Download A lipid that has an ester link between

Phospholipid: A lipid that has an ester link between
phosphoric acid and an alcohol (either glycerol or
sphingosine).
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• The glycolipids are also derived from sphingosine.
They contain no phosphate group, but have an
attached carbohydrate that is a monosaccharide or a
short chain of monosaccharides.
• The classes of membrane lipids overlap.
• Glycolipids and sphingomyelins both contain
sphingosine and are therefore classified as
sphingolipids,
• Glycerophospholipids and sphingomyelins both
contain phosphate groups and are therefore
classified as phospholipids.
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Glycerophospholipids with a phosphate ester link to the
amino alcohol choline are known as phosphatidylcholines, or
lecithins. They are emulsifying agents, substances that
surround droplets of nonpolar liquids and hold them in
suspension in water. You will find lecithin listed as an
ingredient in many foods where it is added to keep oils from
separating out.
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Sphingomyelins are sphingosine derivatives with a
phosphate ester group at C1 of sphingosine. The
sphingomyelins are major components of the coating
around nerve fibers (the myelin sheath) and are
present in large quantities in brain tissue.
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Cerebrosides, glycolipids which contain a
monosaccharide, are particularly abundant in nerve cell
membranes in the brain, where the sugar is D-galactose.
They are also found in other cell membranes, where the
sugar unit is D-glucose.
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• The polar heads and hydrophobic tails
of phospholipids and glycolipids allow
them to form membrane bilayers that
act as barriers separating the interior
of cells from the environment.
• Glycolipids extend their carbohydrate
segments into the fluid surrounding
the cells. They function as receptors
that are essential for recognizing
chemical messengers, other cells,
pathogens, and drugs.
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23.6 Cell Membrane Lipids: Cholesterol
• Animal cell membranes contain significant amounts
of cholesterol. Cholesterol is a steroid, a member of
the class of lipids that all contain the same four-ring
system. In human biochemistry, the major functions
of steroids other than cholesterol are as hormones
and as the bile acids that are essential for the
digestion of fats and oils in the diet.
• The body of a 60 kg person contains about 175 g of
cholesterol that serves two important functions: as a
component of cell membranes and as the starting
material for the synthesis of all other steroids. Some
cholesterol is obtained from the diet, but cholesterol
is also synthesized in the liver. An adult can make
about 800 mg of cholesterol per day.
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Cholesterol is a nearly flat molecule. Except for its –
OH group, cholesterol is hydrophobic. Within a cell
membrane, cholesterol molecules are distributed
among the hydrophobic tails of the phospholipids.
Because the cholesterol molecules are more rigid
than the hydrophobic tails, they help to maintain the
structure of the membrane.
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23.7 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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23.8 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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23.9 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 short-lived 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 are responsible for some of the
pain and swelling that accompany inflammation.
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