Slide 1Selective Permeability of Lipid BilayersMany Factors Affect Membrane PermeabilityKey ConceptsSolute Movement across Lipid BilayersDiffusion along a Concentration GradientOsmosisOsmosis and Relative Solute ConcentrationOsmosis in Hypertonic, Hypotonic, and Isotonic SolutionsSlide 10Slide 11The Fluid-Mosaic Model of Membrane StructureMembrane ProteinsSlide 14Membrane Proteins Affect Ions and MoleculesIon Channels and the Electrochemical GradientFacilitated Diffusion via Channel ProteinsSlide 18Facilitated Diffusion via Carrier ProteinsSlide 20Active Transport by PumpsSlide 22Secondary Active TransportSummary of Membrane TransportSlide 25Plasma Membrane and the Intracellular EnvironmentCholera – Disaster at the plasma membraneWhat role does the cell membrane play in the body’s response to CAFFEINE?© 2011 Pearson Education, Inc.LIPIDS:3 types of lipids found in cells:Structure of membrane Lipids:Membrane Fluidity:© 2011 Pearson Education, Inc.Selective Permeability of Lipid Bilayers•The permeability of a structure is its tendency to allow a given substance to pass across it. Phospholipid bilayers have selective permeability. –Small or nonpolar molecules move across phospholipid bilayers quickly.–Charged or large polar substances cross slowly, if at all.© 2011 Pearson Education, Inc.Many Factors Affect Membrane Permeability•Many factors influence the behavior of the membrane:–Number of double bonds between the carbons in the phospholipid’s hydrophobic tail–Length of the tail–Number of cholesterol molecules in the membrane–Temperature© 2011 Pearson Education, Inc.Key ConceptsPlasma membranes are made up of selectively permeable bilayers of phospholipids. Phospholipids are amphipathic lipid molecules – they have hydrophobic and hydrophilic regions.Ions and molecules diffuse spontaneously from regions of higher concentration to regions of lower concentration. Movement of water across a plasma membrane is called osmosis.In cells, membrane proteins are responsible for the passage of insoluble substances that can’t cross the membrane on their own.© 2011 Pearson Education, Inc.Solute Movement across Lipid Bilayers•Materials can move across the cell membrane in different ways.–Passive transport does not require an input of energy.–With concentration gradient–Active transport requires energy to move substances across the membrane.–Against concentration gradient•Small molecules and ions in solution are called solutes, have thermal energy, and are in constant, random motion.–This random movement is called diffusion. –Diffusion is a form of passive transport.© 2011 Pearson Education, Inc.Diffusion along a Concentration Gradient•A difference in solute concentrations creates a concentration gradient.Molecules and ions move randomly when a concentration gradient exists, but there is a net movement from high- concentration regions to low-concentration regions. Diffusion along a concentration gradient increases entropy and is thus spontaneous.•Equilibrium is established once the molecules or ions are randomly distributed throughout a solution.-Molecules are still moving randomly but there is no more net movement.© 2011 Pearson Education, Inc.Osmosis•Water moves quickly across lipid bilayers.–The movement of water is a special case of diffusion called osmosis.•Water moves from regions of low solute concentration to regions of high solute concentration.–This movement dilutes the higher concentration, thus equalizing the concentration on both sides of the bilayer.Osmosis only occurs across a selectively permeable membrane.© 2011 Pearson Education, Inc.Osmosis and Relative Solute Concentration•The concentration of a solution outside a cell may differ from the concentration inside the cell.–An outside solution with a higher concentration is said to be hypertonic to the inside of a cell.–A solution with a lower concentration is hypotonic to the cell.–If solute concentrations are equal on the outside and inside of a cell, solutions are isotonic to each other.© 2011 Pearson Education, Inc.Osmosis in Hypertonic, Hypotonic, and Isotonic Solutions•In a hypertonic solution, water will move out of the cell by osmosis and the cell will shrink.•In a hypotonic solution, water will move into the cell by osmosis and the cell will swell.•In an isotonic solution, there will be no net water movement and the cell size will remain the same.© 2011 Pearson Education, Inc.Osmosis Can Modify the Shapes of CellsIsotonic solution: equal solute concentration (and equal water concentration)Hypertonic solution: higher solute concentrationHypotonic solution: lower solute concentrationDiffusionWater moleculescrossing away from involvesOsmosisConcentrationgradientsequalize from Semipermeable membranesno netmovement betweenIsotonic solutionsHypertonic solutionsHypotonic solutionstoward involves moving from SolutemoleculesAreas of high to low concentrationFig A6.1On pg A:5© 2011 Pearson Education, Inc.The Fluid-Mosaic Model of Membrane Structure•Although phospholipids provide the basic membrane structure, plasma membranes contain as much protein as phospholipids. •The fluid-mosaic model of membrane structure suggests that some proteins are inserted into the lipid bilayer, making the membrane a fluid, dynamic mosaic of phospholipids and proteins.© 2011 Pearson Education, Inc.Membrane Proteins•Integral proteins are amphipathic and so can span a membrane, with segments facing both its interior and exterior surfaces. –Integral proteins that span the membrane are called transmembrane proteins. –These proteins are involved in the transport of selected ions and molecules across the plasma membrane. –Transmembrane proteins can therefore affect membrane permeability.•Peripheral proteins are found only on one side of the membrane.–Often attached to integral proteins© 2011 Pearson Education, Inc.• Freeze –Fracture preparations allow biologists to view membrane Proteins: See Fig 6.18Amphipathic ProteinsAre stable in lipidBilayers.© 2011 Pearson Education, Inc.Membrane Proteins Affect Ions and Molecules •The transmembrane proteins that transport molecules are called transport proteins. There are three broad classes of transport proteins, each of which affects membrane permeability: 1. Channels2. Carrier proteins or transporters3. Pumps© 2011 Pearson Education, Inc.Ion Channels and the Electrochemical Gradient•Ion