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01 Membrane Lipids

VIEWS: 4 PAGES: 23

									Stephen Fish, Ph.D.
Marshall University J. C. E. School of Medicine
Fish@Marshall.edu
Note to instructors:
I use these PowerPoint slides in cell biology lectures that I give to
first year medical students. Copy the slides, or just the
illustrations into your own teaching media. We all know that
teaching science often requires compromises and simplification
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course. Please feel free to offer suggestions for improvements,
corrections, or additional illustrations. I would be pleased to hear
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them at any resolution.

Stephen E. Fish, Ph.D.
Membrane Lipids
                    Generic cell




95% of cellular membrane is in intracellular organelles
Membranes are made of lipids and
           proteins
    Three membrane lipid types

Phospholipids   Cholesterol   Glycolipids
                    Phospholipids
 •   Phosphatidylcholine
 •   Phosphatidylserine
 •   Phosphatidylethanolamine
 •   Sphingomyelin (serine replaces glycerol backbone)
 •   Phosphatidylinositol (one of the less common types)

Hydrophilic
(soluble)



Hydrophobic
(not soluble
= lipophilic)
                     Cholesterol
• Up to 40% of mammalian cell membranes
• Small polar head makes it weakly amphipathic
                           Hydroxyl group
                           is a very small polar
                           (hydrophilic) head
    Rigid steroid
    hydrocarbon
    ring structure


                                   Flexible
      Nonpolar
                                   hydrocarbon
      (hydrophobic)
                                   chain
                     Glycolipids
• Pattern of sugar residues is variable
• Always in outer leaflet of cell membrane, &
  inner leaflet of organelles


       Hydrophilic
       (soluble)



      Hydrophobic
      (not soluble
      = lipophilic)
The hydrophobic effect & amphipathic lipids


                      Polar (hydrophilic) head

                      Nonpolar (hydrophobic) tail

                    Air




                   Water
Amphipathic lipids under water form
    membranes automatically
     More lipid molecules form a sphere
• Membrane disk (seen on edge) curves to minimize
  hydrophobic core exposure at edges
• Enough lipids automatically forms a liposome
• Similarly, punctured membranes seal themselves
          Why does it do that?

• Compare the amount of exposed edge of
  curved & flat membrane for each size
• Curved membrane exposes less of the
  hydrophobic interior at the edges
Membrane self assembly is useful to
             science
         Motion of lipids in membranes

• Molecular movement
  from heat
  – Rotate in place
  – Hydrophobic tails flex
  – Flip flop across
    membrane
     • Phospholipids rare
     • Cholesterol common
  – Lateral movement
• Phase transition- cool
  down & motion stops
        Membrane phase transition
• Lipid lateral, rotational, & flexional movement
  from heat spreads apart & maintains fluidity
• Cooling reduces movement & lipids stick
  together (congeals like bacon grease)
  – Stiffens, is easily torn & can’t reseal
  – Interferes with membrane protein function

            Liquid            Solidified
            when warm         when cooled
     Types of lipids affect phase transition
• Unsaturated phospholipids with bend in tail
  – Spread lipids out & makes more fluid
  – Lowers temperature of phase transition
• Cholesterol with bend & rigid steroid rings
  – Lowers temperature of phase transition
  – Increases range over which transition occurs
  – Net affect- membrane is more fluid but stiffer (tougher)
    Lipid asymmetry in leaflets of the cell
               membrane
              Extracellular leaflet has more
              Phosphatidylcholine
              Sphingomyelin, &
              Glycolipids (opposite for organelles)


Equal for
Cholesterol


              Intracellular leaflet has more
              Phosphatidylserine,
              Phosphatidylethanolamine, &
              Phosphatidylinositol
Some lipids in the outer leaflet of cellular
  membrane aggregate to form rafts
• Rafts have lower phase transition
  temperature & are stiffer
• Function discussed later with the membrane
  proteins associated with the rafts
Lipid rafts have higher concentrations of -

• Sphingolipid
• Cholesterol
• Glycolipids
  Although a minor membrane component
   (~2%), glycolipids have some special
                 functions



• Gangliosides in neurons
  – Oligosaccharides with negatively charged
    sialic acid residues
  – Attract positive ions, e.g. Ca++
  – Affects electrical properties & signaling
   Glycolipids help form insulation for
    nervous system electrical activity
• Negatively charged gangliosides in glia
  membrane
• Repel negative ions & attract positive ions
• Myelin insulation greatly increases the speed
  of action potentials
Membrane proteins are next and
       Sherman says




                         Lipids &
                         proteins,
                         OH BOY!

								
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