Biology 102 Lectures 6 & 7: Biological Membranes

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Transcript of Biology 102 Lectures 6 & 7: Biological Membranes

  • Biology 102Lectures 6 & 7: Biological Membranes

  • Lecture outlineRelationship of membrane structure and functionFunctionsStructure: The fluid-mosaic model of membranesMovement of substances across membranesPrinciples of DiffusionPassive and active transport of solutesOsmosisEndocytosis and exocytosisSpecialization of cell surfaces

  • 1. Membrane structure and functionBiological membranes Thin barrier separating the inside of the cell (or structure) from the outside environment

    Functions (focus on plasma membrane)Selectively isolate the cells contents from the external environmentRegulate the exchange of substances between the inside and outside of the cellCommunicate with other cells

  • Fluid-mosaic model of membrane structure

  • The phospholipid bilayer is the fluid portion of the membraneDouble layer Polar head group: hydrophilic exteriorNon-polar hydrocarbon tails: hydrophobic interiorUnsaturated hydrocarbon chains: maintains fluidity

  • Phospholipid bilayer as a barrierHydrophilic molecules cannot pass freely through the membranes hydrophobic interiorSugars, amino acids, charged ions (i.e. Na+ and Cl-) are some examplesThough polar, H20 is so small it does pass through.

    Many hydrophobic molecules can pass freely through the membranes hydrophobic interiorSteroid hormones and other lipids are some examples

  • Cholesterol molecules are part of the lipid bilayerAdds strengthAdds flexibilityAffects fluidityDecreases fluidity at moderate temperaturesHarder for phospholipids to movePrevents solidification at low temperaturesKeeps phospholipids from binding to each other

  • A mosaic of proteins is embeded in the membrane

    Glycoproteins: proteins with attached carbohydrates

  • Types of membrane proteinsTransport proteinsFor passage of materials through the plasma membraneChannel vs. carrier proteins

    Receptor proteinsBind molecules and trigger cellular responsesExample: hormones

    Recognition proteinsSelf vs. non-self (glycoprotein-based) recognitionMarkers during development

  • 2. Movement of substances across membranesDefinitionsConcentrationNumber of molecules in a given volumeGradientDifferences in concentration between two regions of space. This causes molecules to move from one region to the other (if no barrier to movement)DiffusionNet movement of molecules from regions of high concentration to regions of low concentrationConsidered as movement down its concentration gradient

  • Diffusion of Dye in WaterTime 0Steep Concentration GradientTime 1Reduced Concentration GradientDispersingTime 2No Concentration GradientRandom Dispersal

  • Passive vs. active transportPassive transportMovement of molecules down their concentration gradientsRequires no net energy expenditureThe gradients themselves provide energyActive transportMovement of molecules against their concentration gradientsRequires energy!

  • Focus: Passive transportSimple diffusionFacilitated diffusionOsmosis

    Remember that no energy is required, and molecules move down their concentration gradients

  • Focus: Passive transportSimple diffusionMolecules simply cross cell membrane on their own, down their concentration gradients

    Possible only for molecules that can cross the lipid bilayer on their ownLipid-soluble molecules Examples: ethyl alcohol, vitamin A, steroid hormonesVery small moleculesExamples: water, carbon dioxide

    Rate depends uponConcentration gradientSizeLipid solubility

  • Focus: Passive transport (cont.)Facilitated diffusionMolecules move down their concentration gradients (as for simple diffusion), butTransport proteins assist these molecules in crossing the membraneNo net energy expenditure! (This is a type of diffusion)

  • Focus: Passive transport (cont.): Facilitated diffusion via a channel

  • Focus: Passive transport (cont.): Facilitated diffusion via a carrier proteinCarrier protein has binding site for moleculeMolecule enters binding siteCarrier protein changes shape, transporting molecule across membraneCarrier protein resumes original shape(Inside Cell)(Outside Cell)Diffusion Channel ProteinDiffusion GradientMolecule in Transit

  • Focus: Passive transport (cont.)OsmosisMovement of water from a high [water] to an area of low [water] concentration across a semi-permeable membrane Note here that water can pass through, but glucose cannotThink about which way water will move (blackboard demo)

  • The effects of osmosisCompare solute and water concentrations outside vs. inside the cell (sketches)

  • Focus: Active TransportMovement via active transport proteinsEndocytosisExocytosis

    Remember that energy is required, and molecules are moved against their concentration gradients

  • Focus: Active transport 1. Movement via active transport proteinsATP required (has own binding site)Note movement of particles (Ca++) against their concentration gradient

  • Focus: Active transport2. EndocytosisThree types of endocytosisPinocytosiscell drinkingExtracellular fluid taken in

    Receptor-mediated endocytosisSpecific for particular moleculesMolecules bind to receptors.Receptor-molecule complex taken in

    PhagocytosisLarge particles engulfed

  • Focus: Active transport3. Exocytosis

  • 3. Specialization of cell surfacesConnections between cellsDesmosome: Membranes of adjacent cells glued together by proteins and carbohydratesTight junction: Cells sealed together with proteins

  • 3. Specialization of cell surfaces (cont.)Communication between cellsGap junctions: Channels connect adjacent cellsPlasmodesmata: Continuous cytoplasm bridges between two cells (plants)Note also cell walls. Only certain cell types have cell walls!

    Shows successive stages in the dispersal of red dye molecules after being dropped into water.

    Note that the molecules to be transported (little balls) are in higher concentration outside the cell than inside the cell.The molecules are of a type that cannot get to the inside of the cell by going directly through the membrane.The protein channel provides a passage way for the molecules to follow the concentration gradient.