Chapter 7: Membrane Structure and Function

1. Chapter 7: Membrane Structure and Function

2. Essential Knowledge • 2.b.1 – Cell membranes are selectively permeable due to their structure (7.1 & 7.2). • 2.b.2 – Growth and dynamic homeostasis are maintained by the constant movement of molecules across membranes (7.3-7.5).

3. Plasma Membrane • The membrane at the boundary of every cell • Functions as a selective barrier for the passage of materials in and out of cells. • Called a semi-permeable membrane(regulates crossing of materials)

4. Membrane Composition • Lipids • Most numerous • Phospholipids and cholesterol • Proteins • Very large (in size) • Peripheral and integral • Carbohydrates

5. Historical Cell Membrane Models

6. Davson-Danielli Model 1935 • Lipid bilayer • Proteins coat the surfaces • Sometimes called the “sandwich” model • Evidence: • Biochemical work • TEM pictures showed double line • Accepted until 1960s

7. Problems w/ Davson model • Not all membranes in a cell were the same • How could the proteins stay in place? • Protein placement was confusing • Result - the model was questioned and tested by scientific process

8. Fluid Mosaic Model 1972 • Current/New model to fit the new evidence • Example of “Science as a Process” • Refers to the way the lipids and proteins behave in a membrane

9. “Fluid” • Refers to the lipid bilayer • Molecules are not bonded together, so are free to shift. • Must remain "fluid" for membranes to function. • Fluid = dynamic, changing • Cell membrane will remain fluid until temperature drops to extreme levels

10. Ways to keep the membrane “fluid” • Lipid changes or shifts: • Plants: • Cold hardening (shift to unsaturated fatty acids – remember unsaturated fats are kinked in shape) • Animals: • Hibernating - Cholesterol amount increases – eat more fatty foods right BEFORE they hibernate

11. Sat = NO double bonds Unsat = double bonds

12. “Mosaic” • Proteins: float in a sea of lipids • Proteins form a collage or mosaic pattern that shifts over time

13. Evidence for fluid-mosaic • TEM pictures of fractured membranes • Cell fusion studies • Tagging of membrane proteins by antibodies

14. MEMORIZE THIS!!!

15. Protein Function in Membranes • Main function: • Determine cell’s specific function • Other functions: • Transport • Enzymatic activity • Receptor sites for signals (hormones) • Cell adhesion • Cell-cell recognition (immunity) • Attachment to the cytoskeleton (cilia and flagella)

16. 2 types of transport: Channel Carrier

17. Types of Membrane Proteins • 1) Integral • Inserted into the lipid bilayer • Go through ENTIRE bilayer • 2) Peripheral • Not embedded in the lipid bilayer • Attached to the membrane surface

19. Question? • How do the integral proteins stick to the membrane? • By the solubility of their amino acids • Protein folding/structure type

20. Hydrophilic Amino Acids Hydrophobic Amino Acids Hydrophilic Amino Acids

21. Membranes are Bifacial • The lipid composition of the two layers is different • The proteins have specific orientations. • Carbohydrates are found only on the outer surface

23. Membrane Carbohydrates • Branched oligosaccharides form glycolipids and glycoproteins on external surface • Glyco = carbohydrate • Made from modification of existing cellular molecules • Glycolipids = Lipids+carbohydrates • Glycoproteins = Proteins + carbohydrates • Function: recognition of "self" vs "other”

24. 1 2 2 3 3 3 4 Carbohydrates

25. Questions • How do materials get across a cell's membrane? • Do they use energy/no energy? • Do the molecules move against/with concentration gradient? • REMEMBER: Cell membrane is regulatory membrane (semi-permeable)

26. Problems with using cell membrane to move materials • 1) Lipid bilayer is hydrophobic • Hydrophilic materials don't cross easily • Ex: ions, H2O, polar molecules • Hydrophobic materials will cross easily • Ex: CO2, O2, hydrocarbons • 2) Large molecules don't cross easily • Too big to get through the membrane (without assistance or through the use of energy) • Proteins play HUGE role in getting certain molecules across (hydrophilic)

