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CHAPTER 7

CHAPTER 7. MEMBRANE STRUCTURE AND FUNCTION. MEMBRANE STRUCTURE. THE PLASMA MEMBRNE IS THE BOUNDARY THAT SEPARATES THE LIVING CELL FROM ITS NONLIVING SURROUNDINGS. ITS ABILITY TO DISCRIMINATE IN ITS CHEMICAL EXCHANGES WITH THE ENVIRONMENT MAKES LIFE POSSIBLE. THE PLASMA MEMBRANE.

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CHAPTER 7

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  1. CHAPTER 7 MEMBRANE STRUCTURE AND FUNCTION

  2. MEMBRANESTRUCTURE • THE PLASMA MEMBRNE IS THE BOUNDARY THAT SEPARATES THE LIVING CELL FROM ITS NONLIVING SURROUNDINGS. • ITS ABILITY TO DISCRIMINATE IN ITS CHEMICAL EXCHANGES WITH THE ENVIRONMENT MAKES LIFE POSSIBLE

  3. THE PLASMA MEMBRANE • IS ABOUT 8 nm* THICK • SURROUNDS THE CELL AND CONTROLS TRAFFIC INTO AND OUT OF THE CELL • IS SELECTIVELY PERMEABLE; IT ALLOWS SOME SUBSTANCES TO CROSS MORE EASILY THAN OTHERS • *nm is 1/billionth of a meter

  4. MEMBRANE VIDEO

  5. MEMBRANE STUDIES • 1935: DANIELLI AND DAVSON PROPOSED A MODEL OF CELL MEMBRANE STRUCTURE. NOTICE HOW 2 GENERATIONS OF MEMBRANE MODELS DIFFER

  6. TWO GENERATIONS OF MEMBRANE MODELS

  7. FLUID MOSAIC MODEL • 1972: SINGER AND NICOLSON PROPOSED THIS MODEL WHICH IS CURRENTLY ACCEPTED. THEY PROPOSED: • PROTEINS ARE INDIVIDUALLY EMBEDDED IN THE PHOSPHOLIPID BILAYER • HYDROPHILIC PORTIONS ARE ON OUTSIDE • HYDROPHOBIC PORTIONS ARE ON INSIDE • MEMBRANE IS A MOSAIC OF PROTEINS BOBBING IN A FLUID BILAYER OF PHOSPHOLIPIDS

  8. THE FLUID QUALITY OF MEMBRANES • MEMBRANES ARE HELD TOGETHER BY HYDROPHOBIC INTERACTIONS, WHICH ARE WEAK ATTRACTIONS • MOST MEMBRANE LIPIDS AND SOME PROTEINS CAN DRIFT LATERALLY WITHIN THE MEMBRANE • MOLECULES RARELY FLIP TRANSVERSELY ACROSS THE MEMBRANE BECAUSE HYDROPHILIC PARTS WOULD COME IN CONTACT WITH HYDROPHOBIC CORE

  9. FLUID QUALITY CON’T • MEMBRANES MUST BE FLUID TO WORK PROPERLY. SOLIDIFICATION MAY RESULT IN PERMEABILITY CHANGES AND ENZYME DEACTIVATION

  10. FLUIDITY CON’T • UNSATURATED HYDROCARBON TAILS ENHANCE MEMBRANE FLUIDITY, BECAUSE KINKS AT THE CARBON-TO-CARBON DOULBE BONDS HINDER CLOSE PACKING OF PHOSPHOLIPIDS • CHOLESEROL MODULATES FLUIDITY BY MAKING THE MEMBRANE • MORE FLUID AT LOWER TEMPERATURES

  11. THE FLUIDITY OF MEMBRANES

  12. MEMBRANES AS MOSAICS OF STRUCTURE AND FUNCTION • A MEMBRANE IS A MOSAIC OF DIFF. PROTEINS EMBEDDED AND DISPERSED WITHIN THE MEMBRANE • INTEGRAL PROTEINS: GENERALLY TRANSMEMBRANE PROTEINS WITH HYDROPHOBIC REGIONS THAT SPAN THE INTERIOR • PERIPHERAL PROTEINS: NOT EMBEDDED BUT ATTACHED TO THE SURFACE

  13. DETAILED STRUCTURE

  14. TRANSMEMBRANE PROTEIN

  15. MEMBRANES ARE BIFACIAL • THE MEMBRANE IS MADE AND MODIFIED BY THE E.R. AND GOLGI, AND THIS DETERMINES THE DISTRIBUTION OF LIPIDS, PROTEINS AND CARBOHYDRATES WITHIN THE MEMBRANE

  16. SIDEDNESS OF THE PLASMA MEMBRANE

  17. MEMBRANE SIDEDNESS • THE 2 LIPID LAYERS MAY DIFFER IN LIPID COMPOSITION • MEMBRANE PROTEINS HAVE DISTINCT DIRECTION ORIENTATION • CARBO’S ARE ONLY IN THE MEMBRANE’S EXTERIOR

