Cell membrane is a biological barrier that separates the interior portion of the cell ( i.e. the Cytoplasm, karyon and the other cell cell organs ) from the outer environment, therefore permits cellular individualism and besides gives form to the cell. This membrane is a mixture of lipoids, protein and saccharides, therefore is a complex construction. The membrane is semi-permeable and therefore merely allows selective ions and molecules to travel through it into the cell or leave the cell. This is achieved by formation of concentration gradient across the membrane, which many biological procedures depend upon. The motion of the biological molecules across the membrane is either passive, which happens without the input of cellular energy or active conveyance that requires the cell to utilize energy. The cell membrane besides helps in keeping cell potency.
Proteins of the cell membrane form the indispensable constituent of the biological membrane since they function as pores, channels or transporters. Proteins therefore have the capableness of selective transition across the lipid bilayer. Some proteins that are embedded in the cell membrane act as molecular signals and hence carry out communicating. They act as receptors and receive signals from other cells or from the external environment and arouse a response in the cell. Some proteins function as markers which aid in designation of unknown cells. The membrane besides aids in intercellular interactions.
The lipid bilayer of the cell membrane is merely a few nanometers thick and is non permeable to most molecules that are soluble in H2O, and therefore Acts of the Apostless as a barrier to modulate the conveyance of ions, proteins and other molecules through the membrane. Since the phospholipid bilayer is non permeable to many ions, it helps in the ordinance of salt concentration and pH by modulating the pumping of ions in and out of the cell via proteins called ion channel pumps.
The Fluid mosaic theoretical account is the most widely accepted biological membrane theoretical account that was proposed in the twelvemonth 1972 by Singer and Nicolson. Floating in the phospholipid bilayer are molecules of protein, which is correspondent to icebergs drifting in a sea. The theoretical account is referred to as fluid because of the sidelong gesture of the bilayer supermolecules, and is referred to as mosaic because of the different molecular constituents [ 1 ] [ 2 ] [ 3 ] [ 4 ] .
Purpose of cell membrane
Cell membrane performs the undermentioned maps:
Membrane Transport of Small Molecules:
Transport proteins present in the bilayer can transport polar molecules through the membrane. There are assorted types of membrane conveyance proteins:
Uniport – This merely moves the solute from one side to the other side of the membrane.
Cotransport – This system moves two solutes at the same time across the lipid bilayer. They are two types of this transport-symport ( solutes are sent in the same way ) and antiport ( solutes are passed in opposite waies ) .
These conveyances are come under the class of inactive conveyance where no energy outgo is involved. Here the solute moves from a higher concentration to a lower concentration gradient.
Examples of this include channel proteins, which allow the solute to go through if they are of a specific charge or size. Carrier proteins bind to the solute and aid in its motion through the bilayer. [ 5 ]
Figure 1: Diagram comparing uniport, symport, and antiport
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Membrane Transport of Macromolecules:
There are two chief classs of conveyance of molecules are at that place in cells:
Small molecules like O, ethyl alcohol and C dioxide base on balls through the membrane by simple diffusion ( inactive conveyance ) down a concentration gradient. Transport of supermolecules like proteins, polynucleotides and polyoses is done by active conveyance utilizing ATP, against the concentration gradient.
There are two types of active conveyance:
1 ) Exocytosis – Procedure by which waste substances are removed from the cell by cyst formation and ejection [ 6 ] .
Figure 2: Exocytosis
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2 ) Endocytosis -A The molecule causes the cell membrane to pouch inward, therefore organizing a cyst. Phagocytosis is a type of endocytosis where the whole cell is engulfed. Pinocytosis is another type when the external fluid is engulfed. Receptor-mediated endocytosis occurs when the stuff to be transported binds to specific molecules in the membrane. Example: conveyance of insulin and cholesterin into carnal cells [ 6 ] .
Figure 3: Endocytosis
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Cell signalling across the membrane
Transmembrane signalling occurs through the coevals of a figure of signals like cyclic bases, Ca, phosphoinositides and diacylglycerol. Specific signals of neurotransmitters endocrines and Igs bind to the specific receptors on the membrane, which are largely built-in membrane proteins.
Figure 4: Signal transduction through membrane
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This is the Ca2+-phosphatidylinositol signalling tract which plays a major function in transmembrane signalling in a big figure of different cell types.
This tract leads to the activation of G-proteins. This initiates activation of phospholipase C and the subsequent formation of DAG and IP3A which triggers the coevals of insistent [ Ca2+ ] A spikes [ 7 ] .
Gap junctions are structures that let the little molecules that are up to ~ 1200 Da to be transported from one cell ‘s cytol to the other. These constructions contain proteins called connexins. Six connexins organize a hemiconnexin and two hemiconnexins form a connexon. These connections in the spread junction signifier cylindrical Bridgess through which substances are transported between cells [ 1 ] [ 8 ] .
