membrane_structure_and_function.ppt
- 1. Membrane Structure and Function Don’t be semiknowledgeable in the semipermeable domain
- 2. Chapter 7: Membranes I. Membrane Structure: A. Membrane models have evolved to fit new data: Science as Process B. A membrane is a Fluid Mosaic of lipids, proteins, carbos
- 3. More Membranes II. Traffic Across Membranes a. Molecular organization of membrane=selective permeability b. Passive Transport=Diffusion across a membrane c. Osmosis=passive transport of water d. Balancing water uptake/loss=cell survival e. Specific proteins facilitate transport of selected solutes f. Active transport=pumping solutes against gradient g. Some ion pumps generate voltage across membranes h. Cotransport: A membrane protein couples the transport of one solute to another i. Exocytosis/Endocytosis transport large molecules
- 6. General Membrane Characteristics • Held together by hydrophobic interactions • Most lipids/proteins can drift laterally • Molecules rarely flip transversely • Phospholipids move faster than proteins • Some proteins are connected to the cytoskeleton, can’t move far • Unsaturated Hydrocarbon tails on lipids increase fluidity • Cholesterol decreases fluidity at warmer temps, more fluid at colder temps. (plant survival adaptation)
- 7. The Fluidity of Membranes
- 8. MOSAICISM • Membranes are mosaics of floating proteins in a lipid bilayer. 2 ways: –Integral Proteins: transmembrane, have both hydropohilic and hydrophobic parts –Peripheral Proteins: Attached to membrane’s surface by: • Attachment to integral proteins or ECM fibers (outside) • Attachment to filaments of cytoskeleton (inside)
- 9. The Structure of a Transport Protein
- 10. Membranes are Bifacial • 2 lipid layers may differ in composition • Membrane proteins have distinct directional orientation • Inside of vesicles, ER, Golgi is the same as the outside of the membrane
- 11. Sidedness of the Plasma Membrane
- 12. Membrane Carbohydrates • Allow Cell to Cell Recognition: The ability of a cell to recognize if other cells are alike or different from itself. Immunity repercussions? This cell-cell recognition is the basis for: *sorting an embryo’s cells into tissues/organs *rejection of foreign cells by immune sys.
- 13. OH! More Carbos • Act as markers on cell’s surface for recognition purposes • Usually branched oligosaccharides (<15 monomers) • Some are covalently bonded to lipids (glycolipids) • MOST are covalently bonded to proteins (glycoproteins) • Vary between species, individuals of same sp., and between cells in same organism
- 14. The Detailed Structure of Membranes
- 15. Traffic Across Membranes • Selective permeability depends on solubility characteristics of the lipid bilayer, and presence of specific integral proteins
- 16. Permeability of the Lipid Bilayer NONPOLAR Molecules • Dissolve in membranes, cross with ease • Smaller of 2 will cross faster POLAR Molecules • Small, polar, uncharged molecules can slip through • Larger, polar, uncharged will not get through easily (glucose) • All ions have trouble getting through hydrophobic layer
- 17. Transport Proteins • Hydrophilic Substances like ions and moderately sized polar molecules can avoid going through the hydrophobic core of the membrane by going through transport proteins: Integral membrane proteins that transport specific ions or molecules across the membrane. They may provide a hydrophilic tunnel or may bind to, and physically carry a substance across. These are specific
- 18. Some Functions of Membrane Proteins
- 19. REVIEW ON YOUR OWN: • Passive transport: Concentration Gradient, Net directional movement, diffusion • Osmosis: Hypertonic, hypotonic, isotonic, osmotic concentration, osmotic pressure • Water balance in organisms without cell walls: Live in isotonic environment, osmoregulation • Water balance in cells with walls: Turgidity, plasmolysis
- 20. Proteins Facilitate Passive Transport • Facilitated Diffusion: specific transport proteins help solutes diffuse across membrane – Is passive transport (down conc. Gradient) – Helps many polar molecules/ions get through the lipid bilayer – SPECIFIC-Have binding site like active site – Can be saturated with solute: rate limited – Can be due to conformational changes – Can be gated channels
- 21. ACTIVE TRANSPORT • Endergonic process by which a transport protein pumps a molecule across a membrane AGAINST its concentration gradient. • These maintain concentration gradients across membranes • Use ATP as energy source. • Ex. Sodium-Potassium Pump
- 24. Water Balance in Cells w/Walls
- 25. MEMBRANE POTENTIAL • Voltage across membranes happens when anions/cations are unequally distributed across cell membranes • Potential ranges from -50 to -200 mv • Negative sign indicates the inside of the cell is – charged. • Affects traffic of charged subs. across membrane, favors diffusion of anions out, cations in.
- 26. Passive Transport of Ions Depends On: • Concentration gradient of the Ion • Effect of the membrane potential of the ion The Electrochemical Gradient is the diffusion gradient created from the combined effects of both of the above
- 27. Factors Contributing to Membrane Potential: • Neg. charged proteins in the cells interior • Plasma membrane’s selective permeability to various ions • The Sodium-Potassium Pump is an ELECTROGENIC PUMP: a transport protein which generates voltage across a membrane. Na+/K+ ATPase is the major one in animals, a Proton pump is the major one in Plants, bacteria, fungi (also Mitochondria, Chloroplasts use it to make ATP)
- 29. Na, Na, Na, Na, K, K, K Goodbye!
- 30. COTRANSPORT • A process where a single ATP-powered pump actively transports one solute, and indirectly drives the transport of other solutes against their concentration gradients
- 31. Cotransport
- 32. Exocytosis/Endocytosis Transport el Moleculo LARGO Exocytosis • Exportation of macromolecules by the fusion of vessicles w/membrane • Vessicle comes from ER or Golgi • Used by secretory cells to export products Endocytosis • Importation of macromolecules into a cell by forming vessicles from membrane • Used by cells to incorporate extracellular substances
- 33. ENDOCYTOSIS • Phagocytosis: “Cell eating” solid particles involved. Cell engulfs them with pseudopods. The vessicle then fuses w/ a food vacuole • Pinocytosis: “Cell drinking” fluid droplets involved • Receptor-Mediated endocytosis: stay tuned..
- 34. Receptor-Mediated Endocytosis • Importation of specific macromolecules into the cell by the inward budding of vessicles formed from COATED PITS • A layer of CLATHRIN , a fibrous protein, lines and reinforces the coated pit, probably causing it to deepen the pit to form a vessicle • This is specific, and is ligand/receptor triggered • Ex. Cholesterol and LDL’s