chapter 7 membrane structure & function. slide 2 of 38 7.1 plasma membrane cell’s barrier to...
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Slide 2 of 38
7.1 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
CO2 and O2
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
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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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
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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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
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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?
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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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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
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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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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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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
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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
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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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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”