1MembranesMembranes

Ch. 5 Outline - MembranesCh. 5 Outline - Membranes

Membrane ModelsMembrane ModelsFluid-MosaicFluid-Mosaic

Plasma Membrane Structure and FunctionPlasma Membrane Structure and FunctionPhospholipidsPhospholipidsProteinsProteins

Plasma Membrane PermeabilityPlasma Membrane PermeabilityDiffusionDiffusionOsmosisOsmosisTransport Via Carrier ProteinsTransport Via Carrier Proteins

Cell Surface ModificationsCell Surface Modifications

2MembranesMembranesStructure and Function:Structure and Function:The Phospholipid BilayerThe Phospholipid Bilayer

The plasma membrane is common to all cellsThe plasma membrane is common to all cellsSeparates:Separates:

Internal living cytoplasmic fromInternal living cytoplasmic fromExternal environment of cellExternal environment of cell

Phospholipid bilayerPhospholipid bilayer::External surface lined with hydrophilic polar External surface lined with hydrophilic polar phosphatephosphate heads heads

Cytoplasmic surface lined with hydrophilic Cytoplasmic surface lined with hydrophilic polar polar phosphate phosphate headsheads

Nonpolar, hydrophobic, fatty-acid tails Nonpolar, hydrophobic, fatty-acid tails sandwiched in betweensandwiched in between

3The Fluid Mosaic Model

4MembranesMembranes

Fluid-Mosaic ModelFluid-Mosaic Model

Four components:Four components:Basic membrane referred to as Basic membrane referred to as phospholipid phospholipid bilayerbilayer

Protein moleculesProtein molecules­ Float around like icebergs on a seaFloat around like icebergs on a sea­ Membrane proteins may be Membrane proteins may be peripheralperipheral or or integralintegral Peripheral proteinsPeripheral proteins are found are found onon the membrane the membrane

surfacesurface Integral proteinsIntegral proteins are partially or wholly embedded are partially or wholly embedded

((transmembranetransmembrane) in the membrane) in the membrane­ Some have carbohydrate chains attachedSome have carbohydrate chains attached

CholesterolCholesterol – helps stiffen membrane – helps stiffen membrane

5Transmembrane Proteins

6Phospholipid & CholesterolMolecules

7MembranesMembranes

Fluid-Mosaic ModelFluid-Mosaic Model

Carbohydrate chains:Carbohydrate chains:­ Give the cell a “sugar coat”, Give the cell a “sugar coat”, glycocalyx.glycocalyx.­ Protects cellProtects cell­ Facilitates adhesion between cellsFacilitates adhesion between cells­ Facilitates reception of signal moleculesFacilitates reception of signal molecules­ Functions in cell-to-cell recognitionFunctions in cell-to-cell recognition

8The Fluid Mosaic Model

9MembranesMembranes

Fluidity of Plasma MembraneFluidity of Plasma Membrane

Fluidity of membrane is dependent on its lipid Fluidity of membrane is dependent on its lipid components and is critical to proper components and is critical to proper functioning.functioning.­ Body temperatureBody temperature – consistency of olive oil – consistency of olive oil..­ More unsaturated fatsMore unsaturated fats = more fluid membrane = more fluid membrane­ Fluidity means that cells are Fluidity means that cells are pliable.pliable.­ Proteins free to “Proteins free to “driftdrift” laterally in phospholipid ” laterally in phospholipid bilayer.bilayer.

10Lateral Migration of

Membrane Proteins

11MembranesMembranes

Functions of Membrane ProteinsFunctions of Membrane Proteins

Channel ProteinsChannel Proteins:: TubularTubular AllowAllow passage of molecules through membrane passage of molecules through membrane

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

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

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

Enzymatic Enzymatic ProteinsProteins:: CarryCarry out metabolic reactions directly out metabolic reactions directly

12Membrane Protein Diversity

13MembranesMembranes

Permeability of MembranePermeability of Membrane

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

Passive TransportPassive Transport::NoNo ATP requirement ATP requirementMolecules follow Molecules follow concentration gradientconcentration gradient

Active TransportActive TransportRequires carrier protein or help of vesiclesRequires carrier protein or help of vesiclesRequires energy in form of ATPRequires energy in form of ATP

14Types of Membrane Transport:Overview

15MembranesMembranesTypes of Transport:Types of Transport:DiffusionDiffusion

A A solutionsolution consists of: consists of:A A solventsolvent (liquid) , and (liquid) , andA A solutesolute (dissolved solid) (dissolved solid)

DiffusionDiffusionNet movement of solute molecules down a Net movement of solute molecules down a concentration gradientconcentration gradient

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

EquilibriumEquilibrium::­ When NET change stopsWhen NET change stops­ Solute concentration uniform – no gradientSolute concentration uniform – no gradient

16Types of Membrane Transport:Diffusion

17Types of Membrane Transport:Diffusion Across Lung

18MembranesMembranesTypes of Transport:Types of Transport:Osmosis – Passive transportOsmosis – Passive transport

OsmosisOsmosis:: Special case of diffusionSpecial case of diffusion Focuses on solvent (water) movement rather than Focuses on solvent (water) movement rather than

solutesolute Diffusion of water across a differentially (selectively) Diffusion of water across a differentially (selectively)

permeable membranepermeable membrane­ Solute concentration on one side high, but water Solute concentration on one side high, but water

concentration lowconcentration low­ Solute concentration on other side low, but water Solute concentration on other side low, but water

concentration highconcentration high Water diffuses both ways across membrane but Water diffuses both ways across membrane but

solute can’tsolute can’t Net movement of water is toward low water (high Net movement of water is toward low water (high

solute) concentrationsolute) concentrationOsmotic pressureOsmotic pressure is the pressure that develops due is the pressure that develops due

to osmosisto osmosis

19Types of Membrane Transport:Osmosis

20MembranesMembranesTypes of Transport:Types of Transport:OsmosisOsmosis

Isotonic Solution – normal cell environmentIsotonic Solution – normal cell environment Solute and water concentrations equal on both Solute and water concentrations equal on both sides of membranesides of membrane

