MEMBRANE STRUCTURE AND FUNCTION OVERVIEW CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION OVERVIEW

Figure 8.2 Two generations of membrane models I. MEMBRANE STRUCTURE A. MEMBRANE MODELS HAVE EVOLVED TO FIT NEW DATA: SCIENCE AS A PROCESS CURRENT MEMBRANE MODEL: FLUID MOSAIC MODEL Figure 8.2 Two generations of membrane models

B. A MEMBRANE IS A FLUID MOSAIC OF LIPIDS, PROTEINS, AND CARBOHYDRATES INTEGRAL PROTEINS ARE EMBEDDED IN THE LIPID BILAYER PERIPHERAL PROTEINS ARE ATTACHED TO THE SURFACE THE INSIDE AND OUTSIDE MEMBRANE FACES DIFFER IN COMPOSITION CARBOHYDRATES LINKED TO PROTEINS AND LIPIDS IN THE PLASMA MEMBRANE ARE IMPORTANT FOR CEL-CELL RECOGNITION

Figure 8.5 The detailed structure of an animal cell’s plasma membrane

II. TRAFFIC ACROSS MEMBRANES A. A MEMBRANE’S MOLECULAR ORGANIZATION RESULTS IN SELECTIVE PERMEABILITY A CELL MUST EXCHANGE SMALL MOLECULES AND IONS WITH ITS SURROUNDINGS, A PROCESS CONTROLLED BY THE PLASMA MEMBRANE HYDROPHOBIC SUBSTANCES ARE SOLUBLE IN LIPID AND PASS THROUGH MEMBRANES RAPIDLY SMALL POLAR MOLECULES SUCH AS H2O ALSO PASS THROUGH THE MEMBRANE LARGER POLAR MOLECULES AND IONS REQUIRE SPECIFIC TRANSPORT PROTEINS TO HELP THEM ACROSS

B. PASSIVE TRANSPORT IS DIFFUSION ACROSS A MEMBRANE DIFFUSION: THE SPONTANEOUS MOVEMENT OF A SUBSTANCE DOWN ITS CONCENTRATION GRADIENT Figure 8.9 Diffusion

C. OSMOSIS IS THE PASSIVE TRANSPORT OF WATER WATER FLOWS ACROSS A MEMBRANE FROM THE SIDE WHERE SOLUTE IS LESS CONCENTRATED (HYPOTONIC) TO THE SIDE WHERE SOLUTE IS MORE CONCENTRATED (HYPERTONIC) IF THE CONCENTRATIONS ARE EQUAL (ISOTONIC), NO NET OSMOSIS OCCURS

Figure 8.10 Osmosis

D. CELL SURVIVAL DEPENDS ON BALANCING WATER UPTAKE AND LOSS CELLS LACKING CELL WALLS (AS IN ANIMALS) ARE ISOTONIC WITH THEIR ENVIRONMENTS OR HAVE ADAPTATIONS FOR OSMOREULATION 8.11 Water Balance of Living Cells

8.13 Facilitated Diffusion E. SPECIFIC PROTEINS FACILITATE THE PASSIVE TRANSPORT OF SELECTED SOLUTES IN FACILITATED DIFFUSION, A TRANSPORT PROTEIN SPEEDS MOVEMENT OF A SOLUTE ACROSS A MEMBRANE DOWN ITS CONCENTRATION GRADIENT 8.13 Facilitated Diffusion

E. ACTIVE TRANSPORT IS THE PUMPING OF SOLUTES AGAINST THEIR GRADIENTS SPECIFIC MEMBRANE PROTEINS USE ENERGY, USUALLY IN THE FORM OF ATP, TO DO THIS WORK 8.15 Passive and Active Transport Compared

G. SOME ION PUMPS GENERATE VOLTAGE ACROSS MEMBRANES IONS CAN HAVE BOTH A CONCENTRATION (CHEMICAL) GRADIENT AND AN ELECTRIC GRADIENT (VOLTAGE) THESE FORCES COMBINE IN THE ELECTROCHEMICAL GRADIENT, WHICH DETERMINES THE NET DIRECTION OF IONIC DIFFUSION ELECTROGENIC PUMPS, SUCH AS SODIUM-POTASSIUM PUMPS AND PROTON PUMPS, ARE TRANSPORT PROTEINS THAT CONTRIBUTE TO ELECTROCHEMICAL GRADIENTS

8.14 The Sodium - Potassium Pump

H.     IN COTRANSPORT, A MEMBRANE PROTEIN COUPLES THE TRANSPORT OF ONE SOLUTE TO ANOTHER ONE SOLUTE’S “DOWNHILL” DIFFUSION DRIVES THE OTHER’S “UPHILL” TRANSPORT 8.17 CONTRANSPORT

I. EXOCYTOSIS AND ENDOCYTOSIS TRANSPORT LARGE MOLECULES EXOCYTOSIS: TRANSPORT VESICLES MIGRATE TO THE PLASMA MEMBRANE, FUSE WITH IT, AND RELEASE THEIR CONTENTS ENDOCYTOSIS: LARGE MOLECULES ENTER CELLS WITHIN VESICLES PINCHED INWARD FROM THE PLASMA MEMBRANE PHAGOCYTOSIS PINOCYTOSIS RECEPTOR-MEDIATED ENDOCYTOSIS

8.18 THREE TYPES OF ENDOCYTOSIS IN ANIMALS