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Transcript
Membrane Structure & Function Lecture 3 Fall 2008 Cellular Membranes Present in all cell types Function: • Separates the internal from the external environment • Regulate chemical exchanges within the environment – Chemical reactions more efficient • Dynamic selective barrier • Mosaic of lipids & proteins 1 Proteins Protein • Complex polymer made from amino acids • Each protein type has unique 3D shape • Many different protein types (tens of thousands) Functions: • Structural • Storage • Muscle movement (Contractile proteins) • Transport • Enzymes (catalyze chemical reactions) • Defense • Signaling • Cell-cell communication/recognition 2 3 Proteins Amino acids • 20 types • Basic structure of each amino acid is the same • Unique side group • Only left-handed amino acids found in living organisms See Fig. 5.17 4 Proteins Polypeptide chains • Amino acids form polypeptide chains – long chain of amino acids in unique sequence • Sequence critical – Change function of protein See Fig. 5.22 Protein Structure 5 4 levels of structure • Primary – Unique sequence of amino acids in polypeptide chain • Secondary – Folding of polypeptide chain –hydrogen bonding • Tertiary – Total 3D structure • Quaternary – If composed of more than one polypeptide chain Fig. 3.24 Shape sensitive to changes in the environment (e.g., heat, ph) Denaturation: loss of structure in a protein 6 Lipids Lipid • Nonpolar • Hydrophobic – Does not dissolve readily in water Functions • Energy storage • Photosynthetic pigments • Cell-cell signaling (hormones) • Waterproof coating • Act as vitamins • Plasma membrane Three types • Fats – Dietary fats • Steroids – Hormones • Phospholipids – Plasma membrane 7 Lipids Phospholipid structure • Head – – – – Hydrophilic (polar) Glycerol Phosphate group (negatively charged) Other polar/charged molecules • Tail – Two fatty acids – Hydrophobic (non-polar) • Diversity – Fatty acids – Groups attached to phosphate group in head Fig. 5.13 8 Membrane Structure & Function Phospholipid bilayer • Phospholipid – Amphipathic – hydrophobic & hydrophilic region • Bilayer – two layered membrane • Spontaneous self-assembly of phospholipids • Formation of cell membranes critical for evolution of cells Fig. 5.14 Membrane Structure & Function Fluid Mosaic Model • Dynamic – Molecules move freely past one another • Mosaic – Many proteins embedded in phospholipid bilayer [Read “Membrane Modes: Scientific Inquiry”, pgs. 126-127] Fig. 7.3 9 10 Membrane Fluidity Phospholipid movement • Lateral • Flip-flop Protein movement • Lateral (some) Temperature & Fluidity • At lower temperature, membrane solidifies • Unsaturated hydrocarbon tails fluid at lower temps • Cholesterol acts as temperature buffer – At high temps, decrease fluidity – At low temps, prevents solidification Fig. 7.5 11 Fig. 7.7 Membrane Proteins Membrane protein functions • Transport • Enzyme activity • Signal transduction • Cell-cell recognition • Intercellular joining • Attachment See Fig. 7.9 12 13 Membrane Proteins Integral proteins • Penetrate hydrophobic core – Transmembrane: span the membrane – Partial • Amphipatic • Directional Peripheral proteins • Not embedded • Attached to membrane surface • Associated with integral proteins Fig. 7.8 14 Cell-Cell Recognition • Glycoproteins – Protein + carbohydrate • Glycolipid – Lipid + carbohydrate Fig. 7.9 15 The Endomembrane System: A Closer Look Membranes • Distinct inside & outside faces • Lipid layers can be different in composition • Proteins have specific orientation • Asymmetrical arrangement of proteins, lipids & associated carbohydrates in PM determined by ER & GA Fig. 7.10 Transport in cells How do substances move into and out of cells? 1. Path • Way of getting from one place to another 2. Driving force • Concentration gradient • Electrochemical gradient • Pressure gradient 16 Transport in cells: Paths • Permeability of lipid bilayer • How do other substances enter/exit cell? – Proteins Type of Molecule Example Permeability Water H 2O Yes (slow) Gases CO2, O2 Yes Small, uncharged polar molecules Ethanol Yes Large, uncharged polar molecules Glucose No (slow) Large charged molecules ATP, Amino Acids No Ions H+, K+ No 17 Transport in cells: Driving Force • Concentration gradient – Variation across space in the concentration of a dissolved substance, from a region of high concentration to a region of low concentration • Electrochemical gradient – The combined effect of a concentration gradient and an electrical gradient. Affects the movement of ions across plasma membranes • Electrical gradient – differences in electrical charges across a plasma membrane (e.g., Na+) • Pressure gradient – Differences in pressure, from areas of high pressure to areas of low pressure (e.g., cardiovascular system) 18 Diffusion • Tendency of molecules or ions (solutes) to spread out and equalize their concentrations • Individual molecules move constantly & randomly, but overall tendency is directional • A substance tends to diffuse down its concentration gradient • Down = from an area of high concentration to an area of low concentration • Spontaneous - does not require input of energy 19 20 Diffusion • A substance tends to diffuse down its concentration gradient • Continues until dynamic equilibrium reached – Concentration of molecules