Cell Membrane
Structure and
composition
CELL
• All living organisms are composed
of cells. cell posses a membrane
cytoplasm, a nucleus and various
inclusion bodies such as
mitochondria ,centrosome, golgi
apparatus and various other
structures of granular nature.
STRUCTURE OF CELL
MEMBRANE
• Plasma membrane form closed
compartment around cellular protoplasm
to separate one cell from the other and
thus permit cell individuality thickness of
cell membrane is from7 to10nm .according
to Davson –Danielle molecular membrane
model, membrane consist of bimolecular
layer of lipid and a monomolecular
thickness of protein layers.
Basic Structural Organization of
Membrane Lipid Bilayer
• Lipids form a bilayer in which
nonpolar regions of lipid
molecules in each layer face core
of bilayer and their polar head
groups face outward, interacting
with aqueous phase on either side.
• Proteins are embedded in this bilayer sheet,
held by hydrophobic interactions between
membrane lipids and hydrophobic domains
in the proteins. Some proteins protrude
from only one side of membrane; others
have domains exposed on both sides
• The orientation of proteins in bilayer is
asymmetric, giving membrane “sidedness”:
protein domains exposed on one side of
bilayer are different from those exposed on
other side, reflecting functional asymmetry .
Characteristics structures
of outer lipid layer.
• 1- RAFTS
• 2- CAVEOLAE
Lipids Rafts
• Rafts are areas of the exoplasmic leaflets of lipid
Bilayer enriched in cholesterol and sphingolipid.
They are involved in signal transduction
• The rafts are made by the clustering of
sphingolipid with cholesterol in the outer
monolayer of the plasma membrane. The rafts
are less fluid and are thicker than their
neighboring phospholipids rich region.
• The rafts appear to have receptors and signaling
protein.
CAVEOLAE
• Caveolae are formed by curving
inward of the inner leaflets of
plasma membrane and integral
protein namely caveolin. Forces
the formation of caveoli. Which
are probably involved in
membrane transformed rigidity.
COMPOSITION OF CELL
MEMBRANE
Cell Membrane composed of :
• lipids
• proteins
• carbohydrate.
Major lipids in cell membrane
1- PHOSPHOLIPIDS
• 1.Phosphoglycerides.glycerol backbone to which
are attached two fatty acids in ester linkage and
phosphorylated alcohol fatty acids having even
no 16 to 18 C can be saturated or unsaturated.
simplest is phosphatidic acid others are
Phosphatidyl choline
,ethnolamine,serine,glycerol or inositol
• 2.sphingomyline Sphingocine backbone .a fatty
acid attached by an amide linkage to the amino
group of Sphingocine forming ceramide. The OH
of Sphingocine is linked with phosphocholine
e.g. Sphingomyline in myelin sheath.
2-GLYCOSPHINGOLIPIDS
• GLYCOSPHINGOLIPIDS are sugar containing
lipids having backbone of ceramide. They
include galactosyl and glucosyl ceramide
(cerebroside) and the ganlioside.
•
3- STEROLS
• The most common sterol in membrane is
Cholestrol. Cholestrol intercalates among the
phospholipids of the membrane. With its
hydroxyl group at the aqueous interface and the
remainder of the molecule within the leaflet.
MEMBRANE LIPIDS ARE
AMPHIPATIC
• All major lipids in membrane contain both
hydrophobic and hydrophilic regions so
termed as amphipathic.
• MEMBRANE LIPIDS FORM BILAYER.
• Bilayer exists as a sheet in which
hydrophobic region of the phospholipids
are protected from aqueous environment.
while the hydrophilic regions are immersed
in water.
Membrane proteins
• Protein can be amphipatic and form an integral
part of the membrane by having hydrophilic
regions protruding at the inside and outside
faces of the membrane but connected by a
hydrophobic region traversing the hydrophobic
core of Bilayer. An other aspect of the interaction
of lipids and protein is that some proteins are
anchored to one leaflet or another of Bilayer by
covalent linkage to certain lipids.
TYPES OF PROTEIN.
