Membrane Structure & Function cont. I. Membrane Protein Function II. Cellular Transport
Sep 18, 2015
Membrane Structure & Function cont.I. Membrane Protein FunctionII. Cellular Transport
Integral proteins span lipid bilayer called transmembrane proteins
hydrophobic regions consist of one or more stretches of nonpolar amino acids
often coiled into alpha helices
Visualize and draw membrane with transmembrane protein containing 2 helices
LE 7-8EXTRACELLULARSIDEN-terminusC-terminusCYTOPLASMICSIDEa Helix
Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)
LE 7-9aEnzymesSignalReceptorATPTransportEnzymatic activitySignal transduction
LE 7-9bGlyco-proteinCell-cell recognitionIntercellular joining Attachment to thecytoskeleton and extra-cellular matrix (ECM)
The Role of Membrane Carbohydrates in Cell-Cell RecognitionCells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
Carbohydrates covalently bonded to lipids (glycolipids) or more often to proteins (glycoproteins)
Much variability of extracellular carbohydrates among species, individuals, cell types in an individual
Example of Pneumococcus
Synthesis and Sidedness of MembranesMembranes distinct inside and outside faces
Plasma membrane is added to by vesicles from ER & Golgi.
Secreted and integral membrane proteins, lipids and associated carbohydrates transported to membrane by these vesicles.
LE 7-10Plasma membrane:Cytoplasmic faceExtracellular faceTransmembraneglycoproteinPlasma membrane:SecretedproteinVesicleGolgiapparatusGlycolipidSecretoryproteinTransmembraneglycoproteinsER
Transport across cellular membranesTo exchange materials with surroundings in part to take in nutrients and give off waste
Exchange(or transport) regulated: selective permeability
Structure Dictates Membrane PermeabilityHydrophobic (nonpolar) molecules cross membrane rapidlye.g., hydrocarbons, oxygen, CO2 can dissolve in the lipid bilayer and pass through the membrane rapidly
Polar molecules cross slowlye.g. sugars, charged proteins, water
How do hydrophilic substances cross membranes?Transport proteinsSome create hydrophilic channels across membranes for polar molecules or ions to pass through
Example: Aquaporin water channel proteinWith Help!
Carrier proteinsbinds solutes & change the shape of carrier
help to facilitate passage across membrane
highly specific for transported solutes
Examples: glucose transporter is a carrier protein for glucose only
Transport Can be Passive or Active
LE 7-11aMolecules of dyeMembrane (cross section)WATERNet diffusionNet diffusionEquilibriumDiffusion of one solutePassive Transport: Diffusion
Substances diffuse down their concentration gradientHigh to low
Substances reach dynamic equilibrium
No work (no added energy) required
LE 7-11bNet diffusionNet diffusionEquilibriumDiffusion of two solutesNet diffusionNet diffusionEquilibrium
Effects of Osmosis on Water BalanceOsmosis diffusion of water across a selectively permeable membrane
Diffuses across a membrane from the region of lower solute (such as an ion) concentration to the region of higher solute concentration
The direction of osmosis is determined only by a difference in total solute concentration
LE 7-12Lowerconcentrationof solute (sugar)Higherconcentrationof sugarSame concentrationof sugarSelectivelypermeable mem-brane: sugar mole-cules cannot passthrough pores, butwater molecules canH2OOsmosis
Water Balance of Cells Without WallsTonicity ability of a solution to cause a cell to gain or lose water
Isotonic solutionsolute concentration is equal inside and outside the cell --> no net water movement cell remains same size
Hypertonic solution external solute concentration is greater than that inside the cell-->cell loses water
Hypotonic solution external solute concentration is less than that inside the cell--> cell gains water
May expand enough to burst!
LE 7-13AnimalcellLysedH2OH2OH2ONormalHypotonic solutionIsotonic solutionHypertonic solutionH2OShriveledH2OH2OH2OH2OPlantcellTurgid (normal)FlaccidPlasmolyzed
Water Balance of Cells with Walls vs No WallsCell walls help maintain water balance
Plant cell in hypotonic solution swells -->turgid (firm)Animal cell?Plant cell and its surroundings isotonic--> no net water movemen; the cell becomes flaccid (limp), and the plant may wiltAnimal cell?In hypertonic environment, plant cells lose water--> membrane pulls away from the wall: plasmolysisLethal
Animal cell?
Passive Transport Aided by ProteinsFacilitated diffusion transport proteins speed movement of molecules across the plasma membraneChannel proteins Carrier proteins
LE 7-15aEXTRACELLULARFLUIDChannel proteinSoluteCYTOPLASM
LE 7-15bCarrier proteinSolute
Active transport
uses energy to move solutes against their gradientsRequires energy, usually ATPPerformed by specific membrane proteins
Examplesodium-potassium pump
LE 7-16 Cytoplasmic Na+ bonds tothe sodium-potassium pumpCYTOPLASMNa+[Na+] low[K+] highNa+Na+EXTRACELLULARFLUID[Na+] high[K+] lowNa+Na+Na+ATPADPP 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 proteinsoriginal conformation. K+ is released and Na+sites are receptive again;the cycle repeats.K+K+K+K+K+
LE 7-17DiffusionFacilitated diffusionPassive transportATPActive transport
Electrogenic pumps is a transport protein that generates a voltage across a membrane--> opposite charges across membrane (membrane potential)
Example: In animals, Na-K pump
In plant fungi and bacteria, proton pumpRequires ATP (active transport)
LE 7-18H+ATPCYTOPLASMEXTRACELLULARFLUIDProton pumpH+H+H+H+H++++++
Cotransport Coupled Transport by a Membrane ProteinWhen active transport of one solute indirectly drives transport of another
ExamplePlants commonly use the proton gradient generated by proton pumps to drive transport of nutrients into the cell
LE 7-19H+ATPProton pumpSucrose-H+cotransporterDiffusionof H+SucroseH+H+H+H+H+H+++++++
How do large molecules move in and out of cells?Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane via vesicles
ExocytosisTransport vesicles with cargo migrate to the membrane, fuse with it, and are release contents
Example:Many secretory cells use exocytosis to export their products Pancreatic cells (beta-cells) secrete insulin
LE 7-10Plasma membrane:Cytoplasmic faceExtracellular faceTransmembraneglycoproteinPlasma membrane:SecretedproteinVesicleGolgiapparatusGlycolipidSecretoryproteinTransmembraneglycoproteinsER
EndocytosisCell takes in macromolecules by forming vesicles at the plasma membrane
Reversal of exocytosis, involving different proteins
Three types of endocytosis
Phagocytosis (cellular eating): Cell engulfs particle in a vacuole
Pinocytosis (cellular drinking): Cell creates vesicle around fluid
Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation
LE 7-20cReceptorRECEPTOR-MEDIATED ENDOCYTOSISLigandCoatedpitCoatedvesicleCoat proteinCoat proteinPlasmamembrane0.25 mA coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs).