Membranes

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Stephen Fish, Ph.D. Marshall University J. C. E.
School of Medicine Fish_at_Marshall.edu
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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 for
specific student populations, or the requirements
of a specific course. Please feel free to offer
suggestions for improvements, corrections, or
additional illustrations. I would be pleased to
hear from anyone who finds my work useful, and am
always willing to make it better. Also, the
images have been compressed to screen resolution
to keep PowerPoint file size down, and I can
provide them at any resolution. Stephen E.
Fish, Ph.D.
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Membrane Lipids
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Generic cell
95 of cellular membrane is in intracellular
organelles
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Membranes are made of lipids and proteins
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Three membrane lipid types
Phospholipids
Glycolipids
Cholesterol
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Phospholipids

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

Hydrophilic (soluble)
Hydrophobic (not soluble lipophilic)
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Cholesterol

• Up to 40 of mammalian cell membranes
• Small polar head makes it weakly amphipathic

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Glycolipids

• Pattern of sugar residues is variable
• Always in outer leaflet of cell membrane, inner
  leaflet of organelles

Hydrophilic (soluble)
Hydrophobic (not soluble lipophilic)
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The hydrophobic effect amphipathic lipids
Air
Water
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Amphipathic lipids under water form membranes
automatically
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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

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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

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Membrane self assembly is useful to science
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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

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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 cant reseal
• Interferes with membrane protein function

Liquid when warm
Solidified when cooled
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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)

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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
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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

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Lipid rafts have higher concentrations of -

• Sphingolipid
• Cholesterol
• Glycolipids

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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

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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

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Membrane proteins are next and Sherman says
Lipids proteins, OH BOY!