Figure 5: Gap junction
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The Fluid mosaic Model:
This theoretical account is the widely accepted membrane theoretical account. The membrane has a biomolecular lipid bilayer bed. There are proteins that are inserted in it or jump to the surface. Integral membrane protein is the proteins that are embedded in the membrane they play a cardinal function as transporters for assorted molecules that can non come in through the cell membrane. The built-in proteins have an extra-cellular sphere and cytoplasmatic sphere and are separated by a non-polar part that holds it tightly in the membrane. Proteins that are slackly bound to the to the outer membrane are called the peripheral proteins. Many of the proteins that are present and about all the glycolipids have an externally oligosaccharides ironss that are exposed outside the membrane [ 1 ] [ 9 ] .
Fig. 6: Fluid Mosaic Model
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The membrane fluidness really much depends on the lipoids concentration in the membrane. The hydrophobic ironss of the fatty acids are much aligned hence giving it a stiff construction. The passage ( Tm ) is the temperature at which the passage takes topographic point from ordered to perturb province, this is the alteration that happens in the hydrophobic side concatenation. Cholesterol affects the fluidness of the membrane. It increases fluidness below Tm and decreases fluidness above Tm.
Alterations to the fluid mosaic theoretical account province that the lipoids and proteins in the membrane are non indiscriminately distributed. Randomness occurs when interaction energy of these molecules are near to their thermic energies. Since interaction energies can non be in a narrow scope due to big figure of interactions, there is really less opportunity for entropy to happen. Hence the theoretical account was found to be “ more mosaic than fluid ” [ 10 ] . The modified position of membrane theoretical account is shown in figure 7 [ 10 ] .
Figure 7: Modified position of cell membrane theoretical account [ 10 ]
Specialized constructions in the membrane:
There are some particular characteristics in the membrane like lipid tonss, caveolae, tight junction, desmososmes, adherens junctions and microvilli. These are found in the recent old ages of research.
Lipids Raft is the country in the membrane that has comparatively higher concentration of cholesterin, sphingo-lipids and some proteins, than the other parts of the membrane. It plays a major function in cell signal transduction. This is under research that if we increase the sum of this and constellating them closely may increase the overall efficiency of the cell.
Caveolae are particular types of lipid tonss. Many of them have protein called caveolin-1 that is involved in the procedure. They were observed under negatron microscope and were found to be flask-shaped. Proteins that are detected in this besides play a function in signal transduction, illustration is insulin. Proteins found in this besides drama in function in folate receptor. This field is a turning involvement for research.
Tight Junctions are present on the surface of the membrane and their major map is to forestall diffusion of supermolecules between cells. They are present below the apical surface of the epithelial cells. They are made up of assorted proteins including occludin, assorted claudins and junctional adhesion molecules [ 1 ] .
DesmosomesA besides called sunspot adherens are the specialized cell constructions for cell to cell adhesion. Their map is to defy shearing force. They are largely found in simpleA andA stratified squamous epithelial tissue [ 11 ] .
Adherens junctions are the proteins that normally occur at cell- cell junction.They are made up of proteins like cadherins, I?-catenin, I±-catenin and sometimes delta catenin. Their map is to supply strong adhesion between next cells. They hold the cardiac muscleA cells steadfastly together as the bosom beats and do non let it to fall in [ 12 ] .
Microvilli are really little finger like constructions found on the cell membrane. They are chiefly found on the epithelial cells, they increase the surface country of the cells hence increasing the absorbent capacity of the cells. Actin filament extends from the terminal of these microvilli [ 13 ] .
Components of cell membrane
Cell membrane is a complex construction and is composed of proteins, saccharides and lipoids. Different cell membranes have different composings.
Figure 8: Components of cell membrane
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Phospholipids: There are two major category of phospholipids out of which in the cell membrane the phosphoglycerides are the most normally found 1s. A Phospholglycerides are esters that are made up ofA two fatty acids, phosphorous acid and a trifunctional intoxicant.
Phosphoglycerides with sphingomyelin have Sphingosine anchor alternatively of glycerin. They play a function in signal transduction. They are outstanding in myelin sheaths [ 1 ] .
Figure 9: Structure of phospholipid
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Glycosphingolipids: These are sugar incorporating lipoids that are present in the membrane. They have a anchor made of ceramides. These are amphipathic molecules dwelling of a ceramide lipid ground tackle linked to an oligosaccharide concatenation of variable length and composing [ 1 ] . They are required for proper operation of nervous system. Determining their map will assist to understand neurodegenerative upsets, malignant neoplastic disease, immune map and diseases of metamorphosis [ 14 ] .
Figure 10: Structure of glycosphingolipid
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The most import steroid alcohol in the membrane is cholesterin.
