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CHAPTER 4
MEMBRANE
I. Structure of membrane
Membrane is composed of lipid and protein, we call it as bio-membrane.
Bio-membranes include the outer membrane of cell (Cell or plasma
membrane) and the membrane surround organelles inside of cell (Inner
membrane).
Three membrane models:
Unit membrane model
Fluid mosaic model
Lipid rafts model
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Unit membrane model
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Fluid mosaic model
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Cell membrane and its surface structure
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The different permeabilities through the plasma membrane
by different molecules
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The features of membrane structure:
• Hydrophilic polar heads of phospholipid molecules form a layer touched
with water phase, and the hydrophobic non-polar tails of phospholipid
molecules form a layer separated with water phase. As the result, the
structure of phospholipid bilayer is formed.
• Protein molecules inserted the bilayer or combined on the surface are the
foundation for the variety of functions of cells. The interaction of the inserted
protein molecules and sugar or other molecules present numerous and
complex bio-functions to cells.
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II.The chemical composition of
membrane
Membrane lipid:
Phosphatidyl choline
(Lecithin)
Phosphoglycerides Phosphatidyl serine
Phospholipid
(over 50% membrane weight)
Phosphatidyl ethanolamine
Phosphatidyl inositol
Sphingolipids (The major component of the
membrane of brain cells and
neural cells, neurons)
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Phospholipid molecule
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Glycolipid and glycoprotein:
Depending on the species and cell type, the carbohydrate content of the
plasma membrane ranges between 2 and 10 percent by weight.
Cholesterol and neutral lipid:
Cholesterol and neutral lipid exist in the membranes of eukaryotic cells,
and they are important to regulate the floatability and water permeability of
plasma membrane.
Liposome:
Liposome can be manufactured in lab. Usually, 25 – 1000nm size in
diameter liposome can be used to scientific research and clinical therapy.
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Liposomes
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Membrane proteins:
Peripheral membrane protein:
Easy to be separated from membrane without membrane damaged. It is
linked to membrane surface with a weak bond.
Integral membrane protein:
It is a part of membrane structure and plays variety of functions.
Isolate integral protein from cells: Use detergent, such as SDS and Triton.
SDS causes protein molecules changed or damaged, but Triton does not
usually.
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The combination of the proteins and membrane
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The receptor of acetylcholine in the cell membrane
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A pump of K+ in the cell membrane
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III. The movements and floatability of
the membrane lipid and protein
molecules
Movements of membrane lipid:
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Floatability of the membrane lipids and proteins:
Florescence labeled cells fusion by Sendai virus is a good test to show the
floatability of membrane lipid and protein molecules
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Use FRAP (Fluorescence Recovery After Photobleaching) to check the
floatability of the membrane lipids and proteins:
Laser
Quench an area
of fluorescence
on membrane
by laser
The area of
fluorescence was
quenched and
became dark
After a while, the
quenched area of
fluorescence was
recovered and
became bright
because of the
floatability of
membrane
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IV. Dissymmetry of membrane
Names of the parts of membrane:
ES: Extrocytoplasmic surface;
PS: Protoplasmic surface;
EF: Extrocytoplasmic face;
PF: Protoplasmic face.
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Dissymmetry of membrane lipids:
Every type of membrane lipid has different distribution in the membrane.
Lipid raft:
A microdomain on the plasma membrane where the sphingolipids and
cholesterol are very rich. The molecules in lipid raft are bound together so
closely that resists to be extracted out by detergent, so we call these areas as
detergent-resistant membranes (DRMs). Usually, some proteins exist in or
on lipid rafts as receptor or others.
Dissymmetry of membrane protein:
Every molecule of membrane protein exists in the membrane with a
special direction, and they have their favorite existing areas. They work
following the direction absolutely, for examples, signal transduction,
transportation, receptor and activation, and enzyme catalytic reaction.
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Caveolae:
The composition of caveolae is almost same to lipid raft and includes a
special protein, caveolin (21KD). Caveolae is associated with endocytosis
(phagocytosis) and signal transduction.
