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Chapter 8Chapter 8
Membrane Structure & FunctionMembrane Structure & Function
Membrane StructureMembrane Structure
Selective permeabilitySelective permeabilityControls trafficControls traffic
Known as the Known as the plasma membraneplasma membraneAmphipathicAmphipathic - hydrophobic & - hydrophobic &
hydrophilic regionshydrophilic regionsSinger-Nicolson developed the Singer-Nicolson developed the
fluid mosaic modelfluid mosaic model
Membranes are FluidMembranes are Fluid
Structures related to properties & Structures related to properties & functionfunction
Membrane is usually about as fluid as salad oil.
Fluid Mosaic ModelFluid Mosaic ModelLipidsLipids
PhospholipidsPhospholipids - - membrane membrane fluidityfluidityCholesterol Cholesterol - - membrane stabilizationmembrane stabilization
““Mosaic” Structure due to:Mosaic” Structure due to:Proteins:Proteins:
Integral proteinsIntegral proteins - - transmembrane proteins transmembrane proteinsPeripheral proteinsPeripheral proteins - - surface /appendages surface /appendages
Attachments-framework for animal cellsAttachments-framework for animal cells
Membrane carbohydratesMembrane carbohydrates - -~ cell to cell recognition; ~ cell to cell recognition; oligosaccharides (cell markers);oligosaccharides (cell markers); glycoproteins; glycolipids; ABO glycoproteins; glycolipids; ABO blood typingblood typing
Sidedness of Plasma MembranesSidedness of Plasma Membranes
Carbohydrates only on the outside surfaceCarbohydrates only on the outside surface
Proteins may be anchoredProteins may be anchored– Inside to cytoskeletonInside to cytoskeleton– Outside to the extracellular matrixOutside to the extracellular matrix
Molecules that start on the Molecules that start on the insidinside e face of Golgi Complex end up on the face of Golgi Complex end up on the
outsideoutside face of the plasma face of the plasma membranemembrane
Know positioning of these structures; polar Know positioning of these structures; polar and nonpolar regions of membraneand nonpolar regions of membrane
Membrane StructureMembrane StructureMembrane protein functions:Membrane protein functions: TransportTransportEnzymatic activityEnzymatic activitySignal transductionSignal transductionIntercellular joiningIntercellular joiningCell-cell recognitionCell-cell recognitionECM attachmentECM attachment
Traffic Across MembranesTraffic Across Membranes
EasilyEasily hydrophobichydrophobic
With AssistanceWith Assistance– Hydrophilc: Polar, chargedHydrophilc: Polar, charged
ChannelsChannels ShuttlesShuttles
TransportTransport PassivePassive
– No energy expenditureNo energy expenditure– Diffuse through membraneDiffuse through membrane– Diffuse aided by proteinDiffuse aided by protein
Facilitated diffusionFacilitated diffusion
ActiveActive– Energy expenditure—Energy expenditure—
ATPATP
– Usually against concentration GradientUsually against concentration Gradient– PumpsPumps
cotransportcotransport
– Bulk TransportBulk Transport EndocytosisEndocytosis ExocytosisExocytosis
Passive TransportPassive Transport diffusion of a substance across a biological diffusion of a substance across a biological
membranemembrane No energy exertedNo energy exerted
DiffusionDiffusion - - tendency of any molecule to tendency of any molecule to spread out into available space spread out into available space
Concentration gradientConcentration gradient – – moves from high to moves from high to lowlow
OsmosisOsmosis - the diffusion of water across - the diffusion of water across a selectively permeable membrane; a selectively permeable membrane; DOWN the concentration gradient.DOWN the concentration gradient.
