Cell Membrane



PLASMA MEMBRANE 

Communication With Environment.

Cells communicate with their environment by cell membraneThe latter 
(i) acts as a barrier between the cell and its environment, 
(ii) determines what enters or leaves the cell, and 
(iii) detects and responds to the changes in the surroundings. 

   Eukaryotic cells have extensive  intracellular membranes inside them too.

Cell Membrane. 

Every cell, prokaryotic as well as eukaryotic,  is enclosed by a thin covering called the cell membrane or plasma membrane, or plasmalemma. It can be seen only with electron microscope. It is not visible through the light microscope as its thickness is well below the resolving power of this microscope. The cell membrane is a distinct organelle rather than a mere denser layer of the hyaloplasm inside it as held earlier.It is semipermeable  as it  allows only certain substances to enter or leave the cell through it. It is elastic and can restore its former position after being pulled or indented with a microinstrument . It is a living and dynamic organelle because it undergoes constant renewal, can repair minor injuries; may fold, stretch or shrick; and controls the flow of materials through it. The cell membrane is about 70__ 100 A thick.



Intracellular Membranes.

The intracellular membranes, also called subcellular membranes,surround the organelles and Vacuoles,  and form the endoplasmic reticulum. The membranes, thus, compartmentalize the cell. However, neither the cell nor the compartments in it are totally isolated from the surrounding medium. The membranes allow continuous flow of selected materials across them as required from time to time. This helps the cell and the organelles to have contents different from those of the surrounding medium. The    prokaryotic cells lack intracellular membranes. 


Biomembranes.

The plasma membrane and the subcellular membranes, because  of their vital role for the cell, are together referred to as the biological membranes, or biomembranes  

Appearance:

Biomembranes are not visible under  the light microscope. Under electron microscope they appear  to be trilaminar and tripartite having an electron  dense or dark on either side of middle electron transparent layer.

Composition:

Chemically  a Biomembrane  consists of lipids ( 20__ 79%), protein( 20_ 70%) ,Oligosaccharides ( 1.5%) and water  ( 20%). The important lipids of the membrane  include phospholipids of some 100 types, sterols ( e.g., cholesterol), glycolipids, sphingipids( e.g., sphingomyelin, cerebrosides). Carbohydrates in membrane are  carbohydrates  occur almost  invariably in combination with proteins or lipids  in the form of glycoproteins or glycolipids. The glyco portions of these molecules almost invariably protrude to the outside of the cell.Thus the entire outside surface of the cell has a loose carbohydrate coat called called the glycocalyx. Proteins  can be fibrous or globular , structural, carrier,  receptor  or enzymatic.  ATP_  ase esterases, nucleases, etc. There is wide variation  in the lipid_ protein ratio between  different  cell membranes.Myelin, is an exception, in the sense that the lipid predominates ; in the other cell membranes,  there is higher  protein _ lipid ratio ( De_ Robertis). In human beings, the membrane of the erythrocyte has approximately  52 per cent proteins  and 40 per cent lipids.

The lipid molecules in membrane are amphiatic or amphipathic possessing both polar hydrophilic and nonpolar  hydrophobic ends. The hydrophilic  region is in the form of a head while the hydrophobic part contains  two tails of fatty acids which usually  occur towards the center  of the membrane. It results in the formation of a lipid bilayer.  The most common lipid of the bilayer  is phospholipid. Sterols, like cholesterol, provide strength  to the bilayers. Proteins  molecules also possess  both  polar and no polar side chains. The polar hydrophilic  linkages  are towards the outer side while the nonpolar or hydrophobic linkages are either kept folded inside or are used to  establish  connections with hydrophobic part of the lipids.


Molecular Structure of Cell Membrane:

Currently, the fluid Mosaic model for the molecular structure of cell membrane is considered satisfactory. However, it is useful to trace the evolution of this model as this will show how the scientists build on earlier observations. 

