1. Membranes

2. Introduction Properties attributed to living organisms (movement, growth, reproduction &metabolism etc) depend on membranes All membranes – same general structure (lipid & protein mols) Currently accepted concept of membrane: “FLUID MOSAIC MODEL” (membrane is a bimolecular lipid layer) The proteins, most of which float within the lipid bilayer- determine’s the membrane biological functions

4. Membrane structure diff type of cell has its own function – unique membrane structure Proportion & type of lipid and protein varies among cells.

5. Membrane lipids Amphiphatic mol suspend in water: - Hydrophobic – buried in water - Hydrophilic – exposed to water Phospholipid form into bimolecular layers when sufficiently concentrated (basis of cell membrane)

6. Function of membrane lipids: 1) Membrane fluidity Phospholipids in the plasma membrane can move within the bilayer Lateral diffusion - movement of lipids & proteins in membrane, rapid & spontaneously process Transverse diffusion – movement of lipids & proteins from one side of lipid bilayer to the other, rare.

8. Membrane fluidity is determined by the % of unsaturated FAs in its phospholipid molecules. High conc of unsaturated chain- more fluid membrane

9. 2) Selective permeability allow certain molecules or ions to pass through it by diffusion The hydrophobic nature of the membrane makes it impenetrable to the transport of ionic and polar substances. Membrane proteins regulate the movements of ionic and polar substances by binding to the protein carrier or by providing a channel. Nonpolar substances diffuse through lipid bilayer down their concentration gradient

10. 3) Self-sealing capability When lipid bilayer disrupted, immediately & spontaneously reseal Because a break in a lipid bilayer exposes the hydrophobic hydrocarbon chains to water. In living cells, certain protein component of membrane, cystoskeleton, calcium ion also assist in membrane resealing.

11. Lipid bilayer When hydrophobic tails of lipid bilayer exposed to polar water mol, lipid form hydrophilic edges consisting of polar head groups As membrane edges draw closer to each other They fuse and reform the bilayer

12. 4) Asymmetry Biological membranes are asymmetric Lipid composition of each half of a bilayer is diff. Because each side of membrane is exposed to diff. environment. Eg. the human red blood cell membrane possesses more phosphatidylcholine and sphingomyelin on its outside surface.

13. Membrane proteins protein molecule that is attached to, or associated with the membrane Most membranes require proteins to carry out their functions Classified according to their structural relationship to membrane :- 1) Integral proteins 2) Peripheral proteins

14. Integral proteins - are embedded in and/or extend through the membrane. - Can be extracted by disrupting membrane with organic solvents/detergents - Ion channel, proton pump Peripheral proteins - are bound to membranes primarily through interactions with integral proteins (hydrophobic, electrostatic, non covalent) - Can be released from membrane by gentle methods (pH change) - Hormone, enzyme

16. Membrane functions Membranes are involved in: 1) Transport of molecules and ions into and out of cells and organelles 2) Binding of hormones and other biomolecules

17. 1) Membrane transport mechanisms that regulate the passage of solutes such as ions and small molecules through membranes Ions & mols constantly move across cell plasma membranes & organelles movements of most solutes through the membrane are mediated by membrane transport proteins

18. types of membrane transport are passive transport and active transport.

19. a) Passive transport Diffusion of solute through membrane No need of energy Concentration gradient represents the potential energy 3 types 1) simple diffusion 2) facilitated diffusion 3) osmosis

20. 1) simple diffusion molecules move through a membrane down its concentration gradient ([H] to [L]) There is net movement of solute until an equilibrium is reached Higher concentration gradient = faster the rate of solute diffusion Diffusion of gas – proportional to concentration gradient Diffusion of organic mols – depend of molecular weight & lipid solubility

22. 2) facilitated diffusion Transport of large/charged mols from [H] to [L] through special channels or carriers Channels = tunnel-like transmembrane protein Each type is designed for transport specific solute Eg. Aquaporins- are channel proteins specific to water molecules, water molecules are small enough to pass thru lipid bilayers, rate of movements is slow- polar. Ion channels: open and close in response to an electrical/chemical stimulus

23. Carriers – specific solute bind to the carrier on one side of membrane and cause a conformational change in the carrier to shuttle them across membrane The solute is then translocated across the membrane and released.

25. 3) osmosis Passive transport of water across a membrane Ability of water to move to pass through a semi permeable membrane from a solution of lower solute concentration (dilute) to a solution of higher solute concentration (concentrated).

28. Similarities between Simple Diffusion and Facilitated Diffusion 1) Down the concentration gradient (From high concentration to low concentration) 2) No energy is required Differences

29. b) Active transport Energy is required to transport molecules against a concentration gradient Energy derived from ATP hydrolysis, or other energy sources is required to move the mols against concentration gradient 2 types – primary active transport & secondary active transport

31. 1) primary active transport Energy provide directly by ATP hydrolysis Transmembrane ATP-hydrolyzing enzyme use energy from ATP hydrolysis to drive the transport of ion/mols eg Na + -K + pump – primary transporter Na + and K + gradients for maintain cell vol and membrane potential Typically, K+ conc is low outside an animal cell and high inside the cell Na+ conc is high outside an animal cell and low inside the cell. The Na+-K+ pump maintains these conc gradients using the energy of 1 ATP to pump 3 Na+ out and 2 K+ in.

33. 2) secondary active transport Concentration gradient by primary active transport harness to move substances across membrane Eg Na + gradient created by Na + -K + pump is used in kidney tubule cells and intestinal cells to transport D-glucose

35. Bulk transport Exocytosis - Large molecules such as polysaccharides and proteins cross the membrane via vesicles. - During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by cyoskeleton to the plasma membrane. - When the 2 membranes in contact, the bilayers fuse and spill contents to the outside.

37. Endocytosis - During endocytosis, a cell brings in macromolecules by forming new vesicles from the plasma membrane. - Endocytosis is a reversal of exocytosis- but diff protein involved in these processes. - A small area of plasma membrane sinks inward to form a pocket. - As the pockets deepens, it pinches in to form a vesicle containing the material outside the cell.

38. 3 types of endocytosis Phagocytosis Pinocytosis Recepto-mediated endocytosis Phagocytosis (cellular eating) - The cell engulfs a particle by extending pseudopodia around it and package it in a large vacuole. - The content of the vacuole are digested when the vacuole fuses with lysosome.

40. Pinocytosis (cellular drinking) - A cell creates a vesicle around a droplet of extracellular fluid. - All included solutes are taken into the cell- nonspecific process.

41. Receptor-mediated endocytosis Specific. Only allow certain substances. This process is triggered when extracellular substances/ligands bind to to receptor on the membrane surface. The receptor proteins are clustered at the coated pits.

42. Binding of ligands to receptors triggers the formation of a vesicle by the coated pit, bringing the bound substances into the cell.

43. EXP 6: Extraction of lipids Results: A6 = 62.3848g A3A4 = 63.6418g A1A2 = 62.9922g A7A8 = 63.3789 MID-TERM TEST 1 = Friday, 9/11/2012, 4-5 pm @ DKG 2 & 3 Introduction to biochemistry – lipids.