Chapter 7 Membrane Structure & Function

Slide 2 of Plasma Membrane  Cell’s barrier to the external world  Selectively permeable  Allows only certain molecules through  Most molecules CANNOT pass into the cell  Small hydrophobic molecules can pass in or out  CO 2 and O 2

Slide 3 of 38 Plasma Membrane (Page 2)  Main constituents: Phospholipids & Proteins  Membrane proteins  Allow hydrophilic molecules into the cell  Phospholipid bilayer  Barrier to most things  Amphipathic  Hydrophilic & hydrophobic regions  Hydrophilic head & hydrophobic tail

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Slide 5 of 38 Fluid Mosaic Model

Slide 6 of 38 Fluid Mosaic Model  Mosaic due to membrane proteins  Fluid due to the phospholipid bilayer  Unsaturated hydrocarbons = more fluid  Higher Temperature = Higher fluidity  Cholesterol = temperature buffer  Reduces fluidity at warm temps.  Increases fluidity at cold temp.

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Slide 8 of 38 How does it get in?  Phospholipid bilayer is the hydrophobic barrier  Only small & hydrophobic can cross  Hydrophilic molecules are unable to enter  Water & other polar molecules = no entry  For most molecules (large and/or polar) membrane proteins allow passage  Peripheral protein – Bound to 1 surface of the membrane  Integral proteins – embedded in membrane  Transmembrane proteins – completely span the membrane

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Slide 12 of 38 Membrane Proteins  Receptor proteins  Enzymatic proteins  Structural Support  Cell-to-cell OR Cell-to-ECM adhesion  Transport proteins  Type of transmembrane protein  Transport hydrophilic substances across membrane  Aquaporins  Transport protein for moving water across membrane

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Slide 16 of 38 Membrane Carbohydrates  Function in cell-cell recognition  Cell’s ability to distinguish cell types  Basis for rejection of foreign cells by the immune system  Blood types result from membrane carbohydrates

Slide 17 of 38 Questions 1. What do small & nonpolar substances require to cross the membrane? 2. What is an amphipathic molecule? 3. Explain why it is called the “fluid mosaic” model of the membrane? 4. What is the function of membrane proteins? 5. What is the function of a membrane carbohydrate? 6. What does adding cholesterol do to alter a membrane?

Slide 18 of 38 Passive Transport  Diffusion – movement from an area of high concentration to an area of low concentration  Moving down a concentration gradient  No energy expenditure for this transport  Osmosis – diffusion of water across a selectively permeable membrane  Osmosis is only done by water (for us)  Requires a permeable membrane

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Slide 20 of 38 Tonicity

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Slide 22 of 38 Facilitated Diffusion  Transport proteins that allow polar molecules & ions to cross the membrane  Passive transport but requires a membrane protein  Two means of operation  1. Channel Proteins - Hydrophilic channels  2. Carrier Proteins – shape change = translocation

Slide 23 of 38 Facilitated Diffusion (Channel Protein)

Slide 24 of 38 Facilitated Diffusion (Carrier Protein)

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Slide 26 of 38 Questions? 1. How is facilitated diffusion different from simple diffusion? 2. What type of energy does facilitated diffusion require? 3. What are the 2 versions of facilitated diffusion? 4. How is osmosis different from diffusion? 5. What is a concentration gradient? 6. Passive transport goes down or goes against the concentration gradient?

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Slide 28 of 38 Active Transportation  Some transport proteins do facilitated diffusion, other do active transport  Active transport = carrier proteins only  why?  Move substances against their concentration gradient  Lower concentration  Higher concentration  Requires energy or ATP  Maintains separate & different internal environment  Establishes or maintains a gradient

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Slide 30 of 38 Na+/K+ Pump  Watch Na+ / K+ Animation

Slide 31 of 38 Electrochemical Gradients  Not concentration gradient, but electrochemical gradient  Both concentration & charge gradients  Commonly referred to as membrane potential  Negative = Inside  Favored movement:  Anions  Out  Cations  In

Slide 32 of 38 Electrochemical Gradients  Ions move down their concentration & ionic gradients  Transport protein that establishes a voltage across the membrane = Electrogenic Pump  Na+/K+ Pump  Proton Pump

Slide 33 of 38 CoTransport (Secondary Active Transport)  Electrogenic pump creates a concentration gradient  THEN Ions move down their gradient, and ACTIVELY transport another substance

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Slide 35 of 38 Movement of LARGE molecules

Slide 36 of 38 Animation swf

Slide 37 of 38 Phagocytosis & Pinocytosis  Phagocytosis – cell membrane wraps around and takes in a solid particle  Form of endocytosis  Called “cell eating”  Pinocytosis – cell membrane wraps around and takes in small amount of liquid (extracellular fluid)  Form of endocytosis  Called “cell drinking”

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