Direction determined only by a Direction determined only by a difference in difference in total total solute concentrationsolute concentration
Rate influenced byRate influenced byTemperatureTemperatureSteepness of conc. gradientSteepness of conc. gradient
Water BalanceWater BalanceOsmoregulationOsmoregulation - - control of water control of water
balancebalance
Comparison of 2 solutions:Comparison of 2 solutions:Hypertonic - Hypertonic - higher concentration of solutes higher concentration of solutes
Hypotonic -Hypotonic - lower concentration of solutes lower concentration of solutes
Isotonic -Isotonic - equal concentrations of solutes equal concentrations of solutes
Water BalanceWater BalanceCells with Walls (plants, bacteria, Cells with Walls (plants, bacteria,
fungi):fungi):Require Require hypotonic external environmentshypotonic external environments
to keep their to keep their turgor pressureturgor pressure (water pressure (water pressure pushing cell membrane out against cell wall)pushing cell membrane out against cell wall)
Become limp or Become limp or flaccidflaccid when lose turgor when lose turgor pressurepressure
PlasmolysisPlasmolysis - - plasma membrane pulls away plasma membrane pulls away from cell wallfrom cell wall
Water BalanceWater BalanceCells without Walls (animals, most Cells without Walls (animals, most
protist):protist):Require Require isotonicisotonic external environments external environmentsHypertonic environmentsHypertonic environments – cells swell & – cells swell &
may burst with too much water pressure may burst with too much water pressure ((CytolysisCytolysis))
May have May have contractile vacuolescontractile vacuoles (some (some protists; paramecium also have less porous protists; paramecium also have less porous membrane) to control internal water pressure membrane) to control internal water pressure
Contractile vacuoles & Contractile vacuoles & Osmoregulation in ParameciumOsmoregulation in Paramecium
Specialized TransportSpecialized TransportUtilizingTransport proteinsUtilizingTransport proteins (with or without (with or without
channels)channels)
Facilitated diffusionFacilitated diffusion - - passage of molecules passage of molecules and ions and ions with transport proteinswith transport proteins across a across a membrane down the concentration gradient.membrane down the concentration gradient.
Specific for its substrateSpecific for its substrate
Facilitated diffusionFacilitated diffusion Transport of water and certain hydrophilic Transport of water and certain hydrophilic
solutes across; down conc. gradient.solutes across; down conc. gradient. Transport ProteinsTransport Proteins Most-very specificMost-very specific 2 types pf proteins2 types pf proteins
– 1. Channel Proteins1. Channel Proteins– 2. Transport Proteins2. Transport Proteins
Facilitated diffusionFacilitated diffusion
1. Channel Proteins1. Channel Proteins Permits rapid flow across membrane.
1. Aquaporins: plants and animals. Discovered in plants in 1994
2. Ion Channels– Many of these are Gated
Channels-– Stimulus to open Electrical or Chemical
Stimulus
– Example: Nerve cell stimulated by a neurotransmitter molecules (chemical) , opens gate, allows Na+ into the cell.
Diseases linked to Ion ChannelsDiseases linked to Ion ChannelsA multitude of human and animal diseases A multitude of human and animal diseases are caused by dysfunction of ion channels. This may are caused by dysfunction of ion channels. This may be genetic, i.e. caused directly by mutations in genes be genetic, i.e. caused directly by mutations in genes coding for ion channels. Such diseases are called coding for ion channels. Such diseases are called ‘channelopathies’. Examples of channelopathies are ‘channelopathies’. Examples of channelopathies are cystic fibrosis, epilepsy, and arrhythmias, e.g. the cystic fibrosis, epilepsy, and arrhythmias, e.g. the long QT syndrome. Also, diseases may result from long QT syndrome. Also, diseases may result from defects caused by mutations in genes coding for defects caused by mutations in genes coding for regulatory proteins. regulatory proteins.
http://www.sophion.dk/Technology/ion_channelshttp://www.sophion.dk/Technology/ion_channels
LE 7-15aLE 7-15a
EXTRACELLULARFLUID
Channel protein Solute
CYTOPLASM
Facilitated diffusionFacilitated diffusion
2. Transport Proteins2. Transport Proteins Undergoes subtle change in shape.Undergoes subtle change in shape. Alternates between 2 conformations, moving Alternates between 2 conformations, moving
molecule across as changes.molecule across as changes.