1. Overton's  Suggestion. 

In 1895, Charles Overton suggested that cell membrane was composed of  lipids. His suggestion was based on the finding that the lipid__ soluble substances entered the cells much more readily than the water_ soluble materials. 

2. Gorter and Grendel Model.

In 1925, two Dutch scientists, E. Gorter and E.  Grendel proposed that cell membrane was composed of a double layer ( bilayer) of lipid molecules. They measured the surface area and lipid content of a red blood corpuscle and found that the lipid could occur only in a 2 molecule thick layer.

3. Danielle and Davson Model.

In 1935, James Danielli and Hugh Davson proposed that the cell membrane is made of a double layer of phospholipid molecules with a single continuous layer of protein molecules on either surface. In other words, two protein monolayers sandwich the phospholipid bilayer. This view is referred to as the lameller theory, or sandwich model of cell membrane. The phospholipid molecules are set at right angles to the surface and are so arranged in the two layers that their nonpolar,  hydrophobic fatty  acids tails face each other and their polar, hydrophilic phosphate heads face the protein layers. The phospholipid and  protein layers are held together by electrostatic forces, and the two phospholipid layers are kept adhered by van der Waal's ¹ forces.The proteins involved were thought to be globular. 


Drawbacks. 

This model does not explain certain physiological phenomena, such as 
(i) passage of water and water_ soluble materials through phospholipid core, and 
(ii) the active transport of materials through the cell membranes.  Moreover,  this model assumed the cell membrane to be a stable structure with little functional specificity and variability. 

4. Robertson Model.

 J. David Robertson examined electron micrographs² of cell membrane of red blood corpuscles and found it to consist of 3 layers: 2 dark _ staining layers with a light __ staining layer in between. The outer dark__ layers were considered to represent the  protein monolayers and the middle light layer was considered to be the lipid bilayer.Thus, Robertson supported Danielli__ Davson lamellar ( sandwich) model of cell membrane. He, however,  held that protein molecules had extended rather than globular conformations.


 

● Unit Membrane Hypothesis. 

Robertson noted  trilaminer or 3 __ layered ( dark_ light__ dark) structure for all membranes he studied, and proposed the  "unit membrane hypothesis "in 1959. This hypothesis holds that
 (i) a membrane consisting of a phospholipid bilayer sandwiched between 2 protein monolayers is a unit membrane, and 
 
(ii) the various membranes in a cell are unit membranes or multiple of unit membrane. Cell membrane and membranes of endoplasmic reticulum,  Golgi apparatus,  and lysosomes are unit membranes. Mitochondrial and plastic membranes and nuclear envelope are double unit membranes. 
   
    The unit membrane of Robertson is about 75 A° thick, its middle phospholipid bilayer is about 35 A° thick and its peripheral protein layers are about 20 A° thick earth each.

Drawbacks: Robertson model has the same drawbacks as the Danielli _ Davson model.

5. Fluid Mosaic Model.

In early 1972, S. Jonathan Singer and Garth Nicolson propsosed fluid Mosaic model for the membrane structure. According to this model, the  biomembrane is not solid but viscous fluid and the phospholipid and protein molecules are arranged not uniformly but as a mosaic. As per the fluid Mosaic model, the cell membrane consists of a highly viscous, fluid matrix of 2 layers of phospholipid molecules, having globular proteins associated with them. The lipids and proteins vary from one type of membrane to another. The cell membrane also has oligosaccharides on the exposed surface. A good amount of cholesterol is also present in the cell membrane of animal cells.

(i) Phospholipids.

The phospholipid molecules are amphipathic, i.e, have both hydrophilic and hydrophobic regions. They have their polar, hydrophilic heads directed outward and nonpolar, hydrophobic tails pointing inward. This arrangement forms a water resistant barrier that,  which allows only lipid materials to pass through it. The phospholipid bilayer is 35A° thick. The two layers have different kinds of lipids. The lipid molecules can move sideways in their layer, changing places with their neighbours. This property, called  fluidity,allows the membrane to stretch under stress and to reseal itself if it is disrupted. The membrane's fluidity also allow the proteins to move within it.