  18. CELL-CELL RECOGNITION • THE ABILITY OF A CELL TO DETERMINE IF OTHER CELLS IT ENCOUNTERS ARE ALIKE OR DIFF. • IT IS THE BASIS FOR: • SORTING EMBRYO CELLS INTO TISSUE AND ORGANS • REJECTION OF FOREIGN CELLS BY THE IMMUNE SYSTME

  19. CELL-CELL RECOGNITION • CELLS RECOGNIZE OTHER CELLS BY CELL MARKERS FOUND ON THE EXTERNAL SURFACE OF CELL MEMBRANE. THESE MARKERS ARE MAINLY CARBOHYDRATES: • SOME COVALENTLY BONDED TO LIPIDS • MOST COVALENTLY BONDED TO PROTEINS

  20. MEMB. PROT. FUNCTIONS

  21. TRAFFIC ACROSS MEMBRANES • THE SELECTIVE PERMEABILITIY OF A MEMBRANE DEPENDS UPON: • MEMBRANE SOLUBILITY CHARACTERISTICS OF PHOSPHOLIPID BILAYER • PRESENCE OF SPECIFIC INTEGRAL TRNSPORT PROTEINS

  22. PERMEABILITY OF LIPID BILAYER • NONPOLAR (HYDROPHOBIC) MOLECULES • DISSOLVE IN MEMBRANE AND CROSS WITH EASE • SMALLER MOLECULES MOVE FASTER • POLAR(HYDROPHILIC) MOLECULES • SMALL, POLAR UNCHARGED MOLECULES (WATER) WILL PASS • LARGER MOLECULES WILL NOT • ALL IONS, EVEN SMALL ONES (Na+, H+) WILL NOT

  23. TRANSPORT PROTEINS • MEMBRANE PROTEINS THAT TRANSPORT SPECIFIC MOLECULES OR IONS ACROSS THE MEMBRANE • MAY PROVIDE A HYDROPHILIC TUNNEL THROUGH THE MEMBRANE • MAY BIND A SUBSTANCE AND PHYSICALLY MOVE IT ACROSS • ARE SPECIFIC FOR THE SUBSTANCE THEY TRANSLOCATE

  24. DIFFUSION • PASSIVE TRANSPORT-THE DIFFUSION OF A SUBSTANCE ACROSS A MEMBRANE WITH NO ENERGY NEEDED • CONCENTRATION GRADIENT- REGULAR, GRADED CONCENTRATON CHANGE OVER A DISTANCE IN A PARTICULAR DIRECTION

  25. DIFFUSION CON’T • NET DIRECTIONAL MOVEMENT- OVERALL MOVEMENT AWAY FROM THE CENTER OF CONCENTRATION • DIFFUSION- THE NET MOVEMENT OF A SUBSTANCE DOWN A CONCENTRATION GRADIENT • CONTINUES UNTIL EQUILIBRIUM IS REACHED, IN THE ABSENCE OF OTHER FORCES

  26. DIFFUSION ACROSS MEMBRANE

  27. DIFFUSION VIDEO

  28. TONICITY • HYPERTONIC SOLUTION- A SOLUTION WITH A GREATER SOLUTE CONCENTRATION THAN THAT INSIDE A CELL • HYPOTONIC SOLUTION- A SOLUTION WITH A LOWER SOLUTE CONCENTRATION COMPARED TO INSIDE THE CELL • ISOTONIC SOLUTION- A SOLUTION WITH EQUAL CONCENTRATION INSIDE AND OUTSIDE THE CELL

  29. OSMOSIS • OSMOSIS IS THE DIFFUSION OF WATER ACROSS A SELECTIVELY PERMEABLE MEMBRANE • WATER DIFFUSES DOWN ITS CONCENTRATION GRADIENT • DIRECTION OF OSMOSIS IS DETERMINED BY THE DIFF. IN TOTAL SOLUTE CONCENTRATION • IN AN ISOTONIC SOLUTION, WATER MOVES IN BOTH DIRECTIONS, WITH NO NET MOVEMENT

  30. OSMOSIS

  31. OSMOSIS VIDEO

  32. OSMOSIS CON’T • OSMOTIC CONCENTRATION= TOTAL SOLUTE CONCENTRATION OF A SOLUTION • OSMOTIC PRESSURE= MEASURE OF THE TENDENCY FOR A SOLUTION TO TAKE UP WATER WHEN SEPARATED FROM PURE WATER BY A MEMBRANE; CAN BE MEASURED BY AN OSMOMETER

  33. WATER BALANCE OF CELLS WITHOUT WALLS • SINCE ANIMAL CELLS LACK CELL WALLS, THEY ARE NOT TOLERANT OF EXCESSIVE OSMOTIC UPTAKE OR LOSS OF WATER • ISOTONIC ENVIRONMENT-VOLUME REMAINS STABLE • HYPERTONIC ENV-CELLS WILL SHRINK • HYPOTONIC ENV-CELLS WILL SWELL

  34. WATER BALANCE

  35. WATER BALANCE OF CELLS WITH WALLS • PROKARYOTES, SOME PROTISTS, FUNGI AND PLANTS • HYPOTONIC-CELL SWELLS AND BECOMES TURGID (FIRM) • ISOTONIC-CELL BECOMES FLACCID, NO NET MOVEMENT OF WATER, PLANT WILL WILT • HYPERTONIC-CELLS WILL LOSE WATER AND WILL SHRINK (PLASMOLYZE) AND DIE

