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Slide 1 ___________________________________ Chapter 7 ___________________________________ Membrane Structure and Function ___________________________________ PowerPoint® Lecture Presentations for Biology ___________________________________ Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 2 ___________________________________ Overview: The Membrane Barrier • The plasma membrane is the boundary that separates the living cell from its surroundings ___________________________________ • All eukaryotic membranes exhibit selective permeability, allowing some substances to cross them more easily than others ___________________________________ Membrane Channel Protein ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 3 ___________________________________ Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteins • Phospholipids are the most abundant lipid in membranes ___________________________________ • Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions ___________________________________ The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 4 Fig. 7-5 ___________________________________ The Fluidity of Membranes Lateral movement (~107 times per second) Flip-flop (~ once per month) (a) Movement of phospholipids Fluid Unsaturated hydrocarbon tails with kinks Viscous Saturated hydrocarbon tails (b) Membrane fluidity Most of the lipids, and some proteins, drift laterally ___________________________________ ___________________________________ Rarely does a molecule flip-flop transversely across the membrane ___________________________________ Cholesterol (c) Cholesterol within the animal cell membrane ___________________________________ ___________________________________ ___________________________________ Slide 5 Fig. 7-6 ___________________________________ Demonstration of Protein Drift in a Membrane ___________________________________ RESULTS Membrane proteins Mouse cell ___________________________________ Mixed proteins after 1 hour Human cell Hybrid cell ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 6 Fig. 7-5b ___________________________________ •Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids •Membranes must be fluid to work properly; they are usually about as fluid as salad oil Fluid ___________________________________ Viscous ___________________________________ Unsaturated hydrocarbon tails with kinks ___________________________________ Saturated hydrocarbon tails ___________________________________ ___________________________________ ___________________________________ Slide 7 Fig. 7-5c ___________________________________ •The steroid cholesterol has different effects on membrane fluidity at different temperatures •At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids ___________________________________ •At cool temperatures, it maintains fluidity by preventing tight packing ___________________________________ ___________________________________ Cholesterol ___________________________________ ___________________________________ ___________________________________ Slide 8 ___________________________________ Fig. 7-7 Fibers of extracellular matrix (ECM) ___________________________________ Carbohydrate Glycoprotein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Transmembrane protein ___________________________________ Cholesterol Proteins provide most membrane-specific functions Peripheral proteins ___________________________________ Integral protein CYTOPLASMIC SIDE OF MEMBRANE ___________________________________ ___________________________________ ___________________________________ Slide 9 Fig. 7-8 ___________________________________ N-terminus EXTRACELLULAR SIDE What types of side chains are on outside of these helices? ___________________________________ ___________________________________ C-terminus α Helix ___________________________________ CYTOPLASMIC SIDE ___________________________________ ___________________________________ ___________________________________ Slide 10 Fig. 7-9ac ___________________________________ Signaling molecule ___________________________________ Receptor Enzymes ___________________________________ ATP Signal transduction (b) Enzymatic activity (a) Transport (c) Signal transduction ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 11 Fig. 7-9df ___________________________________ ___________________________________ Glycoprotein (d) Cell-cell recognition (e) Intercellular joining Carbohydrates often play important roles on the plasma membrane. Covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) ___________________________________ (f) Attachment to the cytoskeleton and extracellular matrix (ECM) ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 12 Fig. 7-10 ___________________________________ ER 1 Membranes have distinct inside and outside faces Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 4 Secreted protein Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein The asymmetrical distribution of proteins, lipids, and carbohydrates is determined when the membrane is built by the ER and Golgi apparatus ___________________________________ ___________________________________ ___________________________________ Membrane glycolipid ___________________________________ ___________________________________ ___________________________________ Slide 13 ___________________________________ Selective Permeability • A cell must exchange materials with its surroundings and compartmentalize its interior. ___________________________________ • This process is controlled by the membranes. • Membranes are selectively permeable, regulating the cell’s molecular traffic ___________________________________ • What does “Selective Permeability” mean? ___________________________________ _______________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 14 ___________________________________ The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly ___________________________________ estradiol ___________________________________ Charged or strongly polar molecules, such as ions, sugars and proteins, do not cross the membrane easily testosterone ___________________________________ glucose proteins Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 15 Some small molecules, even if polar, can pass through the lipid bilayer - very slowly...... ___________________________________ -oxygen -carbon dioxide -urea ___________________________________ -water ___________________________________ H2O ___________________________________ ___________________________________ ___________________________________ Slide 16 ___________________________________ Diffusion: True or False 1. Diffusion requires that the membrane be permeable to the substance that is diffusing ___________________________________ 2. Diffusion is spontaneous and passive – requiring no Energy 3. A substance moves against its concentration gradient during diffusion 4. The concentration of one substance can affect the diffusion of another 5. Diffusion occurs because populations of molecules move randomly, but individual molecules can be directional (moving in a particular direction across a membrane) ___________________________________ 6. Diffusion results from thermal motion (heat) 7. Osmosis is the diffusion of a solute across a selectively-permeable membrane. ___________________________________ 8. At dynamic equilibrium, as many molecules cross in one direction across the membrane as in the other direction ___________________________________ ___________________________________ ___________________________________ Slide 17 Fig. 7-11 ___________________________________ Membrane (cross section) one solute WATER ___________________________________ Net diffusion Net diffusion Equilibrium Net diffusion ___________________________________ two solutes Net diffusion Net diffusion Net diffusion Net diffusion ___________________________________ Equilibrium Equilibrium Net diffusion ___________________________________ ___________________________________ ___________________________________ Slide 18 Fig. 7-12 Lower concentration of solute (sugar) Higher concentration of sugar ___________________________________ Same concentration of sugar ___________________________________ H2O Selectively permeable membrane ___________________________________ Water bound to sugar creates less “free” water OSMOSIS: the special case of water (solvent) diffusion. ___________________________________ Osmosis ___________________________________ ___________________________________ ___________________________________ Slide 19 ___________________________________ Water Balance of Cells • Tonicity is the ability of a solution to cause a cell to gain or lose water (e.g., potato in lab) – Considers both solute concentration, and – Membrane permeability – Depends on concentration of non-penetrating solutes ___________________________________ ___________________________________ • Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane • Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water ___________________________________ • Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 20 Fig. 7-13 Hypotonic solution H2O Isotonic solution H2O H2O ___________________________________ Hypertonic solution H2O ___________________________________ (a) Animal cell Lysed H2O Normal H2O Shriveled H2O ___________________________________ H2O (b) Plant cell Turgid (normal) Flaccid ___________________________________ Plasmolyzed ___________________________________ ___________________________________ ___________________________________ Slide 21 ___________________________________ Water Balance of Cells with Walls • Cell walls help maintain water balance ___________________________________ • A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) • If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt ___________________________________ • In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 22 • Hypertonic or hypotonic environments can create osmotic problems ___________________________________ • Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments ___________________________________ • The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 23 Fig. 7-14 Filling vacuole ___________________________________ 50 µm ___________________________________ (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. ___________________________________ Contracting vacuole ___________________________________ (b) When full, the vacuole and canals contract, expelling fluid from the cell. ___________________________________ ___________________________________ ___________________________________ Slide 24 ___________________________________ Transmembrane ‘Transport Proteins’ can help hydrophilic substances cross the membrane 1. Facilitated Diffusion ___________________________________ 1. Aqueous channels for charged or polar molecules to diffuse through - or 2. Passive carrier mechanisms that move molecules from higher to lower concentration – usually in either direction ___________________________________ 2. Active Transport 1. ATP-dependent carrier that can move charged or polar molecules against their concentration gradient ___________________________________ 3. All Transport Proteins are very specific for their molecules Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 25 Fig. 7-17 Passive transport ___________________________________ Active transport ___________________________________ ___________________________________ ATP Diffusion Facilitated diffusion ___________________________________ Both passive and active carrier proteins undergo a change in shape that opens the solute-binding site on the pother side of the membrane ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 26 • Channel proteins include ___________________________________ – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels) ___________________________________ What kind of “gating” have you heard about? _____________________________________________ ___________________________________ _____________________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 27 • Active transport allows cells to maintain concentrations of things different from what would occur naturally – We have calcium pumps, hydrogen ion pumps and sodium-potassium pumps, among others – We also have glucose transporters and amino acid transporters – even large protein transporters ___________________________________ ___________________________________ Why do you think a cell would want to do that? ___________________________________ ion pumps?_____________________________________ transporters?