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CHAPTER 4

MEMBRANE

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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 Phosphatidyl ethanolamine

(over 50% membrane weight) 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.

12Liposomes

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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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Laser

Use FRAP (Fluorescence Recovery After Photobleaching) to check the floatability of the membrane lipids and proteins:

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.

Ruffle on MØ

Infolding

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Cilium and flagellium:

The cilia on the trachea endothelial

cells

Cilia

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Some protozoa cell with a flagellium

Flagellium

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Balantidium  coli  Malmsten

Parasited in tissueDischarged in faeces

Cilia

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VII. Cell junction: The junction structure between cell and cell or cell and matrix.

Cell junctions

Occluding junction

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: E-cadherin and other adhesion proteins, such as, α-, β-, and γ-catenin, vinculin, α-actinin, 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).

37Adhesion belt

Occluding junction

(Tight junction)

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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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Desmosome is located below the adhesion belt

Occluding junction

(Tight junction)

Adhesion belt

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A model figure of desmosome structure

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Hemidesmosome binds epithelial cell and matrix together

Epithelial Cell

Matrix

Hemidesmosome

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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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Left: Connexons under electric microscope.

Right: A model of gap junction.

Connexons

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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:

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.

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

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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.

Presynaptic membrane

Neuron excitation

Release out the transmitter The receptors

on postsynaptic membrane

Cause the permeability of postsynaptic membrane

changed

The postsynaptic membrane will be

depolarized

Extend the excitation to next neuron

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Synapse

Presynaptic membrane

Postsynaptic membrane

Synaptic knob

Synaptic cleft

Synaptic vesicle

Excitation

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Model of synapse

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Comparison of junctions

Occluding junctionTight junction Epithelial cells

Septate junction Exists in invertebrates only

Anchoring junction

Junction to actinin

Adhesion belt Epithelial cells

Adhesion plaque Matrix part of epithelial cell

Junction to inter filament

Desmosome Heart muscle cells and skin

Hemidesmosome Matrix part of epithelial cell

Communicating junction

Gap junction Exists in most of animal tissue

SynapseBetween neurons, neuron and muscle cell

Plasmodesmata Between plant cells

 

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Comparison of junctions