  36. PROTEINS HELP WITH PASSIVE TRANSPORT • FACILITATED DIFFUSION=DIFFUSION OF SOLUTES ACROSS A MEMBRANE, WITH THE HELP OF TRANSPORT PROTEINS • PROTEINS ARE SPECIFIC FOR THE SOLUTES THEY TRANSPORT • IS PASSIVE, BECAUSE SOLUTE IS MOVED DOWN ITS CONCENTRATION GRADIENT • HELPS TO MOVE POLAR MOLECULES AND IONS

  37. FACILITATED DIFFUSION

  38. FACILITATED DIFFUSION VIDEO

  39. ACTIVE TRANSPORT • ACTIVE TRANSPORT REQUIRES ENERGY (ATP). TRANSPORT PROTEINS PUMP MOLECULES AGAINST ITS CONC. GRADIENT • PROCESS IS (+DG), CELL EXPENDS ENERGY • HELPS CELLS MAINTAIN STEEP IONIC GRADIENTS ACROSS MEMBRANE (EX: Na+,K+, Mg++, Ca++, Cl-

  40. THE SODIUM-POTASSIUM PUMP-ACTIVE TRANPORT • THIS TRANSPORT SYSTEM MOVES THREE Na+ IONS OUT OF THE CELL FOR EVERY TWO K+ IONS INTO THE CELL. • ATP POWERS THIS BY PHOSPHORYLATING THE TRANSPORT PROTEIN, WHICH CAUSES CHANGE IN PROTEIN CONFORMATION, WHICH THEN EXPELS Na+ OUT, AND BINDS EXTRACELLULAR K+, TO BE TRANSPORTED IN

  41. SODIUM-POTASSIUM PUMP

  42. ACTIVE TRANSPORT VIDEO

  43. VOLTAGE ACROSS MEMBRANES • BECAUSE ANIONS AND CATIONS ARE UNEQUALLY DISTRIBUTED ACROSS THE PLASMA MEMBRANE, ALL CELLS HAVE VOLTAGES ACROSS THEIR MEMBRANES • MEMBRANE POTENTIAL-VOLTAGE ACROSS MEMBRANES • RANGES FROM -50 TO -200 mv • THE CELL’S INSIDE IS NEGATIVE WITH RESPECT TO OUTSIDE • FAVORS DIFFUSION OF CATIONS INTO CELL AND ANIONS OUT OF CELL

  44. ELECTROGENIC PUMP • A TRANSPORT PROTEIN THAT GENERATES VOLTAGE ACROSS A MEMBRANE • SODIUM-POTASSIUM PUMP IS THE MAJOR ELECTROGENIC PUMP IN ANIMALS • PROTON PUMP IS THE MAJOR ONE IN PLANTS, BACTERIA AND FUNGI • MITOCHONDRIA AND CHLOROPLASTS USE A PROTON PUMP TO DRIVE ATP SYNTHESIS IN CELL RESPIRATION AND PHOTOSYNTHESIS

  45. ELECTROGENIC PUMP H+ CONCENTRATION IS NOW ON OUTSIDE, SETTING UP FOR DIFFUSION BACK INTO THE CELL WHICH WILL CREATE ENERGY (CH 9)

  46. COTRANSPORT • PROCESS WHERE A SINGLE ATP-POWERED PUMP ACTIVELY TRANSPORTS ONE SOLUTE AND INDIRECTLY DRIVES THE TRANSPORT OF OTHER SOLUTES AGAINST THE CONC. GRADIENT • 2 TRANSPORT PROTEINS ARE INVOLVED

  47. COTRANSPORT • ATP POWERED PUMP MOVES H+ OUT OF CELL. H+ -SUCROSE COMPLEX DIFFUSES BACK INTO CELL USING 2ND PROTEIN

  48. TRANSPORTING LARGE MOLECULES • EXOCYTOSIS- PROCESS OF EXPORTING MACROMOLECULES FROM CELL BY FUSION OF VESICLES WITH PLASMA MEMBRANE • VESICLE USUALLY BUDDED FROM E.R. OR GOLGI AND MIGRATES TO MEMBRANE • USED BY SECRETORY CELLS TO EXPORT PRODUCTS (EX: INSULIN)

  49. ENDOCYTOSIS • IMPORTING LARGE MOLECULES • VESICLE FORMS FROM A REGION OF MEMBRANE THAT SINKS INWARD; PINCHES OFF CYTOPLASM • THERE ARE 3 TYPES OF ENDOCYTOSIS • PHAGOCYTOSIS: SOLID PARTICLES • PINOCYTOSIS: LIQUID PARTICLES • RECEPTOR-MEDIATED ENDOCYTOSIS: VESICLES FORMED FROM COATED PITS; OCCURS IN RESPONSE TO THE BINDING OF SPECIFIC LIGANS TO RECEPTORS ON CELL’S SURFACE

  50. EXOCYTOSIS VIDEO

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