Download 24.7 Structure of Cell Membranes

24.7 Structure of Cell Membranes
• Phospholipids provide the basic structure of cell
membranes, where they aggregate in a closed,
sheet-like structure the lipid bilayer. The bilayer
is formed by two parallel layers of lipids oriented
so that their ionic head groups protrude into the
aqueous environments on either side of the
bilayer. Their nonpolar tails cluster together in
the middle of the bilayer where they can interact
and avoid water.
• When phospholipids are shaken vigorously with
water, they spontaneously form liposomes, small
spherical vesicles with a lipid bilayer surrounding
an aqueous center.
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• Lipid bilayer: The basic structural unit of cell
membranes; composed of two parallel sheets of
membrane lipid molecules arranged tail to tail.
• Liposome: A spherical structure in which a lipid
bilayer surrounds a water droplet.
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• 20% or more of the
weight of a membrane
consists of protein
molecules, many of
them glycoproteins.
• Peripheral proteins are
associated with just one
face of the bilayer.
• Integral proteins extend
completely through the
cell membrane and may
twist in and out of the
membrane many times
before ending on the
outside with a
hydrophilic sugar group.
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• The overall structure of cell membranes is
represented by the fluid-mosaic model.
• The membrane is described as fluid because it is
not rigid and molecules can move around within
it, and as a mosaic because it contains many
kinds of molecules. Oil floating on water is an
analogy for the fluid-mosaic cell membrane
model.
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24.8 Transport Across Cell Membranes
• Active transport:
Movement of substances
across a cell membrane
using energy.
• Energy from the conversion
of ATP to ADP is used to
change the shape of an
integral protein (the Na/K
pump), simultaneously
bringing two K+ ions into the
cell and moving three Na+
ions out of the cell
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• Passive transport: Movement of a substance
across a cell membrane without the use of
energy, from a region of higher concentration to a
region of lower concentration.
• Simple diffusion: Passive transport by the
random motion of diffusion through the cell
membrane or through channel proteins. Lipid
soluble and small hydrophilic molecules move by
simple diffusion.
• Facilitated diffusion: Passive transport across a
cell membrane with the assistance of a protein
that changes shape.
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24.9 Eicosanoids: Prostaglandins
and Leukotrienes
• The eicosanoids are a group of compounds
derived from 20-carbon unsaturated fatty
acids (eicosanoic acids) and synthesized
throughout the body. They function as shortlived chemical messengers that act near their
points of synthesis (“local hormones”).
• The prostaglandins (named for their discovery
in prostate cells) and the leukotrienes (named
for their discovery in leukocytes) are two
classes of eicosanoids that differ somewhat in
their structure.
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• The prostaglandins all contain a five
membered ring, which the leukotrienes lack.
Prostaglandins and leukotrienes are
synthesized in the body from the 20- carbon
unsaturated fatty acid arachidonic acid.
• Arachidonic acid, in turn, is synthesized from
linolenic acid, helping to explain why linolenic
is one of the two essential fatty acids.
• The several dozen known prostaglandins have
an extraordinary range of biological effects.
They can lower blood pressure, influence
platelet aggregation during blood clotting,
stimulate uterine contractions, and lower the
extent of gastric secretions. In addition, they
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