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CHAPTER 7 MEMBRANE STRUCTURE & FUNCTION
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I Can’s Explain why membranes are selectively permeable Describe the roles of phospholipids, proteins, & carbohydrates in membranes Outline how water will move if a cell is placed in an isotonic, hypertonic, or hypotonic solution Identify how electrochemical gradients are formed
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7.1 The cell (plasma) membrane is selectively permeable Allows some things to cross easier than others Made primarily of phospholipids & proteins Held together by weak interactions that cause the membrane to be fluid The FLUID MOSAIC MODEL describes the the membrane as fluid, with proteins embedded in or associated with the phospholipid bilayer
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PHOSPHOLIPIDS Provide a hydrophobic barrier that separates the cell from its liquid environment Hydrophilic cannot easily enter the cell Hydrophobic can enter more easily
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PROTEINS 2 types of proteins: 1) Integral Completely embedded in the membrane Some are transmembrane proteins that span the membrane completely 2) Peripheral Loosely bound to the surface of the membrane
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Six major functions of membrane proteins: Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)
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CARBOHYDRATES Crucial for cell-cell recognition (for immune function) Important for developing organisms (tissue differentiation) Cell surface carbs vary from species to species and are the reason that blood transfusions must be type-specific
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7.2 Membrane structure results in selective permeability Nonpolar molecules (hydrocarbons, CO 2, & O 2 ) Hydrophobic & can dissolve in the phospholipid bilayer & cross the membrane
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Hydrophobic core of the membrane Impedes the passage of ions & polar molecules (hydrophilic) Hydrophilic substances can avoid the lipid bilayer by passing through transport proteins (transmembrane proteins) Carrier proteins bind to molecules and change shape to shuttle them across the membrane Movement of water Move through special transport proteins called aquaporins by accelerating passage
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7.3 Passive transport is diffusion of a substance across a membrane with NO ENERGY investment Hydrocarbons, CO 2, & O 2 exhibit PT Passive diffusion Travels from high concentration to a less concentration Flows down the concentration gradient Requires NO WORK Relies on thermal motion energy
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Osmosis = diffusion of water across a selectively permeable membrane 3 relationships: 1) Isotonic 2) Hypertonic 3) Hypotonic
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Isotonic No net movement across the membrane Water crosses at the same rate in both directions
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Hypertonic solution Cell will lose water to its surroundings More solutes around the cell so water moves to the higher concentration Cell loses water, shrivels, & dies
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Hypotonic Water will enter faster than it leaves Fewer solutes in the water around the cell Movement of water follows the higher concentration of solutes The cell will swell and possibly burst
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Ions and polar molecules Cannot easily pass across the membrane Called facilitated diffusion Utilizes transport proteins TP are specific for what they transport
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How transport proteins work: 1) provide a hydrophilic channel that molecules can pass through 2) bind loosely to the molecules and carry them through the membrane
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7.4 Active transport uses energy to move solutes against their gradients Moved from less concentrated to higher concentrated (think of uphill movement) Requires energy (usually ATP)
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Sodium-Potassium Pump Pumps sodium out of the cell and potassium into the cell Necessary for proper nerve transmission Major energy consumer in your body
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DIFFUSION OF IONS Membrane Potential The difference in electric charge across a membrane that is expressed in voltage The inside carries a (-) charge Leads to an attraction with a Cation such as sodium This leads to 2 forces called the electrochemical gradient: 1) a chemical force, which is the ion’s [ ] gradient 2) a voltage gradient, attracts + ions and repels - ions
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Electrogenic pump A transport protein that generates voltage across a membrane Na-K pump & Proton pump are examples
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Cotransport An ATP pump that transports a specific solute indirectly drives the active transport of other substances The substance that was initially pumped across the membrane can do work as it moves back across the membrane by diffusion & will bring a second compound against its gradient
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7.5 Bulk transport across the membrane occurs by exocytosis & endocytosis Exocytosis Vesicles from the cell’s interior fuse with the cell membrane Expels the contents of the vesicles
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Endocytosis Cell forms new vesicles from the membrane (reverse of exo) Allows the cells to take IN large molecules 3 types: 1) Phagocytosis 2) Pinocytosis 3) Receptor-mediated endocytosis
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Phagocytosis – “cellular eating” Occurs when the cell wraps pseudopodia around a solid particle and brings it into the cell
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Pinocytosis – “Cellular drinking” Cell takes in small droplets of extracellular fluid within small vesicles Not specific, because it takes in anything
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Receptor-mediated endocytosis VERY SPECIFIC Certain substances (ligands) bind to specific receptors on the cell’s surface (clusters) Causes a vesicle to form around the substance and then pinch off into the cytoplasm
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