V. The functions of plasma membrane
(1) Present a stable inner environment for all life events inside cells
(2) Separate cells into individuals to be proliferated and differentiated
(3) Transport through membrane selectively to recruit materials and supply
energy
(4) There are cell identification sites to the transmembrane communications for
many cell functions
(5) There are binding sites of many enzymes that make enzyme catalytic
reactions efficient and orderly
(6) Intermediate the linkages between cell and cell, cell and matrix
(7) Form special surface structures with different function
(8) Be important and specific to immune and hormone identification
(9) There are some special components we can use to stain to display cell or
select as antigen
(10) Cell fusion happens between membranes firstly
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VI. Specialized structures on cell surface
Cell coat:
A sugar riched layer on the animal cell surface called as glycocalyx also. Cell coat
can be displayed by heavy mental staining under electron microscope. Cell coat is
composed of a lot of oligosaccharide branch chains from the glucoproteins or
glucolipids in the plasma membrane, and actually, it is a part of the cell membrane.
The functions of cell coat:
(1) Protect cell membrane
(2) Cell identification. Many cell surface markers and antigens are from this layer
(3) Determine blood group (blood type). Human blood can be identified as four
groups (A, B, AB, O) based on three blood type antigens, A, B and O, from the
glycosyls of red blood cell coat. Actually, more than 20 blood types exist in human
being. ABO blood type system is basic blood group identification system based on the
antigen A (N-acetylgalactose at sugar chain terminal) and B (galactose at sugar chain
terminal). Blood types: A (antigen A), B (antigen B), AB (antigen A and B), O (No
A, No B).
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Cell coat composed of oligosaccharide chains
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Membrane associated cytoskeleton:
Membrane associated cytoskeleton is a meshwork composed of fibrin
and linked with membrane proteins. Membrane associated cytoskeleton is
located in hypolemmal layer, and it plays roles to maintain the shape of
membrane and cooperate the functions of membrane.
Because red blood cells (RBC) must have the membrane with a very
good deformability because of their roles. Membrane associated cytoskeleton
maintain the deformability of RBC. Reactive oxygen species (ROS) and
other free radicals can damage the membrane associated cytoskeleton of
RBC and cause the deformability changed to form “hard RBC”, that is
associated with some central vascular diseases probably.
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The
meshwork of
RBC
membrane
associated
cytoskeleton
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Microvilli:
Microvillus is a spike from plasma membrane. Microvilli enlarge the surface
area and make the material exchange easy, for example, the microvilli of intestine
endothelial cells.
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Ruffle and infolding:
Ruffles are distributed on macrophages and other cells that are associated with
phagocytosis. Infoldings are located on some cells that have strong function to
exchange the ions or liquid by their membranes.
Infolding
Ruffle on MØ
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Cilium and flagellium:
Cilia
The cilia on the
trachea endothelial
cells
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Some protozoa cell
with a flagellium
Flagellium
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Cilia
Discharged in faeces
Parasited in tissue
Balantidium coli Malmsten
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VII. Cell junction:
The junction structure between cell and cell or cell and matrix.
Occluding junction
Cell junctions
Anchoring junction
Communicating junction
Occluding junction:
Occluding junctions include tight junction and septate junctions. Septate
junction exists between the epithelial cells of invertebrates only. Tight junction
is the majority type of cell junctions and located among epithelial cells. The
function of tight junction is to block off material exchange passing through the
epithelial layer, and protect the tissues or organs.
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The tight junction between epithelial cells
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A model figure of
the tight junction
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Septate junction exists between the
epithelial cells of invertebrates only
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Anchoring junction:
Adhesion belt and focal adhesion
Anchoring junction
Desmosome and hemidesmosome
Adhesion belt: Located at the occluding junction below with a 15-20nm space
between cells. There are some adhesion molecules in the space. They are: Ecadherin and other adhesion proteins, such as, α-, β-, and γ-catenin, vinculin, αactinin, and plakoglobin. The neighbored cells are combined together closely by the
cadherin and adhesion proteins through the actinin filament meshwork.
