Chapter 11 Membrane Structure

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Chapter 11Membrane Structure

• The Lipid Bilayer
• Membrane Proteins

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• Function of a membrane
• a barrier preventing cell content to escape
• nutrients going in
• waste products going out
• Sensors
• Signals to turn on/off genes
• Movement and expansion
• Some organelles are separated by membranes
• ER, Golgi, mitochondria and others
• Fig 11-1, -2, -3

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• Cell membrane is composed of a lipid bilayer with
  embedded proteins
• Membrane lipids are composed of
• A hydrophobic tail (hydrocarbons)
• A hydrophilic head (lipids)
• Fig 11-4, -5

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• Phosphatidyl choline-the most abundant
  phospholipid
• Its hydrophilic part is a choline molecule
  attached to a phosphate
• A hydrophobic part consists of hydrocarbon chains
• Fig 11-6

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• Examples of other membrane lipids, which are
  amphiphatic molecules
• Phosphatidylserine
• Cholesterol
• Galactoserebroside
• Fig 11-7

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• Triacylglycerol-hydrophobic only, in water will
  form a large drop
• Phospholipid-amphiphatic molecule, in water will
  form a bilayer
• A bilayer will assemble itself
• Tear in a membrane is energetically unfavorable
• A bilayer will spontaneously rearrange and seal
  itself
• Membrane is of a fluid nature and in constant
  movement
• Fig 11-10, -11, -12, -13

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• Phospholipids move within the plane of the
  membrane-rotation and flexion
• Enzymes called flippases will transfer
  phospholipid to the opposite monolayer (occurs
  very rarely)
• Fig 11-15

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• Saturated fatty acids-the hydrocarbon tail has
  single bonds only
• Unsaturated fatty acids
• double bonds are present (does not contain the
  maximum of hydrogen bonds)
• each double bond creates a kink
• the more kinks the more the membrane is fluid
• Cholesterol
• Hydrophilic group (polar OH) and a hydrophobic
  tail
• Presence of cholesterol in a membrane controls
  its fluidity-the more cholesterol in the membrane
  the more rigid it becomes
• Fig 11-16

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• The lipid bilayer is asymmetrical
• Different selection of phospholipids and
  glycolipids are facing inside and outside of the
  cell
• Formation of a lipid bilayer is
• rapid and spontaneous in water
• Hydrophobic interactions are the major driving
  force
• Hydrocarbon tails van der Waals forces
• Hydrogen bonds between polar heads and water
  molecules
• Fig 11-17

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Membrane ProteinsFunction

• Allow for transport in/out of the cell
• Anchors the membrane to other macromolecules
• Receptors, ability to detect external signal
• Catalyze reactions
• Fig 11-20

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• Proteins in lipid bilayer-types of association
• Transmembrane proteins-pass through the bilayer
• Membrane associated-entirely in cytosol,
  associated with the inner membrane
• Outside of the bilayer-covalently linked to the
  bilayer lipids
• Attached to other membrane proteins by
  non-covalent bonds
• Fig 11-21, 11-23

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• Aqueous pores allow water soluble molecules to
  cross the membrane
• 5 transmembrane helices form a water filled
  channel
• Hydrophilic amino acids are in the middle of the
  aqueous channel
• Hydrophobic amino acids are on the outside
• Fig 11-24

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• Beta barrel
• Formed from beta sheets
• Porin proteins-aqueous channel
• Fig 11-25, -26, -27

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• Bacteriorhodopsin-a membrane transport protein
  which is a photosynthetic reaction center
• Found in archaebacterium Halobacterium
• Utilizes sunlight
• Composed of retinal covalently attached to one of
  the 7 alpha helices
• Membrane transport protein that pumps hydrogen
  ions outside of the cell and generates excess of
  H ions outside of the cell
• Hydrogen ions flow back to the cell and the ATP
  is generated
• Fig 11-28

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• Photosynthetic reaction center of the bacterium
  Rhodopseudomonas captures energy from sunlight
• A large complex composed of 3 transmembrane
  proteins bound to the fourth protein, cytochrome
• This complex utilizes light energy absorbed by
  chlorophyll and produces high energy electrons
  required for photosynthetic reactions
• Fig 11-29

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• Carbohydrates on the outer cell surface
• Glycoproteins-carbohydrates covalently linked to
  proteins
• Glycolipids-carbohydrates covalently linked to
  lipids
• Fig 11-32

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• Function of glycolipids on the cell surface
• Inflammatory response
• Carbohydrate on the surface of neutrophils (WBC)
  is recognized by lectin (transmembrane protein in
  the endothelial cells lining blood vessels)
• Neutrophils adhere to the blood vessels and
  migrate to the site of inflammation
• Fig 11-23