Lecture 18: Introduction to Membranes

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Lecture 18Introduction to Membranes

• Lipid Structure
• Properties of Lipid Bilayers

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Biological Membranes Define the Boundaries of
the Cell and its Compartments
Membranes are permeability barriers that keep
the cell contents in and unwanted substances
out. Membranes are selectively permeable
specific substances can cross membranes in
a controlled way through protein-based
transport systems. The various organelles
and compartments of the cells are bounded by
membranes to create interior environments suitable
for different functions.
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Common Features of Biological Membranes
Membranes are sheet-like structures, 2 molecules
thick, or 60 to 100 Angstroms across. Lipids and
proteins are the major components of membranes,
occurring in the ratio of 14 to 41. Both can be
covalently modified by carbohydrates. Lipids are
small molecules with both hydrophobic and
hydrophilic groups. They associate to form double
layers called lipid bilayers, which
are impermeable to polar molecules. Proteins
associated with or embedded in the lipid bilayer
carry out the different functions of membranes.
These proteins serve as pumps, channels, receptors
, energy transducers, and enzymes. Both lipid
and protein components of membranes are held
together by noncovalent bonds. Membranes are
asymmetric- the two layers are not equivalent.
Lipids can flow and diffuse within a layer, but
cannot in general cross to the other layer. Most
membranes are electrically polarized, enabling
transport of molecules, energy conversion, and
transmission of signals.
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Schematic Structure of a Membrane Lipid
Schematic Structure of a Lipid Bilayer
Layer 1
Layer 2
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Example Functions of Membranes
PERMEABILITY BARRIERS Regulation of molecular
and ionic compositions of cells and intracellular
organellesa) channels pumps (proteins that act
as selective transport systems)b) electrical
polarization of membrane (due to differences in
ion concentrations on opposite
sides) INFORMATION PROCESSING biological
communication a) signal reception by specific
protein receptors (BINDING)b) transmission/transd
uction of signals (via protein conformational
changes) ENERGY CONVERSION ordered arrays of
enzymes to organize of reaction sequences
a) photosynthesis (conversion of light energy to
provide chemical bond energy) b)
oxidative phosphorylation (oxidation of fuel
molecules to provide chemical bond energy)
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Lipids
Lipids are a diverse group of biological
molecules which share the common solubility
property of being insoluble in water but highly
soluble in organic solvents. Lipids can serve
diverse biological roles (energy
sources, signalling molecules) but we will focus
on membrane lipids- lipids whose primary role is
as components of biological membranes.
Three major types of membrane lipids Phospholipi
ds Glycolipids Cholesterol
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Fatty Acids
Fatty acids are components of phospholipids and
glycolipids. They consist of long hydrocarbon
chains that terminate with a carboxylic acid.
Palmitate 16 carbons (fully saturated)
The hydrocarbons of fatty acids vary in length
(16 and 18 carbons are the most common) and also
in the number and position of double bonds.
Oleate 18 carbons (monounsaturated)
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Description of Fatty Acids
An alkane is a hydrocarbon with no double
bonds. An alkene is a hydrocarbon containing one
or more double bonds. A shorthand notation to
describe the degree of unsaturation 180
represents a fatty acid with 18 carbons and 0
double bonds 181 represents a fatty acid with 18
carbons and 1 double bond 182 represents a fatty
acid with 18 carbons and 2 double bonds
Fatty Acid Nomenclature Substitute oic for e in
name of parent alkane octadecane is an alkane
with 18 carbons octadecanoic acid is the fatty
acid derived from octadecane (180) Substitute
enoic for ene in name of parent alkene
octadecene is an alkene with 18 carbons, 1 double
bond octadecenoic acid is the fatty acid
derived from octadecene (181) Fatty acids
containing more than one double bond are given
the suffixes -dienoic, -trienoic, etc.
octadecadienoic acid has 18 carbons and 2 double
bonds (182) octadecatrienoic acid has 18
carbons and 3 double bonds (183)
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The carbon atoms are numbered from the carboxyl
group. The position of double bonds, and whether
they are cis or trans, can be indicated by the D
notation, e.g. oleate can be referred to as
cis-D9-octadecenoate (cis double bond between
carbons 9 and 10)
cis
trans
Oleate
The last carbon is referred to as the w carbon.
Consuming foods rich in w-3 fatty acids, such as
salmon, is believed to protect against heart
disease.
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Phospholipids and Glycolipids
Phospholipids are a major class of membrane
lipids, and are comprised of a platform or
central backbone compound (glycerol or
sphingosine), fatty acids, a phosphate, and an
alcohol. The presence of a phosphate group is the
primary identifier. These have charged head
groups.
Glycerol
Sphingosine
Glycolipids are another class of membrane lipids.
The platform is sphingosine, with a fatty acid
linked to the amino group and one or more sugars
to the primary hydroxyl. These have polar head
groups.
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Phosphoglycerides
Phosphate-containing lipids built from glycerol
are phosphoglycerides. They contain 2 fatty acids
esterified to two of the hydroxyl groups
on glycerol, and the third hydroxyl is esterified
to phosphoric acid. The phosphate group is
typically further esterified to other alcohols.