Figure 11: Structure of cholesterin
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Proteins in cell membrane
Integral membrane proteins: besides calledA intrinsic proteins t has its some portion of the protein embedded in the phospholipid bilayer. Most of these proteins have hydrophobic side ironss that interact with membrane phospholipids fatty acyl groups. They are called transmembrane proteins if they one or more membrane spanning spheres. The transmembrane proteins of the membrane spanning spheres are made up of I± spirals or multiple I? strands [ 8 ] . These proteins are made up of two hydrophilic and one hydrobhobic part. The hydrophobic part crossbeams through the bilayer. They are asymmetric in nature. The transmembrane part of manyA built-in membrane proteinsA is made up of a package of hydrophobic I±-helices [ 7 ] . Their major function is as transporters, A and are besides structural membrane-anchoring spheres. They function by transporting hydrophilic molecules through the membrane. Many Built-in Proteins Contain Multiple Transmembrane I± lpha Helices [ 8 ] .
Examples: Insulin receptor, Glycophorin, Rhodopsin, CD36 and GPR30.
Figure 12: Built-in and peripheral proteins
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Peripheral membrane proteins:
They are besides called as extrinsic proteins ; they do non interact with hydrophobic nucleus of the membrane phospholipid bilayer. They are bound to the membrane by interaction with the intergral proteins or are bound to the bilayer ‘ outer lipoids polar caputs groups. They are merely present in the cytosolic part of the cell membrane. They play an of import function in signal transduction. Some peripheral proteins are localized to the surface of the plasma membrane, these are called exoplasmic proteins. Peripheral enzymes are involved in the synthesis of different membrane constituents likeA lipidsA , cell wallA oligosaccharidesA , or proteins.
Membrane peripheral proteins are of five types: Structural proteins, channel proteins, conveyance or bearer proteins, enzymes and receptor proteins.
Carbohydrates are attached to membrane lipoids and proteins as short oligosaccharide ironss. Proteins attached with sugar molecules are called glycoproteins and lipoids attached with sugar molecules are called glycolipids. The saccharides form a protective coat called glycocalyx around the cell, which helps in cell acknowledgment.
Glycoproteins are formed by glycosylation of proteins. There are two types: N-glycosylation ( sugar links to nitrogen atom of asparagines residue ) and O-glycosylation ( sugar attaches to hydroxyl group of serine or threonine rsidues ) . Examples of glycoproteins found in the organic structure areA mucins, collagens, beta globulins, immunogloulins, etc.
Glycolipids are lipoids linked to oligosaccharide ironss. Examples include glycosphingolipidsA which contain a hydrophobic ceramide, A A N-acylsphingosineA and carbohydrates. They are by and large located on the outer membrane surface. The composing of the saccharide concatenation depends on the type of the cell and development of the being. [ 9 ]
[ 1 ] Harper
[ 2 ] hypertext transfer protocol: //users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CellMembranes.html
[ 3 ] hypertext transfer protocol: //www.emc.maricopa.edu/faculty/farabee/biobk/BioBookCELL2.html
[ 4 ] hypertext transfer protocol: //www2.estrellamountain.edu/faculty/farabee/biobk/biobooktransp.html
[ 5 ] hypertext transfer protocol: //library.thinkquest.org/C004535/cell_membranes.html
[ 6 ] hypertext transfer protocol: //library.thinkquest.org/C004535/molecule_transport.html
[ 7 ] Chay, Lee, Fan, 1995
Appearance of Phase-locked Wenchbach-like Rhythms, Devil ‘s Staircase and Universality in Intracellular Calcium Spikes in Non-excitable Cell Models
[ 9 ] The Fluid Mosaic Model of the Structure of Cell Membranes Cell membranes are viewed as planar solutions of orientated ball-shaped proteins and lipoids. S. J. Singer and Garth L. Nicolson
[ 10 ] hypertext transfer protocol: //www.cytochemistry.net/cell-biology/membrane3.htm
[ 11 ] hypertext transfer protocol: //en.wikipedia.org/wiki/Desmosome
[ 12 ] hypertext transfer protocol: //users.rcn.com/jkimball.ma.ultranet/BiologyPages/J/Junctions.html
[ 13 ] A Krause J. William ( July 2005 ) .A Krause ‘s Essential Human Histology for Medical Students. Universal-Publishers. pp.A 37-.A ISBNA 9781581124682. Retrieved 25 November 2010.
[ 14 ] ] Glycosphingolipid maps: penetrations from engineered mouse theoretical accounts, Department of the Interior: 10.1098/rstb.2003.1268
Phil. Trans. R. Soc. Lond. B 2003 358, 879-883
[ 15 ]
[ 16 ]
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Gap junction movie:
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cell membrane movie
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