Biological Membranes and Transport

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Chapter 11 Biological Membranes and Transport
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Contents
11.1 The Composition and Architecture of
Membranes 11.2 Membrane Dynamics 11.3 Solute
Transport across Membranes
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FIGURE 111 Biological membranes. Membranes are
flexible, self-sealing, selectively permeable to
polar solutes. An erythrocyte is stained with
osmium tetroxide and viewed with an electron
microscope, the plasma membrane appears as a
three-layer structure, 5 to 8 nm (50 to 80 Å)
thick.
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Each Type of Membrane Has Characteristic Lipids
and Proteins
The relative proportions of protein and lipid
vary with the type of membrane,
reflecting the diversity of biological roles.
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FIGURE 112 Lipid composition of the plasma
membrane and organelle membranes of a rat
hepatocyte.
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FIGURE 113 Fluid mosaic model for membrane
structure. The fatty acyl chains in the interior
of the membrane form a fluid, hydrophobic region.
Integral proteins float in this sea of lipid,
held by hydrophobic interactions with their
nonpolar amino acid side chains. Both proteins
and lipids are free to move laterally in the
plane of the bilayer, but movement of either
from one face of the bilayer to the other is
restricted. The carbohydrate moieties attached
to some proteins and lipids of the plasma
membrane are exposed on the extracellular
surface of the membrane.
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FIGURE 114 Amphipathic lipid aggregates that
form in water.
Depending on the precise conditions and the
nature of the lipids, three types of lipid
aggregates can form when amphipathic lipids are
mixed with water.
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Micelles spherical structures, a few dozen to a
few thousand amphipathic molecules, hydrophobic
regions aggregated in the interior, where water
is excluded, and hydrophilic head groups at the
surface, in contact with water.
Micelle formation is favored when the
cross-sectional area of the head group is greater
than that of the acyl side chain(s). e.g. free
fatty acids, lysophospholipids (phospholipids
lacking one fatty acid), and detergents such as
sodium dodecyl sulfate (SDS)
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Bilayer two lipid monolayers (leaflets) form a
two-dimensional sheet, the cross-sectional areas
of the head group and acyl side chain(s) are
similar, as in glycerophospholipids and
sphingolipids. The hydrophobic portions in each
monolayer, excluded from water, interact with
each other. The hydrophilic head groups interact
with water at each surface of the bilayer.
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Liposome the bilayer sheet is relatively
unstable and spontaneously forms a third type of
aggregate it folds back on itself to form a
hollow sphere, a vesicle or liposome. By forming
vesicles, bilayers lose their hydrophobic edge
regions, achieving maximal stability in their
aqueous environment. These bilayer vesicles
enclose water, creating a separate aqueous
compartment.
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FIGURE 115 Asymmetric distribution of
phospholipids between the inner and outer
monolayers of the erythrocyte plasma membrane.
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• The distribution of a specific phospholipid is
  determined by treating
• the intact cell with phospholipase C, which
  cannot reach lipids in the inner monolayer
  (leaflet) but removes the head groups of lipids
  in the
• outer monolayer. The proportion of each head
  group released provides an estimate of the
  fraction of each lipid in the outer monolayer.
• Plasma membrane lipids are asymmetrically
  distributed between the two monolayers of the
  bilayer.
• Changes in the distribution of lipids between
  plasma membrane leaflets have biological
  consequences. For example, only when the
  phosphatidylserine in the plasma membrane moves
  into the outer leaflet is a platelet able to play
  its role in formation of a blood clot.
• For many other cells types,
  phosphatidylserine exposure on the outer surface
  marks a cell for destruction

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FIGURE 116 Peripheral and integral proteins.
Determine membrane protein topology
(localization relative to the lipid bilayer)
To Remove peripheral proteins changes in pH or
ionic strength, removal of Ca 2 by a chelating
agent, or addition of urea or carbonate. (destroy
electrostatic interactions, hydrogen
bonding) Integral proteins treatment with
detergents, organic solvents, denaturants.
(destroy hydrophobic interactions) Integral
proteins covalently attached to a membrane lipid
treatment with phospholipase C.
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FIGURE 117 Transbilayer disposition of
glycophorin in an erythrocyte.
Many Membrane Proteins Span the Lipid Bilayer
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FIGURE 118 Integral membrane proteins. Types I
and II have only one transmembrane helix the
amino-terminal domain is outside the cell in type
I proteins and inside in type II. Type III
proteins have multiple transmembrane helices in a
single polypeptide. Type IV proteins,
transmembrane domains of several different
polypeptides assemble to form a channel through
the membrane. Type V proteins are held to the
bilayer primarily by covalently linked
lipids. Type VI proteins have both transmembrane
helices and lipid (GPI) anchors.
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FIGURE 119 Bacteriorhodopsin, a
membrane-spanning protein. (PDB ID 2AT9) The
single polypeptide chain folds into seven
hydrophobic helices, each of which traverses the
lipid bilayer roughly perpendicular to the plane
of the membrane. The seven transmembrane helices
are clustered, and the space around and between
them is filled with the acyl chains of membrane
lipids.
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FIGURE 1110 Three-dimensional structure of the
photosynthetic reaction center of
Rhodopseudomonas viridis, a purple
bacterium. The first integral membrane protein
to have its atomic structure determined by x-ray
diffraction methods (PDB ID 1PRC).
Eleven -helical segments from three of the four
subunits span the lipid bilayer, forming a
cylinder 45 Å (4.5 nm) long hydrophobic residues
on the exterior of the cylinder interact with
lipids of the bilayer.
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FIGURE 1111 Hydropathy plots.
The Topology of an Integral Membrane Protein Can
Be Predicted from Its Sequence
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A single hydrophobic sequence a helix that
traverses the membrane bilayer.
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Seven hydrophobic regions Seven transmembrane
helices.
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FIGURE 1112 Tyr and Trp residues of membrane
proteins clustering at the water-lipid interface.
Tyr orange Trp red Charged residues (Lys, Arg,
Glu, Asp) blue
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Fig 11-13. Membrane proteins with -barrel
structure. The first four are from the E. coli
outer membrane. The fifth is Staphylococcus
aureus -hemolysin toxin.
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FIGURE 1114 Lipid-linked membrane proteins.
Covalently attached lipids anchor membrane
proteins to the lipid bilayer.