Nonpolar fatty acid tails
Polar (charged) head group
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The simplest phosphoglyceride is phosphatidate in
which the phosphate is not esterified. Othe
r common phosphoglycerides can be formed by
addition of hydroxyl-containing groups serine,
choline, ethanolamine, inositol.
(lecithin)
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Sphingophospholipids
An example of phospholipids based on sphingosine
are the sphingomyelins. The amino group of
sphingosine forms an amide bond to a fatty acid
and the primary hydroxyl is esterified to
phosphoryl choline or phosphoryl ethanolamine.
Polar (charged) head group
Nonpolar tails
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Glycolipids
Glycolipids are based on sphingosine and also
form an amide bond with a fatty acid. The primary
hydroxyl is esterified to one or more
sugars. Cerebrosides have a single sugar and
gangliosides have a branched chain of up to seven
sugars. Glycolipids are found on the exterior
layer of the plasma membrane with the sugars
ouside the cell.
Nonpolar tails
Polar head group
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Phospholipids (charged)
Phosphoglycerides (based on glycerol)
Sphingophospholipids (based on sphingosine)
Phosphatidyl choline Phospatidyl serine
Sphingomyelins
Glycolipids (based on sphingosine. uncharged)
Cerebrosides Gangliosides
Polar or charged head group
Nonpolar tails
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Cholesterol
An important lipid of entirely different form is
cholesterol. Its backbone is a steroid, a 4-ring
structure, with a hydrocarbon tail, and a polar
hydroxyl group. It is particularly abundant in
some kinds of nerve cell membranes.
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Lipids Spontaneously Form Bilayers
In aqueous solution, the nonpolar tails of
phospholipids and glycolipids tend to associate
to minimize contact with water, but the polar
head groups tend to seek contact with water. Two
ways to satisfy both requirements are to form
micelles or bilayers.
Micelles are formed by fatty acids,
detergents. Typically smaller than 200 Angstroms.
Bilayers are formed by phospholipids. The 2 fatty
acid chains are too large to fit in the interior
of a micelle. Bilayers can be very large, up to
10 Angstroms (1 mm).
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Bilayer Self-Assembly
Bilayers are stabilized primarily by hydrophobic
interactions between the nonpolar tails but also
by Van der Waals interactions and also
hydrogen bonding between the polar head groups
and water.
Bilayers seek to avoid exposure of the
hydrophobic tails of lipids to water so can be
very extensive. To avoid such exposure they form
closed compartments. Any holes that form in the
bilayer are energetically unfavorable so bilayers
are self-sealing.
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Formation of Lipid Vesicles
The self-sealing property of bilayers allows
creation of lipid vesicles or liposomes- small
membrane-bounded compartments containing a
desired substance. These vesicles have clinical
uses such as drug delivery. The ability to create
membranes with concentration gradients across
them is useful for the study of membrane proteins.
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Permeability of Bilayers
Bilayers are nearly impermeable to ions and most
polar molecules other than water. The rate at
which such substances traverse the membrane is
correlated with their solubility in nonpolar
solvents. Polar or charged molecules must be
desolvated (shed bound water) before they can
spontaneously cross the membrane, which is
unfavorable.
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Summary
Biological membranes are composed of lipids and
proteins and form the boundary of the cell and
its compartments. Phospholipids and glycolipids
are formed of fatty acids esterified to a
platform molecule and contain other groups such
as alcohols or sugars. Lipids spontaneously
assemble into bilayers which are
largely impermeable to charged and polar
molecules and which form closed
compartments. Key Concepts Fatty
acids Phospholipids Glycolipids Cholesterol Micell
es Bilayers Vesicles Permeability of bilayers