FIGURE 1115 Two states of bilayer lipids'

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FIGURE 1115 Two states of bilayer lipids.
The structure and flexibility of the lipid
bilayer depend on temperature and on the kinds of
lipids present.
At low temperatures the lipids in a bilayer form
a semisolid gel phase, motion of individual lipid
molecules are strongly constrained the bilayer
is paracrystalline
At high temperatures Individual hydrocarbon
chains of fatty acids are in constant
motion produced by rotation about the
carboncarbon bonds. (liquid-disordered state, or
fluid state)
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At intermediate temperatures the lipids exist in
a liquid-ordered state there is less thermal
motion in the acyl chains of the lipid bilayer,
but lateral movement in the plane of the bilayer
still takes place.
At temperatures in the physiological range (about
20 to 40 C) long-chain saturated fatty acids
(such as 160 and 180) pack well into a
liquid-ordered array, but the kinks in
unsaturated fatty acids interfere with this
packing, favoring the liquid-disordered state.
Shorter-chain fatty acyl groups have the same
effect. The rigid planar structure of the
steroid nucleus, inserted between fatty acyl side
chains, reduces the freedom of neighboring fatty
acyl chains to move by rotation about their
carboncarbon bonds, forcing acyl chains into
their fully extended conformation. The presence
of sterols therefore reduces the fluidity in the
core of the bilayer, thus favoring the
liquid-ordered phase, and increases the thickness
of the lipid leaflet
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Bacteria synthesize more unsaturated fatty acids
and fewer saturated ones when cultured at low
temperatures than when cultured at higher
temperatures. As a result of this adjustment in
lipid composition, membranes of bacteria cultured
at high or low temperatures have about the same
degree of fluidity.
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FIGURE 1116 Motion of single phospholipids in a
bilayer.
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large, positive free-energy change.
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Transbilayer Movement of Lipids Requires Catalysis
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FIGURE 1117 Measurement of lateral diffusion
rates of lipids by fluorescence recovery after
photobleaching (FRAP).
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A lipid could circumnavigate E. coli in one
second.
Some membrane lipids diffuse laterally by up to
1 µm/s.
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FIGURE 1118 Hop diffusion of individual lipid
molecules.
Single particle tracking follow the movement of
a single lipid molecule in the plasma membrane on
a much shorter time scale. Rapid lateral
diffusion within small, discrete regions of the
cell surface.
Movement from one such region to a nearby region
is inhibited.
The track shown here represents a molecule
followed for 56 ms.
The pattern of movement indicates rapid
diffusion within a confined region (about 250
nm in diameter).
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FIGURE 1119 Restricted motion of the erythrocyte
chloridebicarbonate exchanger and glycophorin.
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Atomic Force Microscopy to Visualize Membrane
Proteins
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Single molecules of bacteriorhodopsin in the
purple membranes of the bacterium Halobacterium
salinarum
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E. coli aquaporin
Fo, the proton-driven rotor of the chloroplast
ATP synthase
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FIGURE 1120a Microdomains (rafts) in the plasma
membrane.
The cholesterol sphingolipid microdomains in the
outer monolayer of the plasma membrane, are
slightly thicker and more ordered (less fluid)
than neighboring microdomains rich in
phospholipids and are more difficult to dissolve
with nonionic detergent. They behave like
liquid-ordered sphingolipid rafts. These lipid
rafts are enriched in two classess of integral
membrane proteins two covalently attached long
chain saturated fatty acid and GPI anchored
proteins.
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FIGURE 1120b Microdomains (rafts) in the plasma
membrane.
The greater thickness of raft regions can be
visualized by atomic force microscopy (AFM).
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FIGURE 1121 Caveolin forces inward curvature in
membranes.
The protein caveolin has a central hydrophobic
domain and three palmitoyl groups on the
C-terminal of each monomeric unit, which hold the
molecule to the inside of the plasma membrane.
Caveolin binds cholesterol in the membrane, and
the presence of caveolin forces the associated
lipid bilayer to curve inward, forming caveolae
(little caves).
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Several families of integral proteins in the
plasma membrane provide specific points of
attachment between cells, or between a cell and
extracellular matrix proteins.
FIGURE 1122 Four examples of integral protein
types that function in cell-cell interactions
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Integrins are heterodimeric proteins anchored to
the plasma membrane by a single hydrophobic
transmembrane helix in each subunit. The large
extracellular domains of the a and ß subunits
combine to form a specific binding site for
extracellular proteins such as collagen and
fibronectin. Integrins also serve as receptors
and signal transducers, conveying information
across the plasma membrane in both directions.
Integrins regulate many processes, including
platelet aggregation at the site of a wound,
tissue repair, the activity of immune cells, and
the invasion of tissue by a tumor.
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Cadherins undergo homophilic (with samekind)
interactions with identical cadherins in an
adjacent cell.
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Immunoglobulin-like domain
Immunoglobulin-like proteins can undergo either
homophilic interactions with their identical
counterparts on another cell or heterophilic
interactions with an integrin on a neighboring
cell.
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Selectins have extracellular domains that, in
the presence of Ca 2 , bind specific
polysaccharides on the surface of an adjacent
cell. Selectins are present primarily in the
various types of blood cells and in the
endothelial cells that line blood vessels. They
are an essential part of the blood-clotting
process.
Lectin domain (binds carbohydrates)
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FIGURE 1123 Membrane fusion (fusion of two
membrane segments without
loss of continuity).

