2. Structure of Cell Membrane
Structure and function of cell are critically dependent on membrane
•Separates interior of cell from outside
•Define internal compartments
Formation of biological membrane depend on
•Properties of lipids
•All biological membranes share a common structural organization
Bilayer of lipids with associated proteins
Cell Membrane
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3. Membrane structure
Phopholipids are major structural component of membranes consisting of two hydrophobic fatty acid chain linked to phosphate joined to polar head groups
Cell Membrane
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4. Contd….
Lipid constitute approximately 50% of mass of cell membrane, composition of cell membranes varies with type of membrane
e.g
• inner membrane of mitochondria (75%) of proteins
•Plasma membrane 50% lipid & 50% proteins
•Plasma membrane of Ecoli consist of phosphotidylethanolamine which constitutes 80% of total lipids
Mammalian plasma membranes are complex containing four different phospholipids
Phosphotidylcholine
Phosphotidyl ethanolamine
Phosphotidyl serine
Sphingomylein
Glycolipids and cholestrol 40% of total membrane lipids
50-60% of total membrane lipids
Cell Membrane
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5. Contd….
An important property of lipid bilayer is that they behave as two dimensional fluids
◦Lipids and proteins are free to rotate & move in lateral directions
Fluidity of lipid bilayer is determined by
◦Lipid composition
◦Temperature
Interaction between shorter fatty acid chains are weaker than those with longer chains
Membranes containing shorter fatty acid chains
◦Less rigid and remain fluid at < temperature
Membrane fluidity is also increased by
◦Unsaturated fatty acids
Cell Membrane
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6. Contd….
cholesterol plays a distinct role in determining membrane fluidity because of its hydrocarbon ring structure
The rigid hydrocarbon rings of cholesterol therefore interact with the regions of the fatty acid chains that are adjacent to the phospholipid head groups
This interaction decreases the mobility of the outer portions of the fatty acid chains, making this part of the membrane more rigid.
On the other hand, insertion of cholesterol interferes with interactions between fatty acid chains, thereby maintaining membrane fluidity at lower temperatures
Cell Membrane
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8. Membrane proteins
Proteins constitute approximately 25-75% of mass of membranes
Fluid Mosaic Model
In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer
Cell Membrane
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9. Contd….
Phospholipids provide basic structural organization of membrane
Membrane proteins carry out specific functions of membranes
Proteins divided into two broad classes based on
nature of their association with cell membrane
•Integral membrane Proteins
•Peripheral membrane proteins
Cell Membrane
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11. Contd….
Like phospholipids transmembrane proteins (intergral) are amphipathic molecules with their hydrophilic portions exposed to extracellular enviroment on both sides of membrane
Within membrane
◦nonpolar amino acids
hydrophobic
anchors protein into membrane
On outer surfaces of membrane
◦polar amino acids
hydrophilic
extend into extracellular fluid & into cytosol
Cell Membrane
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12. Contd…
Proteins can also be anchored in membranes by lipids that are covalently attached to the polypeptide chain
Distinct lipid modifications anchor proteins to the cytosolic and extracellular faces of the plasma membrane
Proteins can be anchored to cytosolic face of plasma membrane by addition of
14 carbon fatty acid (myristic acid)
16 carbon fatty acid (palmitic acid)
15-20 carbon prenyl groups to side chains of cysteine residues
Alternatively, proteins are anchored to the extracellular face of the plasma membrane by the addition of glycolipids to their carboxy terminus.
Cell Membrane
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13. The Glycocalyx
The surface of cell is covered by carbohydrate coat known as glycocalyx formed of
Oligosaccharides of glycolipids
Transmembrane glycoproteins
•Functions
•Protect cell surface from ionic and mechanical stress
•Barriers to invading microrganism
•Oligosaccharides of glycocalyx participate in cell-cell interactions
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Cell Membrane
14. A well studied example of physiological interaction involving the glycocalyx involves
•Adhesion of white blood cells (leukocytes) to endothelial cells that line the blood vessels
Allows leukocytes to leave the circulatory system and mediate inflammatary response in injured tissues
Initial step in interaction is mediated by
◦Selectins (transmembrane protein)
Recognizes carbohydrate on cell surface
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Cell Membrane
15. •Two members of selectin family
E-selectins
P-selectins
•L-selectin is expressed by leukocytes and recognizes an oligosaccharide on surface of endothelial cells
•Oligosacchride exposed on cell surface thus provide a set of markers that help identify distinct cell types
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Expressed by endothelial cells and platelets bind to oligosaccharides on surface of leukocytes
Cell Membrane
18. Functions of Cell Membrane
1. Selective permeability
Enables cells to control and maintain its internal composition
Small uncharged molecules diffuse through membrane
Bilayer is impermeable to
•large polar molecules
•charged molecules such as ions
Many such molecules are able to cross lipid bilayer via the action of specific transmembrane proteins (transporters) which determines
◦Selective permability of membranes
Transmembrane proteins contain multiple membrane spaning regions
Functions of cell membrane
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20. There are two general classes of membrane transport proteins
Channel proteins:
Form open pores allowing passage of any small molecule by free diffusion
◦e.g : Na, K, Ca, Cl
Pores are not permanently open rather they can be selectively opened or closed in response to
Extracellular signals
◦Such channels are extensively studied in
Muscle & nerve cells
Response to extracellular signals
Functions of cell membrane
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21. Carrier proteins
•Selectively bind and transport small molecules such as glucose
•Carrier proteins act like enzymes facilitate the passage of small molecules across membrane
◦Carrier proteins bind specific molecules
◦Undergo conformational change that open channel
Functions of cell membrane
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23. Mechanism of Transport
Passive transport
Molecules transported by either proteins cross the membrane in energetically favorable direction
•Determined by concentration
•Electrochemical gradient
Active Transport:
Molecules can be transported in energetically unfavorable direction if their transport in that direction is coupled with ATP hydrolysis
Functions of cell membrane
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25. Types of membrane proteins
In addition to transport membrane proteins have diversified roles in cell
•Cell-cell recognition proteins
•Integrins
•Intercellular junction proteins
•Signal transduction proteins
Functions of cell membrane
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26. Cell-cell recognition proteins - identify type of cell and identify a cell as “self” versus foreign
•Most are glycoproteins
Carbohydrate chains vary between species, individuals, and even between cell types in a given individual.
Glycolipids also play a role in cell recognition
Functions of cell membrane
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27. Contd…
Integrins are a type of integral protein
•The cytoskeleton attaches to integrins on the cytoplasmic side of the membrane
•Integrins strengthen the membrane
Intercellular junction proteins - help like cells stick together to form tissues
Functions of cell membrane
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28. Enzymatic proteins.
Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface.
Enzymes needed for metabolic pathways can be aligned adjacent to each other to act like an assembly line for the reactions.
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