Hypotonic SolutionHypotonic Solution Concentration of solute Concentration of solute lowerlower than on other side than on other side Cells placed in a hypotonic solution will swellCells placed in a hypotonic solution will swell May cause cells to break – May cause cells to break – LysisLysis Creates Creates turgor pressureturgor pressure in plant cells in plant cells

Hypertonic SolutionHypertonic Solution Concentration of solute Concentration of solute higherhigher than on other side than on other side Cells placed in a hypertonic solution will shrink –Cells placed in a hypertonic solution will shrink – Plasmolysis (plants) & crenation (animals)Plasmolysis (plants) & crenation (animals)

21Osmotic Effects on Cells

22MembranesMembranesTypes of Transport:Types of Transport:Carrier ProteinsCarrier Proteins

Facilitated Transport – no cell energy neededFacilitated Transport – no cell energy needed­ Explains passage of small molecules like Explains passage of small molecules like glucose and amino acids (even though polar)glucose and amino acids (even though polar)

­ Can’t get through membrane lipidsCan’t get through membrane lipids

­ Combine with carrier proteinsCombine with carrier proteins

­ FollowFollow concentration gradient concentration gradientActive Transport – requires extra cell energyActive Transport – requires extra cell energy­ Moves molecules Moves molecules againstagainst concentration gradient concentration gradient

­ Molecules combine with carrier proteinMolecules combine with carrier protein

­ Example: sodium-potassium pumpExample: sodium-potassium pump

23Types of Membrane Transport:Facilitated Transport

24Active Transport:The Sodium-Potassium

Pump

25MembranesMembranesTypes of Transport:Types of Transport:Membrane-Assisted TransportMembrane-Assisted Transport

Macromolecules transported into or out of the Macromolecules transported into or out of the cell inside vesicles = active transportcell inside vesicles = active transportExocytosisExocytosis – Vesicles fuse with plasma – Vesicles fuse with plasma membrane and membrane and secretesecrete contents outside contents outside

EndocytosisEndocytosis – Cells engulf substances into – Cells engulf substances into pouch which becomes a vesicle inside cellpouch which becomes a vesicle inside cell­ PhagocytosisPhagocytosis – Large, solid material taken into – Large, solid material taken into vesiclevesicle

­ PinocytosisPinocytosis – Liquid or small, solid particles go – Liquid or small, solid particles go into vesicleinto vesicle

­ Receptor-MediatedReceptor-Mediated –form of pinocytosis using a –form of pinocytosis using a receptor protein in a receptor protein in a coated pitcoated pit

26Membrane-Assisted Transport:Exocytosis

27Membrane-Assisted Transport:Three Types of Endocytosis

28MembranesMembranesCell Surface Modifications:Cell Surface Modifications:JunctionsJunctions

Cell Surfaces in AnimalsCell Surfaces in Animals

Junctions­Between­CellsJunctions­Between­Cells

Anchoring­junctions­attach­adjacent­cellsAnchoring­junctions­attach­adjacent­cells

­ Adhesion JunctionsAdhesion Junctions

Many intercellular filaments between cells (heart, Many intercellular filaments between cells (heart, bladder, stomach = tissues that get stretched)bladder, stomach = tissues that get stretched)

­ DesmosomesDesmosomes

Single point of attachment between cells (skin)Single point of attachment between cells (skin)

29MembranesMembranesCell Surface Modifications:Cell Surface Modifications:JunctionsJunctions

Cell Surfaces in AnimalsCell Surfaces in Animals

­ Tight JunctionsTight Junctions

Form impermeable barriers (like a zipper)Form impermeable barriers (like a zipper)

Serve as barriers (blood-brain barrier, intestines, Serve as barriers (blood-brain barrier, intestines, kidney tubules)kidney tubules)

­ Gap JunctionsGap Junctions

Two plasma membrane channels are joined. Two plasma membrane channels are joined. Allows communication when small molecules and Allows communication when small molecules and ions pass between two cells (muscle cells)ions pass between two cells (muscle cells)

30Cell-Surface Modifications:

Junctions

31MembranesMembranes

Cell Surface ModificationsCell Surface Modifications

Extracellular MatrixExtracellular MatrixExternal­meshwork­of­polysaccharides­and­External­meshwork­of­polysaccharides­and­proteins­(collagen,­elastin,­cartilage,­bone)proteins­(collagen,­elastin,­cartilage,­bone)

Found­in­close­association­with­the­cell­that­Found­in­close­association­with­the­cell­that­produced­themproduced­them

PlantPlant Cell WallsCell WallsPlants­have­freely­permeable­cell­wall,­with­Plants­have­freely­permeable­cell­wall,­with­cellulosecellulose­as­the­main­component­as­the­main­component­ PlasmodesmataPlasmodesmata­channels­penetrate­cell­wall­channels­penetrate­cell­wall­ Each­contains­strands­of­cytoplasm­that­allow­Each­contains­strands­of­cytoplasm­that­allow­passage­of­small­molecules­between­cellspassage­of­small­molecules­between­cells

32Cell-Surface Modifications:

Extracellular Matrix

33Cell-Surface Modifications:

Plasmodesmata