Facilated DiffusionFacilated Diffusion
2. Carrier Proteins: Undergo conformational 2. Carrier Proteins: Undergo conformational change, change, solute is transported as the protein solute is transported as the protein changes shape.changes shape.,,
Active transportActive transportPumpPump substance across membranes, substance across membranes,
againstagainst its concentration gradient, its concentration gradient, through a a Carrier Protein, with the help through a a Carrier Protein, with the help of of cellular energy (ATP)cellular energy (ATP)
Example: How cells maintain a higher Example: How cells maintain a higher concentrations of K+ concentrations of K+ insideinside cell cell
Active Transport SystemsActive Transport Systems
1. 1. The sodium-potassium pumpThe sodium-potassium pump.. Specific kind of Active TransportSpecific kind of Active Transport Esp. Esp. important in animalsimportant in animals Exchanges Na+ for K+Exchanges Na+ for K+ Transports Transports
– 3 Na3 Na+ out for every + out for every 2 K2 K+ into the cell.+ into the cell.
Restores normal Restores normal electrochemical gradientelectrochemical gradient following following an action potential. an action potential.
LE 7-16LE 7-16
Cytoplasmic Na+ bonds tothe sodium-potassium pump
CYTOPLASMNa+
[Na+] low[K+] high
Na+
Na+
EXTRACELLULARFLUID
[Na+] high[K+] low
Na+
Na+
Na+
ATP
ADP
P
Na+ binding stimulatesphosphorylation by ATP.
Na+
Na+
Na+
K+
Phosphorylation causesthe protein to change itsconformation, expelling Na+
to the outside.
P
Extracellular K+ bindsto the protein, triggeringrelease of the phosphategroup.
PP
Loss of the phosphaterestores the protein’soriginal conformation.
K+ is released and Na+
sites are receptive again;the cycle repeats.
K+
K+
K+
K+
K+
Maintenance of Membrane Potential Maintenance of Membrane Potential by Ion Pumpsby Ion Pumps
Membrane PotentialMembrane Potential: The voltage (charge : The voltage (charge separation) across a membraneseparation) across a membrane– Cytoplasm of a cell is negativeCytoplasm of a cell is negative– Extracellular fluid is positiveExtracellular fluid is positive
Ranges from -50- -200mVRanges from -50- -200mV
Favors diffusion of cations into the cell, and anions out of Favors diffusion of cations into the cell, and anions out of cellscells
..
The The electrochemical gradientelectrochemical gradient: a combination : a combination of 2 forces that influence the movement of of 2 forces that influence the movement of ions across membranes.ions across membranes.– A chemical forceA chemical force (the ion’s concentration (the ion’s concentration
gradient)gradient)– An electrical force (the effect of the membrane An electrical force (the effect of the membrane
potential on the ion’s movement).potential on the ion’s movement).
– SO…….ions move across membranes down SO…….ions move across membranes down their electrochemical gradienttheir electrochemical gradient..
• An An electrogenic pumpelectrogenic pump is a transport protein is a transport protein that generates voltage across a membrane.that generates voltage across a membrane.
The main electrogenic pump of The main electrogenic pump of animals is a animals is a sodium-potassium pump.sodium-potassium pump.
The main electrogenic pump of plants, fungi, The main electrogenic pump of plants, fungi,
and bacteria is and bacteria is a proton pump—a proton pump—activelyactively transports H+ out of cells.transports H+ out of cells.
LE 7-18LE 7-18
H+
ATP
CYTOPLASM
EXTRACELLULARFLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
CotransportCotransport An ATP powered pump indirectly drives the An ATP powered pump indirectly drives the
Active Transport of several other solutes.Active Transport of several other solutes.