Membrane must remain fluid to function properly. If it solidifies, its permeability changes and enzymatic proteins may become inactive. A cell can alter lipid composition of its membranes to some extent as an adaptation to changing temperature. For example, in wheat the percentage of unsaturated phospholipid increases before winter sets in.

The proportions of cell membrane contents vary, depending upon the tissue and the organism involved 






(ii) proteins .

The proteins are two types: peripheral,  and integral, or intrinsic.

(a) peripheral proteins. 

The peripheral proteins are located superficially and do not always cover the entire surface of the phospholipid matrix. They include spectrins. These proteins are loosely bound to the membrane surface, often to the exposed parts of the integral proteins. The peripheral proteins can be easily extracted. Each protein layer is 20 A° thick.

 (b) Integral Proteins. 

The integral protein form about 70% of the total membrane proteins.They are embedded in the phospholipid matrix. They include glycophorins.They are tightly held in place by strong hydrophilic or hydrophobic interactions or both with the polar heads and nonpolar tails of lipid molecules and are difficult to remove from the membrane. Some integral proteins have very large molecules that lie throughout the phospholipid molecules and project on both the surfaces.These are often called transmembrane, or tunnel, proteinsThese are believed to have channels or to enclose channels between them for the passage of water_ soluble materials. Other integral proteins have small molecules that lie in the outer or inner layer of phospholipid molecules and project on one surface only. No protein is believed to be entirely embedded in the matrix. Scattered nature of the protein molecules gives the " mosaic" part of the model's name. Some integral membrane protein molecules passing through the lipid bilayer are immobile, because they are attached to the cytoskeletal elements of the cell, namely, microfilaments,  microtubules and intermediate filaments ( Heslop __ Harrison and Linskens, 1984). On the outer side, some membrane proteins are attached to fibers of the extracellular matrix.
   
      The proteins provide the structural and functional specificity to the cell membrane. Moreover, most proteins freely shift sideways in the fluid lipid matrix to make the membrane dynamic.Therefore, the cell membrane was described as " protein icebergs in a sea of lipids" by the authors of the fluid Mosaic theory. The protein monolayers give elasticity and mechanical support to the lipid matrix.

(iii) Oligosaccharides 

The Oligosaccharide molecules, usually branched, are present on the exposed surface of the cell membrane. They are associated with protein as well as lipid molecules, respectively forming glycoproteins and glycolipids. These form a cell coat, the  glycocalyx,on the free surface of the cell. The glycoproteins can recognize self ( cells similar to them) from non self ( foreign cells). For the utility of this role of the glycoproteins, see box ahead 

( iv) Cholesterol. 

Cholesterol molecules are inserted between the phospholipid molecules of plasma membrane of animal cells. They are more stiff than phospholipid and, therefore, help stabilize the membrane. Some bacteria contain penta cyclic sterol like molecules, called hopanoids, in the cell membrane. The hopanoids play the same role as the cholesterol. 

 ●Role of Protein Molecules:The protein molecules of the cell membrane may act as carrier,  receptor and enzyme molecules. 

(a) Carrier Molecules. 

These protein molecules of the plasma and subcellular membranes bind with and transport specific molecules into or out of the cell and cell organelles. This provides for selective exchange of materials between the cell and the extracellular fluid, and between the cell organelles and the intracellular fluid.The carrier protein molecules are called permeases.

 

(b) Receptor Molecules. 


The glycoproteins on the cell surface act as receptors that recognize and bind specific molecules in the cell's surroundings. The molecules to be recognized may be hormones, other chemical signals or another cell's membrane. Molecules of certain hormones, such as epinephrine and insulin, bind to the cell membrane receptors to produce their effect in the cells. The cell surface receptors on egg and sperm of the same species recognize and bind to each other for fertilization. The membrane receptors also play a role in cellular interactions mentioned ahead.

