Membrane Fluidity and Permeability

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Membrane Fluidity and Permeability

• Permeability depends on FA chain length and
  double bonds of membrane lipids due to
  hydrophobic interactions btwn tails
• unsaturated double bonds, cause kink in FA
  chain
• kinks prevent close packing of FA tails
• ? spacing ? interactions btwn tails
• 109.5, not free to rotate
• saturated no double bonds
  
• maximum of H
• packs tightly
• ? spacing stronger interactions
• ? S
• 120, freely rotates

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Membrane Fluidity and Permeability

• Unsaturated and/or shorter FA tails ? fluidity
  ? permeablility
• Saturated and/or longer FA tails ? fluidity ?
  permeablility

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Membrane Fluidity and Permeability

• Effect of temperature
• at 25C bilayer is "fluid" (like olive oil)
• ? temp. packs tightly ? fluidity ?
  permeability
• membrane solidifies if temp. is low enough
• ? temp. ? fluidity
  
  ? permeability

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Membrane Fluidity and Permeability

• Cholesterol small, amphipathic steroid
• fills space btwn phosopholipid tails allowing
  closer packing
• thus chloesterol ? lipid fluidity and permeability

Polar
Nonpolar
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Diffusion

• Diffusion directed, spontaneous mvmt
  of
  molecules/ions from HI ? LO
• down conc. gradient
• due to random motion of molecules
• ? entropy
• 2 types molecules
• molecules diffuse across membrane
• each along own conc. gradient
• equilibrium est. where each type molecule
  has equal conc. on
  each side of membrane
• move at equal rates

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Osmosis

• Osmosis diffusion of water when 2 solns. with
  different solute are separated by membrane
  permeable to water but not solutes
• water molecules bound to solutes not free to
  diffuse
• water molecules diffuse from HI free water to
  LO free water
• water moves from area w/ ? solute (? water)
  to area w/ ? solute (? water)
• net mvmt of water toward ? solute

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Osmosis

• Osmosis can cause shrinking or swelling of cells
  depending on conc. of solutes and water
• hypertonic has ? solute and ? water
  relative to another soln.
• hypotonic has ? solute and ? water relative
  to another soln.
• isotonic soln. having same solute and water

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Osmosis

• If internal soln. is hypotonic to
  soln. outside
• water moves out of cell into
  external soln.
  
• vesicle shrinks
• If internal soln. is hypertonic to
  soln. outside
• water moves into cell from external soln.
• vesicle swells/bursts
• If internal soln. is isotonic to soln. outside
• no net water mvmt
• volume stays same

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Membrane Proteins

• More lipids than proteins in cell membranes
• but proteins comprise as much mass
• func. signalling, transport, enzymatic
• Membrane proteins form channels/
  transporters/pumps
• affect membrane permeability
• Fluid Mosaic model (Singer-Nicolson, 1972)
  proteins inserted into lipid bilayer, making
  membrane a mosaic of phospholipids/proteins
• amphipathic proteins w/ nonpolar AA
  can integrate
  into bilayer

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Membrane Proteins

• Integral membrane proteins (a.k.a. transmembrane
  proteins) span membrane
• Peripheral membrane proteins on membrane
  surface
• often attached to integral proteins

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Diffusion via Membrane Proteins

• Membrane proteins can form channels/transporters/p
  umps
• affect membrane permeability
• active transport vs. passive transport/facilitated
  diffusion
• against or down electrochemical gradient
• requires ATP or not
• ? conc. differences

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Diffusion via Membrane Proteins

• Ions move as response to electrochemical gradient
  via ion channels and ionophores (ion mover)
• ? charge differences by enabling small, charged
  compounds to cross membranes
• ion channels
• aquaporins channel for water, gated channels
  open/close
• ionophores (ion carriers/transporters)
  (facilitated diffusion)
• bind, diffuse across membranes, release on other
  side

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Facilitated Diffusion via Membrane Proteins

• Transport proteins also ? differences of other
  molecules btwn inside and outside of cell
• occurs only down an electrochemical gradient
• e.g. glucose

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Active Transport

• Pumps create electrochemical gradient across
  membranes
• active transport
• Na/K pump uses ATP to transport Na and K
• 3 Na binds to Na/K pump inside of cell
• phosphorylation of Na/K pump by ATP change in
  protein shape
• Na released outside the cell
• 2 K binds to Na/K pump on exterior of cell
• K released inside the cell (dephosphorylates and
  returns to original shape)

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Summary