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CHAPTER 7 MEMBRANE STRUCTURE & FUNCTION. I Can’s  Explain why membranes are selectively permeable  Describe the roles of phospholipids, proteins, &

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Presentation on theme: "CHAPTER 7 MEMBRANE STRUCTURE & FUNCTION. I Can’s  Explain why membranes are selectively permeable  Describe the roles of phospholipids, proteins, &"— Presentation transcript:


2 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

3 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


5 PHOSPHOLIPIDS  Provide a hydrophobic barrier that separates the cell from its liquid environment  Hydrophilic cannot easily enter the cell  Hydrophobic can enter more easily

6 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

7  Six major functions of membrane proteins:  Transport  Enzymatic activity  Signal transduction  Cell-cell recognition  Intercellular joining  Attachment to the cytoskeleton and extracellular matrix (ECM)


9 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

10 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

11  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

12 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

13 Osmosis = diffusion of water across a selectively permeable membrane 3 relationships: 1) Isotonic 2) Hypertonic 3) Hypotonic

14  Isotonic  No net movement across the membrane  Water crosses at the same rate in both directions

15  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

16  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


18  Ions and polar molecules  Cannot easily pass across the membrane  Called facilitated diffusion  Utilizes transport proteins  TP are specific for what they transport

19  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


21 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)

22  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


24 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

25  Electrogenic pump  A transport protein that generates voltage across a membrane  Na-K pump & Proton pump are examples


27  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



30 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

31  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

32  Phagocytosis – “cellular eating”  Occurs when the cell wraps pseudopodia around a solid particle and brings it into the cell

33  Pinocytosis – “Cellular drinking”  Cell takes in small droplets of extracellular fluid within small vesicles  Not specific, because it takes in anything

34  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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