(c) Enzyme Molecules. 

Each membrane carries enzyme molecules needed for its functions. For example, the inner mitochondrial membrane carries  enzymes comprising the electron transport chain for respiration. 

● Cellular Interactions.  All cells have name tags in the form of glycoproteins which project from their surfaces. Some surface molecules distinguish cells of one species from cells of another. Other surface molecules distinguish individuals within a species from one another. Still other surface molecules distinctively mark cells of different tissues within an individual. Thus, a bone cell's surface is different cells sort themselves out and aggregate and adhere together into groups to form tissues and organs during embryological development. Isolated cells of a sponge, Isolated cells of two different species of sponges, if mixed up, separate and reaggregate species-wise, and form two new sponges. These instances confirm that the outer surface of plasma membrane "recognize " the chemicals present on the similar plasma membrane surface and adhere together. The glycoproteins are also important in the body's defence against foreign bodies. Phagocytes seem to distinguish foriegn bodies ( nonself) from the animal's own cells (self) in a similar manner. The rejection of transplanted organs and reaction on the transfusion of incompatible blood have their basis in the capacity  of cell membranes to recognize their own kind. Cell surfaces change over time, with the number of surface molecules decreasing as a cell ages.

 Comparison with Lamella Models:Fluid Mosaic model does not suffer from the drawbacks of lamellar models as

(i) it assumes biomembranes to be fluid and dynamic, 

(ii) it explains the passage of water and water_ soluble materials through the phospholipid core via protein channels, 

(iii) it explains active transport of material via biomembranes with the help of permeases.

(iv) it assigns functional specificity and variability to the cell and organelle membranes due to the presence of different types of phospholipids and proteins in them,


(v) it enables the biomembranes to have cellular interactions due to the presence of glycoproteins on the exposed surface.


● Asymmetry of cell.Membrane:The cell membrane is asymmetrical because 

(i) the two lipid layers have different lipids, 

(ii) the two faces of the membrane have different proteins,  and

(iii) the exposed surface of the cell membrane bears glycoproteins and glycolipids. Hence, the membrane has distinct 'cytoplasmic 'and 'exoplasmic'or'exterior' ' surfaces, each with a different function. 

 ●Membrane Specificity The plasma membrane and organelle membranes have similar functions due to their similar basic structure. However, the kind, relative amount¹ and arrangement of  phospholipid and protein molecules vary in the  membranes of different cells and organelles. This gives specificity to each membrane. 

●Ce Membranes Fluid and Dynamic This is indicated by the following properties of the cell membranes __ 

(i) The constitution molecules can freely move in the membranes. 

(ii) The cell membranes are constantly renewed during the cell's life.

(iii) They can repair minor injuries. 

(iv) They grow with the growth of the cell and cell organelles. 


(v) They expand and contract during cell movement and during changes in cell shape.

(vi) They allow interactions of cells such as recognition of self and fusion of cells.

(vii) They control the flow of materials through them.

Experimental Demonstration. 

Fluid nature of cell membrane has been demonstrated by a simple experiment by Frye and Edidin in 1970. A mouse cell was labelled with a green fluorescent antibody or a pigment fluorescein¹and a human cell was labelled with red fluorescent antibody or a dry rhodamine.These cell were then fused, the composition cell so formed, called cybrid²,was immediately seen under a microscope. Its one half looked green and the other half red. It was kept at 37°C and reexamined after some time. It was found that the green and red colors had fully mixed up.This could happen only if the lipid bilayer was fluid,  and the labelled proteins could move in it. In the fused cell kept immediately at 0°C, the colors did not mix because lipids solidify at low temperature. This further proved the fluid nature of the cell membrane. When temperature was raised, the lipid bilayer regained fluidity and the colour mixed 





































































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