane:The voltage-gated Na+ channel_ _++++_ __ _++++_ _At rest: the membrane voltage is at -80 mV (polarized: inside is negative) and the voltage-gated Na+ channels are closedVoltage-gated Na+ channels and the action potentialPhysiology of the nerve impulseThe rising phase:• the voltage-gated Na+ channels open if the membranevoltage reaches the threshold potential (-60 mV) due toa depolarizing stimulus current• the inflow of Na+ causes membrane to depolarize to +40 mV = action