the same on each side of the membrane – Individual molecules continually in motion Fig. 7.11 21 Diffusion • Each substance diffuses down its own concentration gradient • Independent! See Fig. 7.11 Transport in cells: Passive Passive transport • Passive = no extra energy (ATP) required Path • Through plasma membrane – Gasses; small, uncharged polar molecules; water Driving force • Simple diffusion • Concentration or electrochemical gradients (H+, glucose) 22 Osmosis Osmosis • Special case of diffusion • Diffusion of water across a selectively permeable membrane • Equalizes its concentration gradient – Path: plasma membrane permeable to water – Driving force: concentration gradient 23 24 Osmosis Tonicity • Ability of a solution to cause a cell to gain or lose water Hypotonic • Solution with a lower concentration of solute • Solution with the higher concentration of water Hypertonic • Solution with a higher concentration of solute • Solution with the lower concentration of water Isotonic • The solute concentrations are equal on either side of the membrane • No net movement of water across the plasma membrane Water Balance – Animal Cells • Isotonic state is the functional norm • Osmoregulation – to keep cells in isotonic state with external environment See Fig. 7.13 25 26 Water Balance – Plant Cells Functional state for plants - in hypotonic solution • Turgid (“firm”) – Turgor pressure – pressure within a plant cell caused by the pressure of the incoming water vs. the pressure of the cell wall – Cell wall prevents plasma membrane from bursting (Plasmolysis) See Fig. 7.13 Transport in Cells: Passive Path • Diffusion – Directly through plasma membrane How do other molecules get inside/outside cells? Transport or carrier proteins • Facilitated diffusion – Through transport proteins – Highly specific (channels, carriers) Fig. 7.5 27 Transport in Cells: Passive Driving force • Concentration or electrochemical gradients (H+, glucose) • No extra energy (ATP) required 28 Transport in Cells: Passive Channel Proteins • Provide hydrophilic corridor – Aquaporin • A channel protein that facilitate movement of water across a membrane – Ion channels • Specific to ions • May be gated (open/close in response to electrical stimulus) Carrier Proteins • Change conformation in response to stimulus 29 Active Transport 30 • Allows movement of a substance against its concentration or electrochemical gradient • Cell can maintain an internal environment that is different from the external environment • Requires energy (ATP) Path • Carrier proteins (Pumps) – Change conformation (shape) when they bind with ATP See Fig.7.17 Driving Force – Requires energy (ATP) 31 Active Transport Sodium-potassium pump • Most important pump in animals • Pumps ions against their steep concentration gradients • ATP Fig. 7.16 32 Active Transport & Membrane Potential • Membrane potential – Voltage across a membrane – Cytoplasm more negative in charge relative to extracellular fluid – Unequal distribution of anions & cations • Sodium-potassium pump – Moves 3 sodium (Na+) out of the cell – Moves 2 potassium (K+) into the cell – Generates an electrochemical gradient across membrane = electrogenic pump Active Transport & Membrane Potential • Electrochemical gradients – The combined effect of a concentration gradient and an electrical gradient. Affects the movement of ions across plasma membranes • Electrical gradient – differences in electrical charges across a plasma membrane (e.g., Na+) • Due to membrane potential – Favors passive transport of cations into cell – Favors passive transport of anions out of cell 33 Active Transport & Membrane Potential Most important pump in plants: proton (H+) pump • Moves H+ outside cell • Generates an electrochemical gradient across membrane – More positive on the outside of the cell Fig. 7.19 34 Active Transport: Coupled Transport Paths • Cotransporters – Brings two substances into a cell • Exchangers – Brings one substance into a cell and another substance out of a cell Driving force – Uses the movement of one molecule (H+) going down/with its concentration gradient to move another molecule against its concentration gradient – Linked to pumps Fig. 7.19 35 Transport of large molecules Large molecules (e.g., proteins) too big for transporters • Use vacuoles formed from plasma membrane – Exocytosis – secretion of cellular contents to the outside of a cell by fusion of vacuoles (vesicles) to the plasma membrane 36 Transport of large molecules Endocytosis • Uptake of extracellular material by engulfing and pinching off the plasma membrane to form a small membrane-bound vacuole (vesicle) in the cell 3 Types of Endocytosis • Pinocytosis • Phagocytosis • Receptor-mediated cytosis 37 Transport of large molecules Pinocytosis (“cellular drinking”) • Uptake of extracellular fluid by endocytosis • Low specificity Fig. 7.20 38 Transport of large molecules Phagocytosis (“cellular eating”) • The engulfment and uptake of a particle or cell by an extension of another cell’s plasma membrane. • Moderate specificity Fig. 7.20 39 Transport of large molecules Receptor-mediated cytosis • Endocytosis triggered by the binding of certain macromolecules outside the cell to membrane proteins • High specificity • Ligand – molecule that binds specifically to a receptor site of another molecule Fig. 7.20 40