____________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 28 • What other way have we already learned that big molecules, like proteins, lipids and carbohydrates, can be moved in or out or around within a cell? ___________________________________ _____________________________________ ___________________________________ _____________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 29 Bulk transport across the plasma membrane occurs by exocytosis and endocytosis ___________________________________ • Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins ___________________________________ • Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles ___________________________________ • Bulk transport requires energy ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 30 Fig. 7-16-1 ___________________________________ EXTRACELLULAR FLUID Example: Active Transport [Na+] high [K+] low ___________________________________ Na+ Na+ - natural gradients? ___________________________________ CYTOPLASM Na+ [Na+] low [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 31 Fig. 7-16-2 ___________________________________ ___________________________________ Na+ Na+ Na+ P ADP ___________________________________ ATP 2 Na+ binding stimulates phosphorylation by ATP. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 32 Fig. 7-16-3 ___________________________________ Na+ Na+ ___________________________________ Na+ ___________________________________ P 3 Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 33 Fig. 7-16-4 ___________________________________ ___________________________________ ___________________________________ P P 4 K+ binds on the extracellular side and triggers release of the phosphate group. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 34 Fig. 7-16-5 ___________________________________ ___________________________________ ___________________________________ 5 Loss of the phosphate restores the protein’s original shape. ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 35 Fig. 7-16-6 ___________________________________ ___________________________________ ___________________________________ K+ is released, and the cycle repeats. ___________________________________ Why do we want Na+ outside of the cell and K+ inside? ___________________________________ ___________________________________ ___________________________________ Slide 36 Neurons and muscle cells in animals and phloem cells in plants rely on electrical signaling. ___________________________________ • Electricity is the energy created by the movement of charged particles – it’s named for the example of electrons ___________________________________ • When a cell uses electricity it does it by allowing ions that it has concentrated by active transport to rush from one side of the membrane to the other through channel proteins ___________________________________ • The opening and closing of the channels determines when the electrical current is flowing • Voltage is a measure of how many ions are on the move ___________________________________ • Membrane potential is a measure of how many ions have been actively concentrated across a membrane Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 37 • Concentrated ions diffuse faster than uncharged molecules ___________________________________ • Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: ___________________________________ – A chemical force (the ion’s concentration gradient) – An electrical force (the effect of the membrane potential on the ion’s movement) ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 38 • An electrogenic pump is a transport protein that generates voltage across a membrane ___________________________________ • The sodium-potassium pump is the major electrogenic pump of animal cells ___________________________________ • The main electrogenic pump of plants, fungi, and bacteria is a proton pump • Mitochondria and chloroplasts use a proton pump to help make ATP ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 39 Fig. 7-18 ___________________________________ – ATP EXTRACELLULAR FLUID + – + ___________________________________ H+ H+ Proton pump H+ – + ___________________________________ H+ H+ – + CYTOPLASM – ___________________________________ H+ + Where else have you heard of a proton pump? ___________________________________ ___________________________________ ___________________________________ Slide 40 Cotransport: Coupled Transport by a Membrane Protein ___________________________________ • Cotransport occurs when active transport of a solute indirectly drives transport of another solute ___________________________________ • Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell ___________________________________ ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 41 Fig. 7-19 – + – ___________________________________ H+ ATP H+ + H+ Proton pump ___________________________________ H+ – H+ + – H+ + ___________________________________ H+ Diffusion of H+ Sucrose-H+ cotransporter H+ Sucrose – – ___________________________________ + + Sucrose ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 42 • In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation ___________________________________ • A ligand is any molecule that binds specifically to a receptor site of another molecule ___________________________________ ___________________________________ Animation: Receptor-Mediated Endocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ Slide 43 Fig. 7-20c ___________________________________ RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle ___________________________________ Coated pit Ligand ___________________________________ A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Coat protein ___________________________________ Plasma membrane 0.25 µm ___________________________________ ___________________________________ ___________________________________ Slide 44 Fig. 7-UN3 ___________________________________ What will happen? “Cell” 0.03 M sucrose 0.02 M glucose ___________________________________ Environment: 0.01 M sucrose 0.01 M glucose ___________________________________ 0.01 M fructose ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 45 ___________________________________ You should now be able to: 1. Define the following terms: amphipathic molecules, aquaporins, diffusion ___________________________________ 2. Explain how membrane fluidity is influenced by temperature and membrane composition ___________________________________ 3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________ ___________________________________ ___________________________________ Slide 46 4. Explain how transport proteins facilitate diffusion ___________________________________ 5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps ___________________________________ 6. Explain how large molecules are transported across a cell membrane ___________________________________ Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings ___________________________________ ___________________________________ ___________________________________