AP Biology (An Introduction)

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Chapter 7
Membrane Structure Function
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7.1 Plasma Membrane

• Cells 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

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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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Fluid Mosaic Model
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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

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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
• Cells 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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Tonicity
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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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Facilitated Diffusion (Channel Protein)
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Facilitated Diffusion (Carrier Protein)
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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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Na/K Pump
? Watch Na / K Animation
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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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Movement of LARGE molecules
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Animation-05-03.swf
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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

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