Membrane Structure and the Lipid bilayer

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Chapter 10
Lecture 23, Fall 2004

• Membrane Structure and the Lipid bilayer

pages 583-593
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• Importance of membranes
• Plasma membrane separates cellular components
  from the environment.
• Allows organelles to execute specialized
  functions by keeping the contents of the
  organelle separate from the rest of the cell.
• Provides boundaries that establish
  electrochemical gradients. These are used to make
  ATP and to generate nerve impulses.

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Biological membranes are thin films composed
mainly of amphipathic lipids and proteins. Most
of the molecules are held together by noncovalent
interactions.
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• Lipids
• Lipid molecules organized into a bilayer
  approximately 5 nm thick.
• Each layer of the bilayer is called a leaflet.
• Lipids diffuse rapidly in the plane of the
  bilayer, but with the exception of cholesterol,
  lipids do no flip from one leaflet to the other
  without assistance from specific proteins.
• Self sealing - when one microinjects something
  into a cell, the membrane seals automatically
  when the needle is withdrawn.
• Purified phospholipids spontaneously form
  bilayers in water.

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• Membrane Proteins
• Proteins embedded in the membrane span from one
  side to the other.
• Proteins diffuse in the plane of the bilayer
  unless they are anchored to something.
• Proteins will not flip-flop.

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• Lipids provide the basis for the spontaneous
  formation of membranes.
• Lipids are amphipathic meaning they have a
  hydrophobic part (nonpolar tail) and a
  hydrophilic part (polar head).
• This example is a phospholipid which is the most
  abundant type of lipid found in animal cell
  membranes.

Polar head
Hydrocarbon tail derived from fatty acid.
Nonpolar tail
Unsaturated hydrocarbon tail
saturated hydrocarbon tail
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Amphipathic molecules pack so as to minimize the
interaction between water and the nonpolar part
of the molecule. The two hydrocarbon tails give
phospholipids a cylindrical shape that causes the
molecules to pack as a bilayer in water.
Minimum contact between water and the hydrocarbon
chains is achieved by forming the bilayer into a
closed compartment.
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Recall that a major driving force in protein
folding is the coalescing of hydrophobic regions
caused by water.
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Polar residues remain in contact with water
because of hydrogen bonding and charge
interactions.
Nonpolar residues are forced together in an
aqueous environment because they interact
unfavorably with water.
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The movements of the lipids are restricted by the
need to maintain favorable interactions between
water and the polar head groups, and to avoid
unfavorable interactions between water and the
nonpolar tails.
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The mobility or fluidity of molecules within
the plane of the membrane is influenced by the
composition of the lipids. Cis-double bonds and
short hydrocarbon chains increase fluidity
because the hydrocarbon chains dont pack as well
as long unsaturated hydrocarbon chains.
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Four major phospholipids are found in mammalian
plasma membranes.
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I want you to know the structure of
phosphatidylserine and its constituent parts.
Serine
phosphate
glycerol
Chemical structures will be drawn in class
fatty acid
fatty acid
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• Another type of lipid called cholesterol
  decreases fluidity because it restricts the
  movement of the hydrocarbon chains.

• Important chemical characteristics of
  cholesterol
• Hydroxyl group constitutes the polar head group.
• OH is attached to rigid steroid ring.
• One hydrocarbon tail.

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In addition to phospholipids and cholesterol,
animal membranes contain glycolipids. Sugars
constitute the polar head group. Gangliosides
are common in nerve cells where they influence
the electrical properties of cell membranes.
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• Some lipids are asymmetrically distributed.
• glycolipids are restricted to the extracellular
  leaflet.
• phosphatidylserine is restricted to the
  cytoplasmic leaflet in a healthy cell.

• An example of a function for the asymmetry
• Dead cells are distinguished from live cells
  because phosphatidylserine becomes exposed on the
  outer leaflet in dead cells. The asymmetry is
  maintained by a phospholipid translocator that
  transports PS to the inner leaflet. This is
  inactivated in dying cells. A second protein
  called scramblase becomes active and transfers
  phospholipids nonspecifically in both directions
  resulting in exposure of PS on the outside of the
  cell. Macrophages detect the PS and destroy the
  dead cell.

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Lipid rafts provide an example of an uneven
distribution of lipids in the plane of the
membrane
Sphingomyelin and glycolipids attract each other
and coalesce into lipid rafts. Certain
proteins involved in cell signaling are grouped
together in these rafts. Cell signaling will be
discussed next semester. For now, recognize that
the grouping of the lipids and proteins into a
lipid raft occurs because of weak interactions
between the molecules and because the molecules
are free to diffuse laterally in the plane of the
membrane so they can interact with each other.