Chapter 7: Membrane Structure & Function Plasma membrane Composition: primarily lipids...

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Transcript of Chapter 7: Membrane Structure & Function Plasma membrane Composition: primarily lipids...

  • Chapter 7: Membrane Structure & Function

    Plasma membraneComposition: primarily lipids (phospholipids) & proteins with some carbohydrates (glycolipids or glycoproteins for cell recognition)Arranged in a fluid mosaicLipid bi-layer with embedded proteins

  • Discovery of plasma membrane structure1915- Red blood cell membranes analyzed; lipid & protein composition discovered1925- Gorter & Grendel suggest membrane is phospholipid bi-layer1935- Davson & Danielli suggest proteins sandwich phospholipids (FALSE)1950s- Electron Microscopes used to study membrane structure1972- Singer & Nicolson suggest proteins are dispersed (float) within the lipid bi-layer (further shown by freeze-fracture electron microscopy

  • Fluidity of membranesMembrane held together by weak hydrophobic interactions; most lipids & some proteins can drift within their layer of the membraneProtein movement/non-movement may be dependant on the proteins connection/lack of connection to the cytoskeletonTemperature affects level of fluidityFluidity affects permeability**cholesterol & unsaturated fats increase fluidity (added to membrane to prep for cooler temps)

  • Concept CheckWhat would happen to the fluidity of the membrane in the following scenarios?Increase in unsaturated phospholipids?Increase in saturated phospholipids?A decrease in temperature?An increase in cholesterol levels?

  • Membrane proteinsDetermine most of the membranes specific functionsTypes:Integral proteins: penetrate through the hydrophobic core of the lipid bi-layer (transmembrane proteins)Peripheral proteins: not embedded in bi-layer; attached to the surface of the membrane

  • Functions of membrane proteinsTransportEnzyme activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton & extracellular matrix (ECM)

  • Carbohydrates & the membraneCarbohydrates in the membrane < 15 sugar unitsTypes:GlycolipidsGlycoproteinsFunction: cell-cell recognition

  • Synthesis of membranesSee text book figure 7.10

    1. synthesis of membrane proteins & lipids in the ER; Carbohydrate added to make glycoproteins2. Inside Golgi apparatus glycolipids are made and glycoproteins are modified3. Transmembrane proteins, glycolipids, & secretory proteins are transported in vessicles4. Vessicles fuse with the membrane releasing secretory proteins & placing glycoproteins & glycolipids on the outside of the membrane

  • **Outside of plasma membrane is made from the inside of the ER, & Golgi vessicle membranes(When vessicles formed in ER & Golgi fuse with membrane to release material they become part of the membrane)

  • Concept CheckOn which side of the membrane are carbohydrates found? How is this location useful to the carbohydrate function in the membrane?

  • Selective permeabilityFluid mosaic model explains how membrane can regulate passage of materials Hydrophobic (non-polar) molecules can diffuse through lipid bi-layer easily Polar molecules & ions which are impeded by the lipid bi-layer pass through specific transport proteins

  • Passive TransportNo energy requiredDiffusionMolecules will move from high to low concentrationDiffusion of molecules is unaffected by the concentration of other substancesRate is determined by membrane permeability to the molecule

  • Passive Transport (contd.)OsmosisDiffusion of water across a selectively permeable membraneTonicity=ability of a solution to cause a cell to gain or lose waterisotonic: no net movement of waterHypertonic: net loss of waterHypotonic: net gain of water

  • Passive Transport (contd.)Facilitated diffusionMovement of molecules down their concentration gradient with the assistance of specific transport proteins in the membraneTypes of transport proteins:Channel proteins: corridors for passage of specific ion or moleculeAquaporins (water channel proteins)Ion channels/gated channels (electrical or chemical signal causes opening or closing Carrier proteins: change shape to translocate substances across the membrane

  • Concept CheckWhat would happen to a Paramecium that swam from a hypotonic environment to an isotonic one?Why do water molecules need aquaporins to cross the membrane? Why dont substances like oxygen and carbon dioxide require transport proteins?

  • Active TransportMolecules move against the concentration gradient (low to high)Energy requiredUses carrier transport proteins

  • Active transport: sodium-potassium pump

  • Na+ in cell binds to proteinATP binds to proteinProtein changes shapeNa+ moves out of cellK+ outside cell binds to proteinP from ATP is removed (dephosphorylation)Original protein shape is restored

  • Active transport: electrogenic pump

  • H+ pumped out through protein with the help of ATPOutside cell becomes +, inside Charge difference across the membrane is used to do work

  • Active transport: Cotransport

  • Same as electrogenic pump but when H+ moves back into cell by diffusion it carries another molecule with it (i.e. sucrose)

  • Passive vs. Active Transport

  • Concept CheckWhy is the sodium-potassium pump not considered a cotransporter?

    Which solute(s) will exhibit net diffusion into the cell?Which solute(s) will exhibit net diffusion into the cell?Which solution cell or environment is hypertonic?In which direction will there be a net osmotic movement of water?After the cell was placed in the beaker did it become for flaccid, or more turgid?

  • Bulk TransportExocytosis: cell secretes macromolecules through the fusion of vesicles with the plasma membraneEndocytosis: cell takes in macromolecules by forming new vesicles from the plasma membranePhagocytosis (cellular eating)Pinocytosis (non-specific cellular drinking)Receptor-mediated endocytosis (specific uptake)

  • Concept CheckAs a cell grows, its plasma membrane expands. Is this a result of exocytosis or endocytosis? Explain. After a neuron has been stimulated by neurotransmitters from a neighboring neuron, the neuron takes in the neurotransmitters by endocytosis. Is it by pinocytosis or receptor-mediated endocytosis? Explain.