CHAPTER 6The Importance of MembranesLipids: What Is a Lipid?PowerPoint PresentationThree Types of Lipids Found in CellsSlide 6The Structure of Membrane LipidsSlide 8Phospholipids and WaterPhospholipid BilayersSlide 11Selective Permeability of Lipid BilayersSlide 13Many Factors Affect Membrane PermeabilityBond Saturation and Membrane PermeabilitySlide 16Other Factors That Affect PermeabilitySlide 18Slide 19Slide 20Slide 21Fluidity of the MembraneSlide 23Solute Movement across Lipid BilayersDiffusion along a Concentration GradientSlide 26OsmosisSlide 28Osmosis and Relative Solute ConcentrationOsmosis in Hypertonic, Hypotonic, and Isotonic SolutionsSlide 31The Fluid-Mosaic Model of Membrane StructureSlide 33Membrane ProteinsSlide 35Slide 36Membrane Proteins Affect Ions and MoleculesIon Channels and the Electrochemical GradientSlide 39Facilitated Diffusion via Channel ProteinsSlide 41Slide 42Facilitated Diffusion via Carrier ProteinsSlide 44Slide 45Active Transport by PumpsSlide 47Slide 48Slide 49Slide 50Secondary Active TransportSummary of Membrane TransportSlide 53Plasma Membrane and the Intracellular EnvironmentKey Concepts© 2011 Pearson Education, Inc.CHAPTER 6© 2011 Pearson Education, Inc.The Importance of Membranes •The plasma membrane, or cell membrane, separates life from nonlife.•The plasma membrane separates the cell’s interior from the external environment. Membranes function to: –Keep damaging materials out of the cell–Allow entry of materials needed by the cell–Facilitate the chemical reactions necessary for life© 2011 Pearson Education, Inc.Lipids: What Is a Lipid? •Lipids are carbon-containing compounds that are found in organisms and that are largely nonpolar and hydrophobic. •Hydrocarbons are nonpolar molecules that contain only carbon and hydrogen.•Lipids do not dissolve in water because they have a major hydrocarbon component called a fatty acid. –A fatty acid is a hydrocarbon chain bonded to a carboxyl(—COOH) functional group. •Fatty acids and isoprene are the key building blocks of lipids.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Three Types of Lipids Found in Cells •Lipid structure varies widely. •The three most important types of lipids found in cells:1. Fats are composed of three fatty acids linked to glycerol.–Also called triacylglycerols or triglycerides2. Steroids are a family of lipids with a distinctive four-ring structure. –Cholesterol is an important steroid in mammals. 3. Phospholipids consist of a glycerol linked to a phosphate group (PO42–) and to either two chains of isoprene or two fatty acids.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.The Structure of Membrane LipidsMembrane-forming lipids contain both a polar, hydrophilic region and a nonpolar, hydrophobic region.•Phospholipids are amphipathic:–The “head” region, consisting of a glycerol, a phosphate, and a charged group, contains highly polar covalent bonds. –The “tail” region is comprised of two nonpolar fatty acid or isoprene chains.•When placed in solution, the phospholipid heads interact with water while the tails do not, allowing these lipids to form membranes.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Phospholipids and Water•Phospholipids do not dissolve when they are placed in water.•Water molecules interact with the hydrophilic heads but not with the hydrophobic tails. –This drives the hydrophobic tails together.•Upon contact with water phospholipids form either:–Micelles–Heads face the water and tails face each other.–Phospholipid bilayers (lipid bilayers)© 2011 Pearson Education, Inc.Phospholipid Bilayers•Phospholipid bilayers form when two sheets of phospholipid molecules align. The hydrophilic heads in each layer face a surrounding solution, while the hydrophobic tails face one another inside the bilayer.•Phospholipid bilayers form spontaneously, with no outside input of energy required.© 2011 Pearson Education, Inc.© 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.© 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.Bond Saturation and Membrane Permeability•Double bonds between carbons in a hydrocarbon chain can cause a “kink” in the hydrocarbon chain, preventing the close packing of hydrocarbon tails, and reducing hydrophobic interactions.–Unsaturated hydrocarbon chains have at least one double bond. –Hydrocarbon chains without double bonds are termed saturated.•Saturated fats have more chemical energy than unsaturated fats.•Membranes with unsaturated phospholipid tails are much more permeable than those formed by phospholipids with saturated tails.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Other Factors That Affect Permeability•Hydrophobic interactions become stronger as saturated hydrocarbon tails increase in length.–Membranes containing phospholipids with longer tails have reduced permeability.•Adding cholesterol to membranes increases the density of the hydrophobic section.–Cholesterol decreases membrane permeability.•Membrane fluidity decreases with temperature because molecules in the bilayer move more slowly.–Decreased membrane fluidity causes decreased permeability.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.© 2011 Pearson Education, Inc.Fluidity of the Membrane •Individual phospholipids can move laterally throughout the lipid bilayer.–They rarely flip between layers.•How quickly molecules move within and across membranes is a function of temperature and the structure of the hydrocarbon tails in the bilayer.© 2011 Pearson Education, Inc.© 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