MembranesChapter

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Membranes 2

Chapter 5 Outline

Membrane ModelsFluid-Mosaic

Plasma Membrane Structure and FunctionPhospholipidsProteins

Plasma Membrane PermeabilityDiffusionOsmosisTransport Via Carrier Proteins

Cell Surface Modifications

Membranes 3Structure and Function:The Phospholipid Bilayer

The plasma membrane is common to all cellsSeparates:

Internal living cytoplasmic fromExternal environment of cell

Phospholipid bilayer:External surface lined with hydrophilic polar heads

Cytoplasmic surface lined with hydrophilic polar heads

Nonpolar, hydrophobic, fatty-acid tails sandwiched in between

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Unit Membrane

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Membrane Models

Fluid-Mosaic ModelThree components:

Basic membrane referred to as phospholipid bilayer

Protein molecules­ Float around like icebergs on a sea­Membrane proteins may be peripheral or integralPeripheral proteins are found on the inner membrane surface

Integral proteins are partially or wholly embedded (transmembrane) in the membrane

­ Some have carbohydrate chains attachedCholesterol

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Membrane Models:Unit Membrane vs. Fluid Mosaic Model

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The Fluid Mosaic Model

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Phospholipid & CholesterolMolecules

Membranes 9Transmembrane Proteins

Membranes 10

Lateral Migration ofMembrane Proteins

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Functions of Membrane Proteins

Channel Proteins: Tubular Allow passage of molecules through membrane

Carrier Proteins: Combine with substance to be transported Assist passage of molecules through membrane

Cell Recognition Proteins: Provides unique chemical ID for cells Help body recognize foreign substances

Receptor Proteins: Binds with messenger molecule Causes cell to respond to message

Enzymatic Proteins: Carry out metabolic reactions directly

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Membrane Protein Diversity

Membranes 13Types of Transport:Active vs. Passive

Plasma membrane is differentially (selectively) permeable Allows some material to pass Inhibits passage of other materials

Passive Transport:No ATP requirementMolecules follow concentration gradient

Active TransportRequires carrier proteinRequires energy in form of ATP

Membranes 14Types of Membrane Transport:Overview

Membranes 15Types of Transport:Diffusion

A solution consists of:A solvent (liquid) , andA solute (dissolved solid)

DiffusionNet movement of solute molecules down a concentration gradient

Molecules both ways along gradientMore move from high to low concentration than vice versa

Equilibrium:­When NET change stops­ Solute concentration uniform – no gradient

Membranes 16Types of Membrane Transport:Diffusion

Membranes 17Types of Membrane Transport:Diffusion Across Lung

Membranes 18Types of Transport:Osmosis

Osmosis: Special case of diffusion Focuses on solvent (water) movement rather than solute

Diffusion of water across a differentially (selectively) permeable membrane­ Solute concentration on one side high, but water

concentration low­ Solute concentration on other side low, but water

concentration high Water diffuses both ways across membrane but solute can’t

Net movement of water is toward low water (high solute) concentration

Osmotic pressure is the pressure that develops due to osmosis

Membranes 19Types of Membrane Transport:Osmosis

Membranes 20Types of Transport:Osmosis

Isotonic SolutionSolute and water concentrations equal on both sides of membrane

Hypotonic Solution Concentration of solute lower than on other sideCells placed in a hypotonic solution will swellMay cause cells to break – Lysis

Hypertonic SolutionConcentration of solute higher than on other sideCells placed in a hypertonic solution will shrink – Plasmolysis

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Osmotic Effects on Cells

Membranes 22Types of Transport:Carrier Proteins

Facilitated Transport­ Small molecules­Can’t get through membrane lipids­Combine with carrier proteins­ Follow concentration gradient

Active Transport­ Small molecules­Move against concentration gradient­Combining with carrier proteins­Requires energy

Membranes 23Types of Membrane Transport:Facilitated Transport

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Facilitated Transport:The Sodium-Potassium Pump

Membranes 25Types of Transport:Membrane-Assisted Transport

Macromolecules transported into or out of the cell inside vesiclesExocytosis – Vesicles fuse with plasma membrane and secrete contents

Endocytosis – Cells engulf substances into pouch which becomes a vesicle­ Phagocytosis – Large, solid material into vesicle­ Pinocytosis – Liquid or small, solid particles go into vesicle­Receptor-Mediated – Specific form of pinocytosis using a coated pit

Membranes 26Membrane-Assisted Transport:Exocytosis

Membranes 27Membrane-Assisted Transport:Three Types of Endocytosis

Membranes 28Cell Surface Modifications:Junctions

Cell Surfaces in AnimalsJunctions Between Cells

­Adhesion Junctions Intercellular filaments between cells

­ Tight JunctionsForm impermeable barriers

­Gap JunctionsPlasma membrane channels are joined (allows communication)

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Cell-Surface Modifications:Junctions

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Cell Surface Modifications

Extracellular MatrixExternal meshwork of polysaccharides and proteins

Found in close association with the cell that produced them

Plant Cell WallsPlants have freely permeable cell wall, with cellulose as the main component­ Plasmodesmata penetrate cell wall­ Each contains a strand of cytoplasm­Allow passage of material between cells

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Cell-Surface Modifications:Extracellular Matrix

Membranes 32

Cell-Surface Modifications:Plasmodesmata

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Review

Membrane ModelsFluid-Mosaic

Plasma Membrane Structure and FunctionProtein FunctionsPlasma Membrane Permeability­Diffusion

­Osmosis

­ Transport Via Carrier Proteins

Cell Surface Modifications

MembranesEnding Slide Chapter

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