•
•
•
•
1- INTEGRAL
2- Lipid-anchored
3- PERIPHERAL
1- INTEGRAL proteins are usually globular and are
themselves amphipathic. They consist of two hydrophilic
ends and separated by hydrophobic region that passes
through hydrophobic core of Bilayer.
• 2-Lipid-anchored are bound covalently to one or more
lipid molecules. Hydrophobic carbon chain of attached
lipid is embedded in one leaflet of membrane and
anchors protein to membrane bilayer. Polypeptide chain
itself does not enter phospholipids bilayer.
• 3- PERIPHERAL protein interact directly
with the phospholipids in the Bilayer.
• Many hormones receptors molecules are
integral protein and specific polypeptide
hormones that bind to these receptors
molecules may therefore be considered
peripheral protein. Peripheral proteins such
as polypeptides hormones may help
organize the distribution of integral protein
such as their receptor within the plane of
bilayer.
CARBOHYDRATE.
• Membrane also contain
carbohydrates which are
either linked to lipid
(glycolipid) are
proteins(glycoproteins)
GLYCOLIPIDS
• Almost all glycolipid are derivatives of ceramides
to which carbohydrates have been attached
(glycosphingolipids) when one sugar molecule is
added to ceramide a cerebroside is produce. If
an oligosaccharide is added a globoside is
produce. If an acidic N-acetyl neuraminic acid
molecule is added a ganlioside is produce. if
cerebroside is sulfated a sulfoglyco sphingolipid
is produce. Glycolipid are found in cell
membrane of brain and peripheral nervous
tissues with high concentration in myelin sheath.
glyco lipids are very antigenic.
GLYCOPROTEINS.
• Glycoproteins are proteins to
which oligosaccharides are
attached. Membrane- bound
Glycoproteins participate in
cell surface antigenicity.
Fluid mosaic
MODEL
Fluid mosaic model
The fluid mosaic of membrane structure
proposed in 1972 by Singer and Nicolson.
According to this proposition protein and
phospholipids present in cell membrane
undergo redistribution in the plain of cell
membrane. one molecule of phospholipids
can move about several micrometer/sec .
proteins can move laterally in the plane of
cell membrane.
FLUID MOSAIC MODEL
DEPENDS UPON
• 1- LIPID COMPOSITION OF THE MEMBRANE.
• In a lipid bilayer the hydrophobic chain of
fatty acid can be ordered to provide rather
stiff structure.
• 2- TEMPERATURE.
• As temperature increases hydrophobic side
chain undergo a transition from the ordered
state to a disordered one taking on more
fluid like arrangement. The temperature at
which the structure undergo transition from
ordered to disordered is called transition
temperature.
• 3- The longer and more saturated fatty acid
chain interact more strongly with each other
via their longer hydrocarbon chain and thus
cause higher value of Tm. higher
temperature are required to increase the
fluidity of bilayer. On the other hand
unsaturated bond that exist in cis
configuration tend to increase the fluidity of
bilayer by decreasing the compactness of
side chain. The phospholipids of cellular
membrane generally contain at least one
unsaturated fatty acid with at least one cis
double bond.
CHOLESTROL.
• Cholestrol modify the fluidity of the
membrane at temperature below Tm. It
interferes with interaction of hydrocarbon
tails of fatty acids and thus increase fluidity.
• At temperature above Tm it limits disorder
because it is more rigid than the
hydrocarbon tails of fatty acid and can’t
move in the membrane to the same extent
thus limiting fluidity.
• In the lipid bilayer the irregular distribution of proteins
gives mosaic appearance. However the mosaic is not
constant but is fluid, i.e. changeable from moment to
moment. The fluidity of the mosaic is due to week (noncovalent) interactions between lipid and protein
molecules that enable individual lipid and protein
molecules to move free laterally, i.e. in the plane of the
membrane. increased fluidity
• 1-result in increased in membrane permeability to water
and small hydrophilic molecule.
• 2.increased in lateral movements of integral protein
• 3.if the protein involved in transport function transport
of molecules increased.