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Understanding Membrane Structure and Function

Explore the fluid mosaic model, components like phospholipid bilayer and proteins, and passive transport mechanisms across membranes. Learn about membrane fluidity and selective permeability.

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Understanding Membrane Structure and Function

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  1. Chapter 5Membranes

  2. I. The Structure of Membranes (5.1) A. The fluid mosaic model shows proteins embedded in a fluid lipid bilayer

  3. Phospholipid a. Hydrophilic head (glycerol and phosphate group make it polar) b. Hydrophobic tail (2 fatty acid chains are nonpolar)

  4. 2. Globular proteins a. proteins w/ nonpolar segments and polar ends to fit in phospholipid bilayer.

  5. 3. Fluid mosaic model a. random arrangement of proteins “floating” in or on the fluid lipid bilayer. http://www.susanahalpine.com/anim/Life/memb.htm

  6. B. Cellular membranes consist of four component groups 1. Phospholipid bilayer: 2 layers of phospholipids with hydrophobic tails pointed inward.

  7. 2. Transmembrane proteins (aka intergral membrane proteins): allow substances to pass through membrane.

  8. 3. Interior protein network: proteins in the membrane that help support cell shape. a. Peripheral membrane proteins: proteins that are not part of the membrane structure but help control cell movement. 4. Cell surface markers: a. Glycoproteins and glycolipids act as cell identity markers (IDs)

  9. Lipid rafts: sections of the cell membrane that function as signal receptors and involved in cell movement tightly packed by cholesterol.

  10. C. Electron microscopy has provided structural evidence Transmission electron microscopes (TEM) and scanning electron microscopes (SEM) used to study the plasma membrane.

  11. II. Phospholipids: The Membrane’s Foundation (5.2) • Phospholipids spontaneously form bilayers 1. Hydrophilic phosphate group facing out 2. Hydrophobic nonpolar fatty acids face in 3. Closely packed hydrophobic tails prevent water soluble substances from passing through.

  12. B. The phospholipid bilayer is fluid 1. Hydrogen bonds help hold membrane together. 2. Weak forces b/t phospholipids allow movement of proteins and lipids.

  13. C. Membrane fluidity can change 1. Saturated fats and cooler temps make the membrane less fluid (solid) b/c lipids are tightly packed. 2. Unsaturated fats and higher temps make the membrane more fluid b/c lipids are more loosely packed.

  14. http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/

  15. III. Proteins: Multifunctional Components (5.3)

  16. A. Proteins and protein complexes perform key functions • Transporters: move substances across membrane in carriers or channels • Enzymes: carry out chemical reactions • Cell surface receptors: detect messages • Cell surface identity markers: ID • Cell-to-cell adhesion proteins: glue cells together • Attachments to the cytoskeleton

  17. B. Structural features of membrane proteins • The anchoring of proteins in the bilayer: nonpolar sections of protein tied to membrane. α-helix or -sheets • Transmembrane domains: area or areas of protein anchored to nonpolar lipid bilayer. α-helix

  18. 3. Pores: proteins that form pipe-like holes to allow passage for molecules -sheets

  19. IV. Passive Transport Across Membranes A. Passive transport: movement of substances in and out of the cell w/OUT using energy B. Concentration gradient: difference in concentration of a substance b/t the inside and outside of a cell.

  20. C. Transport can occur by simple diffusion 1. Diffusion: random movement of molecules from an area of high concentration to an area of lower concentration.

  21. http://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swfhttp://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swf • http://www.wiley.com/college/pratt/0471393878/student/animations/membrane_transport/index.html

  22. NOTE: Nonpolar molecules like O2 and steroids have a much easier time moving across the lipid bilayer membrane. Few small polar molecules can cross very slowly • http://www.teachersdomain.org/resources/tdc02/sci/life/cell/membraneweb/assets/tdc02_int_membraneweb/tdc02_int_membraneweb_swf.html

  23. D. Proteins allow membrane diffusion to be selective • Channel proteins: provide a passage for polar molecules • Carrier proteins: bind to molecule and help them cross membrane. • Selectively permeable: choose which molecules can cross.

  24. 4. Diffusion of ions through channels a. Ion channels: transport channel proteins move ions (i.e. K+, Ca+2, Cl-) across according to their concentration. b. Gated channels: channels proteins that open or close due to a stimulus. c. Membrane potential: difference in voltage across membrane causes channels to open or close.

  25. E. Carrier proteins and facilitated diffusion • Facilitated diffusion: carrier proteins help move molecules across membrane according to the concentration difference. • Saturation: at high concentrations, all carrier proteins will move molecules across. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

  26. c. Facilitated diffusion is: - specific to certain molecules - passive: high to low concentration - saturates: only moves as fast as # of carrier proteins available.

  27. 6. Facilitated diffusion in red blood cells

  28. C. Osmosis is the movement of water across membranes • Solvent: medium, usually water, in which solute is dissolved. • Solute: substance dissolved in solvent ***Solutes and solvents move by diffusion (high concentration to low concentration)

  29. 3. Osmosis: movement of water to area of higher solute concentration. • Osmotic pressure: the concentration of all solutes in a solution. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

  30. b. Hypertonic: higher solute concentration c. Hypotonic: lower solute concentration d. Isotonic: when two solutions have the same osmotic (solute) concentration.

  31. 4. Aquaporins: water channels • Movement of water is assisted by special water channels. • People with Nephrogenic Diabetes Insipidus have nonfunctional aquaporins and have trouble retaining water.

  32. 5. Osmotic pressure • Hydrostatic pressure: pressure of cytoplasm pushing on cell membrane due to osmosis.

  33. Osmotic pressure: force to stop flow of water, depends on solute concentration inside and outside of cell.

  34. 6. Maintaining osmotic balance a. Extrusion: contractile vacuole pumps out water that constantly enters due to osmosis. b. Isosmotic Regulation: Increasing the internal solute concentration to match the environment.

  35. http://trc.ucdavis.edu/biosci10v/bis10v/week7/paramecium.mov

  36. c. Turgor/ Turgor pressure: plant cells are usually hypertonic to their environment to increase hydrostatic pressure. This increases the pressure of the cell membrane against the cell wall maintaining cell shape.

  37. http://www.teachersdomain.org/resources/tdc02/sci/life/cell/membraneweb/assets/tdc02_int_membraneweb/tdc02_int_membraneweb_swf.htmlhttp://www.teachersdomain.org/resources/tdc02/sci/life/cell/membraneweb/assets/tdc02_int_membraneweb/tdc02_int_membraneweb_swf.html

  38. V. Active Transport Across Membranes (5.5) A. Active transport : movement of substances UP CONCENTRATION GRADIENT that requires ENERGY from ATP.

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