Focal adhesion: Located between the cell and matrix. The actinin filament is
linked together with matrix by integrin. The plasma at the junction site is like a disc
called focal adhesion (adhesion plaque).
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Occluding junction
(Tight junction)
Adhesion belt
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A model figure of adhesion belt structure
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Desmosome: Located at the adhesion belt below with a 30nm space
between cells. Desmosome distribute in the tissues that take strong pulling
force. In desmosome, the plasma adhesion proteins form a density plaque
about 15～20nm. The both sides of plaque are bound together by the inter
filaments. The cadherins (desmoglein and desmocollin) are located in the
middle of desmosome. The inter filaments and cadherins form the
cytoskeleton meshwork.
Hemidesmosome: The structure of hemidesmosome is almost same as
desmosome, but it is located between the epithelial cell surface and matrix.
Hemidesmosome is different from desmosome by ① the hemidesmosome
structure is formed on the cell plasma membrane side, other side is matrix,
that is why we call it as hemidesmosome. ② the junction protein is integrin,
not cadherin. Integrin is the receptor protein of the matrix. ③ the plasma
adhesion protein is keratin.
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Occluding junction
(Tight junction)
Adhesion belt
Desmosome is located below the adhesion belt
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A model figure of desmosome structure
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Epithelial Cell
Hemidesmosome
Matrix
Hemidesmosome binds epithelial cell and matrix together
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Communicating junction:
Communicating
junction
Gap junction
Plasmodesmata junction
Synapse junction
Gap junction:
Gap junction exists in most of animal tissue, and combines cells
together with a 2～4nm space in that there are a lot of connexons.
Connexon is the basic unit of the gap junction structure. Connexon
allows the molecules with smaller weight than 1.5 KD to pass through..
The permeability of gap junction can be regulated at some
experimental conditions.
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A image of gap junction by electric microscope
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Connexons
Left: Connexons under electric microscope.
Right: A model of gap junction.
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Functions of gap junction:
1．Gap junction is associated with the differentiation of embryonic cells. Cells will
differentiate with specific direction controlled by the position information that was
presented by some small molecules passed through gap junction.
2．Gap junction can maintain the balance of metabolism. Example:
Mutated fibroblast
can not take
hypoxanthine to
synthesize
nucleotide
Culture mutated
fibroblast with wild
type fibroblast
together
Both type of cells
can take
hypoxanthine and
synthesize
nucleotide
The gap junction
was damaged
3．Gap junction is associated with the construction of electronic synapse that exists
between smooth muscle cells, heart muscle cells, and the terminals of neurons.
Electron synapse can transfer electric excitation to an adjacent cell without any
transmitter or information molecule.
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Plasmodesmata junction:
It exists between plant cells only for communication.
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Synapse:
It exists between excitable cells and composed of presynaptic membrane,
postsynaptic membrane, and synaptic cleft.
Neuron
excitation
The postsynaptic
membrane will be
depolarized
Presynaptic
membrane
Release out the
transmitter
Cause the permeability of
postsynaptic membrane
changed
The receptors
on
postsynaptic
membrane
Extend the
excitation to
next neuron
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Synaptic knob
Presynaptic membrane
Synaptic cleft
Postsynaptic membrane
Synaptic vesicle
Synapse
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Model of synapse
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Comparison of junctions
Tight junction
Epithelial cells
Septate junction
Exists in invertebrates only
Adhesion belt
Epithelial cells
Adhesion plaque
Matrix part of epithelial cell
Desmosome
Heart muscle cells and skin
Hemidesmosome
Matrix part of epithelial cell
Gap junction
Exists in most of animal tissue
Synapse
Between neurons, neuron and
muscle cell
Plasmodesmata
Between plant cells
Occluding junction
Junction to
actinin
Anchoring
junction
Junction to
inter filament
Communicating junction
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Comparison of junctions
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