• Fusion of two membranes
• they recognize each other
• (2) their surfaces become closely apposed, which
  requires the removal of water molecules normally
  associated with the polar head groups of lipids
• (3) their bilayer structures become locally
  disrupted, resulting in fusion of the outer
  leaflet of each membrane (hemifusion)
• (4) their bilayers fuse to form a single
  continuous bilayer.
• (5) the fusion process is triggered at the
  appropriate time or in response to a specific
  signal (if its receptor mediated).
• Integral proteins fusion proteins mediate these
  events

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Membrane Fusion (example 1) FIGURE 1124 Fusion
induced by the hemagglutinin (HA) protein during
viral infection.
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Membrane Fusion (example 2) FIGURE 1125 Fusion
during neurotransmitter release at a synapse.
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FIGURE 1126 Summary of transport types.
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FIGURE 1127 Movement of solutes across a
permeable membrane. (a) Net movement of
electrically neutral solutes is toward the side
of lower solute concentration until equilibrium
is achieved (simple diffusion).
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(b) Net movement of electrically charged solutes
is dictated by a combination of the electrical
potential (Vm) and the chemical concentration
difference across the membrane net ion movement
continues until this electrochemical potential
reaches zero (electrochemical gradient
electrochemical potential).
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FIGURE 1128 Energy changes accompanying passage
of a hydrophilic solute through the lipid bilayer
of a biological membrane.
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FIGURE 1129 Classification of transporters. The
numbers here correspond to the main subdivisions
in Table 113.
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FIGURE 1130 Proposed structure of GLUT1. (a)
Transmembrane helices are represented as oblique
(angled) rows of three or four amino acid
residues, each row depicting one turn of the
helix. Nine of the 12 helices contain three or
more polar or charged amino acid residues, often
separated by several hydrophobic residues.
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(b) A helical wheel diagram shows the
distribution of polar and nonpolar residues on
the surface of a helical segment. The helix is
diagrammed as though observed along its axis from
the amino terminus.
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(c) Side-by-side association of five or six
amphipathic helices, each with its polar face
oriented toward the central cavity, can produce a
transmembrane channel lined with polar and
charged residues. This channel provides many
opportunities for hydrogen bonding with glucose
as it moves through the transporter.
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FIGURE 1131 Kinetics of glucose transport into
erythrocytes.(a) The initial rate of glucose
entry into an erythrocyte, V0, depends upon the
initial concentration of glucose on the outside,
Sout.
The kinetics of facilitated diffusion is
analogous to the kinetics of an enzyme-catalyzed
reaction. Note that Kt is analogous to Km, the
Michaelis constant.
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(b) Double-reciprocal plot
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FIGURE 1132 Model of glucose transport into
erythrocytes by GLUT1.
The transporter exists in two conformations T1,
with the glucose-binding site exposed on the
outer surface of the plasma membrane, and T2,
with the binding site exposed on the inner
surface. Glucose transport occurs in four
steps. (1). Glucose in blood plasma bind to a
stereospecific site on T1 this lowers the
activation energy. (2). A conformational change
from Sout T1 to Sin T2, effecting the
transmembrane passage of the glucose. (3).
Glucose is now released from T2 into the
cytoplasm. (4). the transporter returns to the T1
conformation, ready to transport another glucose
molecule.
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FIGURE 1 Regulation by insulin of glucose
transport by GLUT4 into a myocyte.