Membrane Structure and Function

1. Membrane Structure and Function Chapter 7

2. Plasma Membrane • The boundary that separates the living cell from its nonliving surroundings. • Surrounds the cell and controls chemical traffic into and out of the cell. • Selectively permeable: allows some substances to cross more easily than others.

3. Phospholipid Bilayer • Polar (hydrophilic) heads of phospholipids are oriented towards the outside of the membrane. • The nonpolar (hydrophobic) tails are oriented towards the inside of the membrane, away from the cell contents and the outside environment. • Proteins are imbedded in the bilayer.

5. Fluid Mosaic Model • Membranes must be fluid to work properly. • Membranes are held together by weak hydrophobic interactions. • Lipids and proteins drift and bob within the bilayer. • Membranes can solidify if the temperature drops too low. Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids.

6. Fluid Mosaic Model Cont. • Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks in the hydrocarbon tails hinder close packing of phospholipids. • Cholesterol, found in plasma membranes of eukaryotes, makes the membrane: • Less fluid at warmer temperatures by restraining phospholipid movement. • More fluid at lower temperatures by preventing close packing of phospholipids.

7. Fluid Mosaic Model Cont. • A membrane is a mosaic of different proteins embedded and dispersed in the phospholipid bilayer. • 1. Integral proteins: inserted into the membrane so their hydrophobic regions are surrounded by hydrocarbon portions of phospholipids. They may be: • Unilateral, reaching only partway across the membrane. • Transmembrane, with hydrophobic midsections between hydrophilic ends exposed on both sides of the membrane. • 2. Peripheral proteins: not embedded but attached to the membrane's surface. May be attached to integral proteins or held by filaments of cytoskeleton.

8. Fluid Mosaic Model Cont. • Additional cell markers (carbohydrates) are found on the external surface of the plasma membrane. • Usually branched oligosaccharides (< 15 monomers). • Some covalently bonded to lipids (glycolipids). • Most covalently bonded to proteins (glycoproteins).

9. Fluid Mosaic Model Cont. • Cell-cell recognition -- Ability of a cell to determine if other cells it encounters are alike or different from itself. • Cell-cell recognition is crucial in the functioning of an organism. • Sorting of an animal embryo's cells into tissues and organs. • Rejection of foreign cells by the immune system. • Variation from species to species, between individuals of the same species, and among cells in the same individual.

11. Permeability of the Membrane • Nonpolar (Hydrophobic) Molecules • • Dissolve in the bilayer and cross it with ease. (hydrocarbons, O2). • • Smaller molecules will cross the membrane faster. • Polar (Hydrophilic) Molecules • • Small, polar molecules (H2O, CO2) are small enough to pass slowly between membrane lipids. • • Larger, polar molecules (glucose) will not easily pass through. • • Ions (Na+, H+) have difficulty penetrating the hydrophobic layer due to charge.

12. Permeability of the Membrane cont • Biological membranes are permeable to specific ions and certain polar molecules of moderate size. These hydrophilic substances avoid the hydrophobic core of the bilayer by passing through transport proteins. • Transport proteins: • • May provide a hydrophilic tunnel through the membrane. • • May bind to a substance and physically move it across the membrane. • • Are specific for the substance they transport.

13. Passive Movement Across the Membrane • Concentration gradient -- concentration change over a distance in a particular direction. • Diffusion -- The net movement of a substance down a concentration gradient (from area of high conc to area of low conc). • Passive transport – Spontaneous diffusion of a substance across a biological membrane; does not require the cell to expend energy; driven by potential energy stored in a concentration gradient.

14. Osmosis is the passive transport of water • Osmosis -- Diffusion of water across a selectively permeable membrane from an area of higher water potential to an area of lower water potential. • Hypertonic solution -- (hyperosmotic) A solution with a greater solute concentration than that inside a cell; water will move out from cell and it may shrivel and die. • Hypotonic solution – (hypoosmotic) A solution with a lower solute concentration compared to that inside a cell; water will move into the cell and it may burst. • Isotonic solution – (isosomotic) A solution with an equal solute concentration compared to that inside a cell; equal water flow in both directions.

16. Pure water = 0. The more solute is dissolved, the lower the water potential (negative #). Equals 0 if both sides of membrane have equal solute concentration. Greater solute conc (lower water potential) = greater osmotic pressure. Water Potential / Osmotic Pressure

17. Water Balance of Cells With Walls • Prokaryotes, some protists, fungi and plants have cell walls outside the plasma membrane. • Plasmolysis -- a walled cell shrivels and the plasma membrane pulls away from the cell wall as the cell loses water to a hypertonic environment; usually lethal. • Turgid -- Firmness or tension found in walled cells that are in a hypoosmotic environment. • Turgid cells provide mechanical support for plants. • In an isotonic environment, there is no net movement of water into or out of the cell; loss of structural support from turgor pressure causes plants to wilt.

18. Facilitated Diffusion • Movement of solutes across a membrane with the help of transport proteins (permeases). • Transport proteins are specific for the solutes they transport. Specific binding site analogous to an enzyme's active site. • Transport proteins can be inhibited by molecules that resemble the solute normally carried by the protein (similar to competitive inhibition in enzymes). • Uniport Transport Protein – carries a single molecule across. • Symport – moves 2 different molecules in same direction. • Antiport – moves 2 molecules in opposite directions.

19. http://www.sp.uconn.edu/~terry/images/anim/symport.gif • http://www.sp.uconn.edu/~terry/images/anim/antiport.gif

20. Active Transport • Energy-requiring process during which a transport protein pumps a molecule across a membrane, against its concentration gradient. • Transport proteins involved in active transport harness energy from ATP to pump molecules “uphill”. • An example is the sodium-potassium pump. • ATP powers the shape change in the protein from Na-receptive to K-receptive. • Three Na+ ions out of the cell for every two K+ ions pumped into the cell.

21. http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.htmlhttp://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/ion_pump/ionpump.html • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120068/bio03.swf::Sodium-Potassium%20Exchange%20Pump

22. Vesicle-Mediated Transport • Large molecules (proteins and polysaccharides) cross membranes by the processes of exocytosis and endocytosis. • Exocytosis -- Process of exporting materials from a cell; vesicle usually budded from the ER or Golgi and migrates to and fuses with plasma membrane . • Used by secretory cells (insulin from pancreas, or neuro-transmitter from neuron).

23. Vesicle-Mediated Transport cont • Endocytosis -- Process of importing materials into a cell; vesicle forms from a region of plasma membrane that sinks inward and pinches off. • Phagocytosis -- (cell eating) Endocytosis of solid particles; forms a food vacuole which fuses with a lysosome. • Pinocytosis -- (cell drinking) Endocytosis of fluid droplets. • Receptor-mediated endocytosis – Large molecules attach to specific receptors on the cell's surface; causes vesicle to form around the substance. • Membrane-embedded proteins cluster in regions called coated pits. • Molecule that binds to the receptor site is called a ligand.

24. http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/exocyt.htmlhttp://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/exocyt.html • http://bio.winona.edu/berg/ANIMTNS/Secrtion.htm • http://student.ccbcmd.edu/~gkaiser/biotutorials/eustruct/phagocyt.html