Cell Membrane Lecture

1. Cell Membranes
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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

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17. Functions of Cell Membrane
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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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19. 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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22. 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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24. 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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