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Chapter 7 Membranes

Chapter 7 Membranes. Functions of membranes. Boundaries and serve as permeability barriers. Sites of specific proteins and therefore of specific functions. Membrane proteins regulated the transport of solutes. Membrane proteins detect and transmit electrical and chemical signals.

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Chapter 7 Membranes

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  1. Chapter 7 Membranes

  2. Functions of membranes • Boundaries and serve as permeability barriers. • Sites of specific proteins and therefore of specific functions. • Membrane proteins regulated the transport of solutes. • Membrane proteins detect and transmit electrical and chemical signals. •   Membrane proteins medicate cell adhesion and cell-to-cell communication.

  3. The Prominence of membrane around and within Eukaryotic cells Figure 7-1

  4. Models of Membranes structure • Lipid nature of membrane: It is the pioneering work of Chlarles Overton in the 1890s. • Lipid monolayer: It is the working of Irving Langmair in 1900s. • Lipid bilayer: 1935s Hugh Davson & James Danielli suggested that proteins are present in membranes and its consist of lipids bilayer that are coated on both sides with thin sheet of proteins .

  5. Models of Membranes structure • Unite membranes: In 1950s J.David Robertson suggested that all cellular membranes share a common underlying structure. • Fluid mosaic model :Its consummation in 1970s was proposed by S.Jonathon singer & Groth Nicolson. • Membrane protein structure: Unwin & Richard Henderson used electron microscopy to determine the three dimensional structure of unfixed crystallized bacteriorhodopsion and its position in the membrane.

  6. Timeline for development of the fluid mosaic model

  7. The fluid mosaic model of membrane structure Figure 7-5

  8. Membrane Lipids • The main classes of membrane lipids: • Glycolipids • Sterols • Phospholipids

  9. The main classes of membrane lipids Figure 7-6

  10. Glycolipids Glycolipids are formed by the addition of carbohydrate groups to lipids. The most common glycolipids are: • Cerebrosides: Also known as neutral glycolipids because of the uncharged sugars as head groups. • Gangliosides *Both of the above types of glycolipids are found mostly in brain cells and nerve cell membranes.

  11. Sterols The major sterols in animal membranes are cholesterol.It maintains and stabilizes membranes of our bodies. Plants have different types of sterols similar in function but different in structure, even fungi have a unique sterol known as ergosterol. *Sterols aren’t fount in the inner membrane of mitochondria or chloroplasts.

  12. Phospholipids • Phospholipids are the most abundant lipids in membranes. There are 2 types of phospholipids: A-Phosphoglycerides: glycerol-based B-Sphingolipids: sphingosine-based • The most common phophoglyerides are: 1-Phosphatydylcholine 2-Phosphatidlyethanolamine 3-Phophatidylamine *The portions of any of these lipids vary according to the type of membrane and its function.

  13. Fatty Acids • Fatty acids are components of all membrane lipids except sterols • The asymmetry of membranes depends partly on the degree of unsaturation in the fatty acids and the different types of lipids there are. • Unsaturated fatty acids fail to tightly pack because of the kinks made from double bonds. kink

  14. Membrane Fluidity Membrane fluidity is caused by movement of lipids from one monolayer to the other. The type of movement's maybe any of the following: • Transverse Diffusion (flip-flop) • Lateral Diffusion (movement in the plane of the membrane) • Or Rotation of phospholipids about their long axis One of the techniques used to overview these movements includes Florescence recovery after photobleaching also known as FRAP.

  15. Membrane Proteins • A technique called freeze fracturing was used to support the fluid mosaic model. • Membranes contain: • Integral • Peripheral • Lipid-Anchored proteins • Fatty acid-anchored membrane proteins • Isoprenylated membrane proteins • GPI-anchored membrane proteins

  16. Figure 7-16a

  17. Membrane Proteins

  18. Peripheral Membrane Proteins • Peripheral membrane proteins are bound to membrane surfaces through weak electrostatic forces and hydrogen bonding with the hydrophilic portions of the lipid bilayer, perhaps with the polar head groups of membrane lipids. • Spectrin, Ankyrin, and Band 4.1 are main peripheral proteins found in erythrocyte membranes. These proteins are found on the inner surface of the plasma membrane and forms a skeletal network that maintains the erythrocyte shape.

  19. Integral Membrane Proteins • Integral membrane proteins have hydrophobic regions embedded within the membrane which makes it difficult to remove them. Because of their affinity for the lipid bilayer they are difficult to extract and study. • Types of integral membrane proteins: • Integral monotopic proteins: Those that protrude only from one side • Transmembrane proteins: Those that have hydrophilic regions protruding from both sides of the membrane, they can be: A- Single pass: Cross the membrane once (Example: Glycophorin) B- Multipass: Cross the membrane several times (Example: Band 3)

  20. Lipid-Anchored Membrane Proteins: Lipid-anchored proteins are located on either side of the surfaces of the lipid bilayers but are covalently bonded to lipid molecules embedded within the bilayer. • Fatty Acid-Anchored Membrane Proteins:Its synthesized in the cytosol and then is attached to a saturated fatty acid usually myristic acid. • Isoprenylated Membrane Proteins: Its synthesized in the cytosol as soluble cytosolic proteins then is modified. • GPI-Anchored Membrane Proteins: GPI (glycosylphophatidylinositol) is a glycolipid found in the outer layer of the plasma membrane. These types of membrane proteins are made in the ER.

  21. Isolation of Proteins • SDS- Sodium Dodecyle Sulphate:Allows integral membrane proteins to be isolated in addition to being fractionated and analyzed by electrophoresis. • Membrane protiens have many functions which include: • Enzymatic • Electron transporters • Transport proteins that transport nutrients • Transport ATPase

  22. Glycosylated Proteins • The addition of carbohydrate groups to the protein is called glycosylation. If the linkage of the carbohydrate is to a nitrogen atom of an amino group then its called N-Linked Glycosylation. If the carbohydrate group is linked to an oxygen atom of a hydroxyl group its known as O-Linked Glycosylation. • Glycoproteins and glycolipids that protrude from the outer surface of plasma membranes form a surface coat called glycocalyx.

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