Example: Sucrose - H+ CotransporterExample: Sucrose - H+ Cotransporter– Sucrose loading in plantsSucrose loading in plants
CO-TRANSPORTCO-TRANSPORT... movement of 2 solutes together ... movement of 2 solutes together - -
Often moves 1 solute passively & other activelyOften moves 1 solute passively & other actively
Ex: 1) H+ pump coupled with sucrose transport Ex: 1) H+ pump coupled with sucrose transport ( ( H+symportH+symport** ) ) 2) 2) epithelial transportepithelial transport* * Na+glucose model ( Na+glucose model ( glucose absorptionglucose absorption** ) )
Replenish ions, sugar, fluid following strenuous Replenish ions, sugar, fluid following strenuous exercise, diarrheaexercise, diarrhea
LE 7-18LE 7-18
H+
ATP
CYTOPLASM
EXTRACELLULARFLUID
Proton pump
H+
H+
H+
H+
H+
+
+
+
+
+
–
–
–
–
–
LE 7-19LE 7-19
H+
ATP
Proton pump
Sucrose-H+
cotransporter
Diffusionof H+
Sucrose
H+
H+
H+
H+
H+
H+
+
+
+
+
+
+
–
–
–
–
–
–
Bulk transport across the Bulk transport across the plasma membrane occurs by plasma membrane occurs by exocytosis and endocytosisexocytosis and endocytosis
Large moleculesLarge molecules, such as polysaccharides and , such as polysaccharides and proteins, cross the membrane via proteins, cross the membrane via vesicles.vesicles.
1. Exocytosis1. Exocytosis In In exocytosisexocytosis: cell secretes : cell secretes
macromolecules.macromolecules. Vessicle buds from Gogi.Vessicle buds from Gogi.
Many secretory cells use this—Many secretory cells use this—– Examples: Pancreas secretes Examples: Pancreas secretes
insulininsulin Neurons secrete Neurons secrete
neurotransmittersneurotransmitters
2. Endocytosis2. Endocytosis Reverse process.Reverse process. The cell takes in macromolecules by forming vesicles The cell takes in macromolecules by forming vesicles
from the plasma membrane.from the plasma membrane.
• Three types of endocytosis:Three types of endocytosis:11. Phagocytosis. Phagocytosis (“cellular eating”): Cell engulfs (“cellular eating”): Cell engulfs particle in a vacuole. Not specific.particle in a vacuole. Not specific.2. 2. PinocytosisPinocytosis (“cellular drinking”): Cell creates (“cellular drinking”): Cell creates vesicle around fluid. Not specific.vesicle around fluid. Not specific.
3. Receptor-mediated endocytosis:3. Receptor-mediated endocytosis: Very SpecificVery Specific Binding Binding of ligandsof ligands to receptors triggers vesicle to receptors triggers vesicle
formationformation
LE 7-20cLE 7-20c
Receptor
RECEPTOR-MEDIATED ENDOCYTOSIS
Ligand
Coatedpit
Coatedvesicle
Coat protein
Coat protein
Plasmamembrane
0.25 µm
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).
Familial hypercholesteremiaFamilial hypercholesteremia
Defective gene/genes required for Defective gene/genes required for regulation, synthesis, transport, recycling, or regulation, synthesis, transport, recycling, or turnover of LDL receptors.turnover of LDL receptors.
Exocytosis -Exocytosis - secretion of macromolecules secretion of macromolecules by the fusion of vesicles with the plasma by the fusion of vesicles with the plasma membranemembrane
Endocytosis Endocytosis -- import of macromolecules by import of macromolecules by forming new vesicles with the plasma forming new vesicles with the plasma membranemembranePhagocytosis –Phagocytosis –cell “eating”cell “eating”Pinocytosis – Pinocytosis – cell “drinking”cell “drinking”
Receptor-mediated endocytosisReceptor-mediated endocytosis (ligands) (ligands)
Bulk transit across membranesBulk transit across membranes
Cotransport (symport)Cotransport (symport) involves more than involves more than one type of particle being transported by in one type of particle being transported by in the the same directionsame direction at the same time by the at the same time by the same mechanism From same mechanism From http://http://