Membrane Transport Chapter 12

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Membrane TransportChapter 12

• Principles of membrane transport
• Carrier proteins and their functions
• Ion channels and the membrane potential

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• Rates of diffusion of molecules through the
  membrane vary
• Rapid diffusion of small, non polar (oxygen,
  carbon dioxide)
• Rapid diffusion of uncharged, polar (water,
  ethanol, glycerol)
• Impermiable for ions (sodium, potassium ions) and
  large polar molecules (amino acids)
• Fig 12-1, -2

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• Membrane transport proteins are divided into two
  kinds
• carrier proteins
• Bind to solutes with great specificity
• capable of active or passive transport
• Active transport is uphill, from low to high
  concentration and requires energy
• Passive transport is downhill, from high to low
  concentration
• Ion channels
• Small molecules of appropriate charge can diffuse
• Fig 12-3, -4

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• Glucose carrier in liver cells is an example of
  carrier protein that mediates passive transport
• After a meal glucose level is high outside of the
  liver cells glucose transported into the liver
  cells
• When you get hungry, large amounts of glucose are
  produced by the breakdown of glycogen
• Fig 12-7

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• Electrochemical gradient
• It is a sum of solute charge and the membrane
  potential
• (A) No membrane potential
• (B) concentration gradient is facilitated by
  membrane potential
• (C) membrane potential decreases the driving
  force
• Fig 12-8

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• Active transport-transport of solutes against the
  concentration gradient
• Coupled transport-coupling of the uphill
  transport of one solute and the downhill
  transport of the other
• ATP-driven pump-uphill transport coupled with ATP
  hydrolysis (ATP-driven sodium pump)
• Light driven pumps-uphill transport coupled to
  light energy (mainly bacteria)
• Fig 12-9

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• Na and K ATP-ase
• Pumps Na out of the cell (against gradient)
• Pumps K into the cell (against gradient)
• Hydrolysis of ATP
• This protein is not only and a carrier abut also
  an enzyme

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• What about the membrane potential?
• Na pump works not only against concentration
  gradient but also against the voltage gradient
• K pump is against concentration gradient but
  electric forces are pulling K inside the cell
• Fig 12-10

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• A gradient of Na generated by the Na - K pump
  can be used to transport another molecule uphill
• Types of coupled transporters
• Symport-both solutes move in the same direction
• Antiport-solutes move in the opposite direction
• Not a coupled transporter
• Uniport-one type of solute in one direction
• Fig 12-13

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• Examples of coupled transport-symport of Na
  -glucose
• Active glucose transport (symport) in the lumen
  of the gut
• Passive glucose transport (uniport) in all other
  cells and tissues
• Fig 12-14, -15

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• Na - K pump helps maintain osmotic balance
• Water molecules can move freely through the
  membrane and strive to equal solute concentration
  
• Movement of water from a low solute concentration
  to a high solute concentration is called osmosis
• The driving force for the water movement is
  equivalent to a difference in water pressure and
  is called osmotic pressure
• An animal cell in water (no cell wall) will burst
• Fig 12-16

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• Animal cells pump out certain ions or unwanted
  solutes
• Plant cells surrounded by a rigid cell wall
  (pressure changes will no make them burst)
• Protozoans (amobea) living in fresh water collect
  water in vacuolas and discharge their content to
  the exterior
• Fig 12-17

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• Ion channels
• Allow for small, water soluble molecules to pass
  through the hydrophilic channel
• Concentration of inorganic ions such as Na , K
  , Ca2 , Cl- is regulated
• Ion selective
• Depends on ion size, charge
• Ion gated
• Briefly open and close
• Fig 12-19

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• Membrane potential
• Voltage difference across a membrane due to a
  slight excess of positive ions on one side and a
  negative ions on the other side
• A typical membrane potential is -60mV
• Open ion channel allows for the flow of ions and
  changes the voltage across the membrane (membrane
  potential)
• Fig 12-20

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• Electricity in a aqueous solution is carried by
  ions, either or - charged
• An ion flow across the cell membrane is
  detectable as an electric current
• An accumulation of ions is detectable as an
  accumulation of electric charge, or membrane
  potential
• Fig 12-27

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• Membrane potential is generated and maintained by
  ion movement
• K high inside the cell
• K low outside the cell
• K leaky channel
• Randomly opens and closes
• When open, K will move freely and flow out
• Electric field created that opposes any other
  movement of K out of the cell
• Imbalance of charges will tend to drive K back
  into the cell
• Fig 12-28

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• A typical neuron is stimulated by a a signal
  which initiates the change in membrane potential
• The axon conducts the signal away from the body
• Dendrites receive signals from axons of other
  neurons
• Fig 12-30

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• An electrical signal is converted into a chemical
  signal at the nerve terminal
• Action potential reaches the nerve terminal
• Plasma membrane channels open and allow Ca2 ions
  to flow in
• Stimulated synaptic vesicle fuses with the cell
  membrane
• Neurotransmitter is released
• Fig 12-40

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• Conversion of a chemical signal into an
  electrical signal
• Released neurotransmitter binds to and opens the
  transmitter gated ion channels
• The resulting ion flow changes the membrane
  potential of the postsinaptic cell
• The chemical signal was converted into the
  electrical one
• Fig 12-41