27. Two Mechanisms for Movement 1. Passive Transport • Active Transport * Both involve concentration gradients (movement of molecules from areas of high/low concentrations)

28. Passive Transport • Movement across membranes that does NOT require cellular energy • Types: • Diffusion • Osmosis • Facilitated Diffusion

29. Diffusion • The net movement of atoms, ions or molecules down a concentration gradient • Ex: smells crossing room • Movement is from: High Low Diffusion movie

31. Equilibrium • When the concentration is equal on both sides • There is no net movement of materials • Molecules are constantly in motion (don’t stop moving!) • However, CONCENTRATION STAYS THE SAME!!!

32. Factors that Affect Diffusion • Concentration a) Of solute or solvent/s 2. Temperature 3. Pressure • Particle size • Smaller size = quicker movement • Mixing a) More = faster diffusion

33. Osmosis • Diffusion of water • Water moving from an area of its high concentration to an area of its low concentration. • No cell energy is used • Passive transport • Relies upon tonicity of solutions (both internally and externally)

34. Tonicity • The concentration of water relative to a cell. 1. Isotonic (same) 2. Hypotonic (below) a) The hypotonic solution has a solute concentration BELOW that of the cell 3. Hypertonic (above) Tonicity Tutorial

36. Isotonic • Isosmoticsolution • Cell and water/solution are equal in solute concentration • No net movement of water in or out of the cell • Water still MOVES, but concentration of solutes stays the same!!! • RESULT: No change in cell size • Ex: Marine mammals

37. Hypotonic • Hypoosmotic solution • Cell's water is lower than the outside water (more solutes) • Water moves into the cell • RESULT: Cell swells, may burst or the cell is turgid • Ex: place raisin in water (raisin will swell, because water rushes in to attempt to EQUAL out concentrations of solutes and water) • Ex: place egg in vinegar/water solution

38. Hypertonic • Hyperosmotic solution • Cell's water is higher insidethan the outside water (less solutes) • Water moves out of the cell • RESULT: Cell shrinks or plasmolysisoccurs • Ex: Placing RBC in salt solution (RBC shrivels to attempt to push water out of cell to = solute concentrations) • Ex: Placing egg in corn syrup • Ex: onion cell w/ salt solution added Osmosis movie

39. Facilitated Diffusion • Transport protein that helps materials through the cell membrane • Polar molecules and ions USE this!!! • Doesn't require energy (ATP) • Still passive transport • Works on a downhill concentration gradient Facilitated Diff movie

40. Channel proteins Carrier proteins

41. Active Transport • Movement across membranes that DOES require cellular energy • Uses ALL carrier proteins • Allows cells to differ in solute concentration • Why? Important in Ps, Rs and hormones • Types: • Carrier-mediated • Endocytosis • Exocytosis

42. Key terms for Active Transport • Membrane potential: • Voltage across membrane (opposite charges and concentration of them) – likelihood charges will pass across the membrane • Electrochemical gradient: • Electro – change in charge • Chemical – change in concentration

43. Carrier-Mediated Transport • General term for the active transport of materials into cells AGAINST the concentration gradient • Movement is: low high • Examples: • Na+ - K+ pump • Electrogenic/H+ pump • Cotransport

44. Na+- K+ pump • Moves Na+ ions out of cells while moving K+ ions in • Occurs in animals • Sodium ions increase outside cell while potassium ions increase inside cell

46. Electrogenic or H+ pumps • Also called Proton pumps. • Create voltages (change of charge) across membranes for other cell processes • Ps and Rs • Used by plants, fungi and bacteria. • Transports H+ OUT of cell • This change in charge (or change in voltage) allows for storage of ATP • Used in later rxns

48. Cotransport • Movement of H+ that allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane • Example - Sucrose transport • This is how plants transport food/sugar to non-photosynthetic organs (like the roots)

49. Outside cell Inside cell

50. Exocytosis • Moves bulk material out of cells • Uses Golgi vesicles to do this • Example: • Secretion of enzymes • Hormone movement • Secretion of insulin by pancreas