Membrane Structures

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Chapter 11

• Membrane Structures

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Plasma Membrane

• The container for the cell
• Holds the cytoplasm and organelles together
• Highly selective barrier for the cell
• Bacteria have a single membrane
• Eukaryotes have outer plasma membrane and
  internal membranes
  
• Endoplasmic reticulum
• Nuclear membrane
• Membrane-bound organelles

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Cell Membranes
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Cell Membrane Functions

• Receives signals from outside the cell for
  internal cellular activities
  
• Imports and exports molecules
• Movement of the cell

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General Structure

• A lipid bilayer that contains 2 sheets of lipids
  interdispersed with proteins
  
• Lipid gives the membrane its structure and can
  act as permeability barrier

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Lipid Structure

• Hydrophilic head H2O loving due to polar
  group in the head
  
• Hydrophobic tail H2O hating due to the long
  hydrocarbon tails

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Review of Lipids
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Lipids

• Most abundant lipid is the phospholipid
• Phospholipids have a PO4 group in the 3rd OH
  group of the glycerol instead of hydrocarbon
  
• This can attach a hydrophilic group
• Choline phosphyltidylcholine
• Polar amino acids like serine - phosphatidylserine

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Amphipathic Molecules

• Contain both a hydrophilic and a hydrophobic
  portion to the molecule
  
• Form a bilayer because of this
• Other molecules are amphipathic
• Steroids
• Glycolipids lipid with a sugar attached rather
  than a phosphate group

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Other Lipid Molecules
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Reminder

• Hydrophilic molecules can dissolve in H2O due to
  the polarity of both of these molecules
  
• H bonds and other non-covalent interactions may
  aid in this

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Reminder

• Hydrophobic molecules will be caged by the
  polar molecules requires energy
  
• Minimize energy requirement if orient the
  hydrophobic portions as far away from water as
  possible
  
• Why when fats or oils are placed in water that
  they usually sit as a glob on the surface

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Membrane

• Amphipathic molecules have both components so the
  hydrophilic head molecules interact with the
  aqueous solution and the hydrophobic tails will
  interact with each other energetically favorable

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Lipid Bilayer

• Due to amphipathic property the membrane can
  reseal after an injury
  
• Bilayer is fluid the orientation of the lipids
  and the outer aqueous surroundings keeps the
  lipids in the bilayer
  
• The lipid can move around the layer like one
  person moving in a crowded room
  
• Not the same as flexible entire membrane bending

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Liposomes

• Can study membranes by using artificial membrane
  structures called liposomes
  
• Can follow the movement of lipids in each of the
  layers

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What We Know

• Lipids cannot move from one layer to another
  without the aid of proteins
  
• Lipids can exchange places with neighbors based
  in thermal motion
  
• Decrease the temp will slow down the movement
• Lipids can rotate around their axis - rapidly

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Importance of Hydrocarbon

• Hydrocarbon tail will determine the fluidity of
  the membrane just as it does in fats and oils,
  also dependent on temperature
  
• 2 components are important
• Length of hydrocarbon chain
• 14 to 24 C but usually 18 to 20 C per tail
• Short increased fluidity
• Level of unsaturation ( of CC bonds)
• 1 tail has 1 or more CC bonds (unsaturated)
• Other tail is saturated (no CC bonds)

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Unsaturated Hydrocarbons

• Each CC bond causes a kink or bend in the tail
• Cant pack tightly in the layer
• More lipids that have unsaturated tails the more
  fluid the membrane

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Cholesterol in the Membrane

• Cholesterol is added to areas that have lots of
  unsaturated lipids to help fill in the gaps
  between the tails
  
• Helps to stiffen and stabilize the bilayer
• Less fluid
• Less permeable
• Only in animal cells

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Membrane Fluidity

• Enables the membrane proteins to diffuse rapidly
• Simple means of distributing lipids and proteins
• Allows membranes to fuse with one another
• Evenly distributed during daughter cell formation

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Membranes are Asymmetrical

• Inner surface is different from the outer
  surface
  
• Types of lipids in each layer
• Proteins in the bilayer have a specific
  orientation due to its function
  
• Both lipids and the proteins in the membrane are
  assymetrically distributed

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New Membrane

• New lipids are added on one side of the membrane
• Enzyme called flippase used to put the lipid in
  the other half of the bilayer
  
• Flippase may be selective for the type of lipids
  that it puts on either surface

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Asymmetry

• New membrane comes from the SER
• Vesicle buds off the SER and when fuses with the
  plasma membrane, the orientation is maintained
  
• Membranes have distinct inner and outer surface
• Inner cytosolic face
• Adjacent to the cytosol
• Outer non-cytosolic face
• Adjacent to the cell exterior or the interior of
  an organelle
  

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Special Lipids

• Glycolipids are found only on the non-cytosolic
  surface
  
• Sugar added in the Golgi apparatus
• No flippase to move the glycolipid to the
  cytosolic surface
  
• Inositol phospholipids are only on the cytosolic
  surface
  
• Functions to relay signals on cytosolic surface
  that pass through the membrane