Chapter 4Chapter 4

Cell MembranesCell Membranes

Chapter 4 2

Cell MembraneCell Membrane

FunctionsFunctions1.1. Selectively choose what’s inside or Selectively choose what’s inside or

outside the cell…creating a outside the cell…creating a concentration gradientconcentration gradient

2.2. Regulate the exchange of substancesRegulate the exchange of substances3.3. Communicate with other cellsCommunicate with other cells4.4. Attach to other cellsAttach to other cells5.5. Regulate biochemical reactionsRegulate biochemical reactions

Chapter 4 3

Phospholipid ReviewPhospholipid Review

Fatty

Aci

d

Fatty

Aci

d

Glycerol

Polar groupPolar groupGlycerol

Fatty

Aci

d

Fatty

Aci

d

Fatty

Aci

d

Change from Change from Triglyceride to Triglyceride to PhospholipidPhospholipid

Chapter 4 4

OligosaccharidesOligosaccharides

CytoplasmCytoplasmCytoplasmCytoplasm

Membrane ProteinsMembrane Proteins

CholesterolCholesterol

OuterOuterSurfaceSurface

Fluid Mosaic ModelFluid Mosaic Modelof Membrane Structureof Membrane Structure

Chapter 4 5

Membrane Proteins Form a MosaicMembrane Proteins Form a Mosaic

Proteins are embedded in the Proteins are embedded in the phospholipid bilayerphospholipid bilayer• Some proteins can float and driftSome proteins can float and drift• Other proteins are anchored by protein Other proteins are anchored by protein

filaments in the cytoplasmfilaments in the cytoplasm• Many proteins have attached Many proteins have attached

carbohydrates (carbohydrates (glycoproteinsglycoproteins))

Chapter 4 6

Chapter 4 7

Membrane Proteins Form a MosaicMembrane Proteins Form a Mosaic

Categories of membrane proteinsCategories of membrane proteins• Receptor ProteinsReceptor Proteins• Recognition ProteinsRecognition Proteins• Enzymatic ProteinsEnzymatic Proteins• Attachment ProteinsAttachment Proteins• Transport ProteinsTransport Proteins

Chapter 4 8

Membrane Proteins Form a MosaicMembrane Proteins Form a Mosaic

Receptor ProteinsReceptor Proteins• Trigger cellular responses upon Trigger cellular responses upon

binding specific molecules, e.g. binding specific molecules, e.g. hormoneshormones

Recognition ProteinsRecognition Proteins• Serve as identification tags on the Serve as identification tags on the

surface of a cellsurface of a cell

Chapter 4 9

Chapter 4 10

Membrane Proteins Form a MosaicMembrane Proteins Form a Mosaic

EnzymesEnzymes• Promote chemical reactions that Promote chemical reactions that

synthesize or break apart biological synthesize or break apart biological moleculesmolecules

Attachment ProteinsAttachment Proteins• Anchor the cell membrane to inner Anchor the cell membrane to inner

cytoskeleton, to proteins outside the cytoskeleton, to proteins outside the cell, and to other cellscell, and to other cells

Chapter 4 11

Membrane Proteins Form a MosaicMembrane Proteins Form a Mosaic

Transport ProteinsTransport Proteins• Include Include channelchannel and and carrier proteinscarrier proteins• Regulate import/export of hydrophilic Regulate import/export of hydrophilic

moleculesmolecules

Chapter 4 12

Transport ProcessesTransport Processes

How stuff gets across cell membrane.How stuff gets across cell membrane.Passive transportPassive transport

• Follows Follows concentration gradientconcentration gradient

• Does not require energyDoes not require energy

Facilitated transportFacilitated transport

• Follows concentration gradientFollows concentration gradient

• Requires gateway proteinRequires gateway protein

Chapter 4 13

DiffusionDiffusion

All molecules constantly vibratingAll molecules constantly vibrating

Those in gas and liquid move Those in gas and liquid move randomlyrandomly

Mixtures tend to become uniformMixtures tend to become uniform

•Move from high concentration to lowMove from high concentration to low

• Concentration gradientConcentration gradient

• Doesn’t require energyDoesn’t require energy

Chapter 4 14

Diffusion of Dye in WaterDiffusion of Dye in Water

Time 0Time 0SteepSteep

ConcentrationConcentrationGradientGradient

Time 1Time 1ReducedReduced

ConcentrationConcentrationGradientGradient

DispersingDispersing

Time 2Time 2NoNo

ConcentrationConcentrationGradientGradient

RandomRandomDispersalDispersal

Chapter 4 15

Simple DiffusionSimple Diffusion

(extracellular fluid)

(cytoplasm)

Some moleculesSome molecules

diffuse freelydiffuse freelyacrossacross

Some moleculesSome moleculesdiffuse freelydiffuse freely

acrossacross

Chapter 4 16

Facilitated Diffusion: CarriersFacilitated Diffusion: Carriers

Carrier proteinhas binding sitefor molecule

Molecule entersbinding site

Carrier protein changesshape, transportingmolecule across membrane

Carrier protein resumesoriginal shape

(Inside Cell)(Inside Cell)(Inside Cell)(Inside Cell)

(Outside(OutsideCell)Cell)

(Outside(OutsideCell)Cell)

DiffusionDiffusionChannelChannelProteinProtein

DiffusionDiffusionChannelChannelProteinProtein

Diff

usio

nD

iffus

ion

Gra

dien

tG

radi

ent

Molecule inMolecule inTransitTransit

Molecule inMolecule inTransitTransit

Chapter 4 17

OsmosisOsmosisDiffusion of water across membraneDiffusion of water across membrane

Special case of diffusionSpecial case of diffusion• Ex: Pure water on one side; sugar solution Ex: Pure water on one side; sugar solution

on otheron other• Sugar molecules can’t cross, but crowd Sugar molecules can’t cross, but crowd

porespores• Water crosses faster toward sugarWater crosses faster toward sugar

Net water movement toward sugarNet water movement toward sugar

Water follows concentration gradientWater follows concentration gradient

Chapter 4 18

Osmotic FactorsOsmotic Factors

Compare cell’s environment with cell.Compare cell’s environment with cell.If environment is:If environment is:• HypotonicHypotonic::

– ““Hypo-” means environment “<” cellHypo-” means environment “<” cell– Cell gains waterCell gains water

• HypertonicHypertonic::– ““Hyper-” means environment “>” cellHyper-” means environment “>” cell– Cell loses waterCell loses water

• IsotonicIsotonic::– ““Iso-” means environment “=” cellIso-” means environment “=” cell– No change in cell volumeNo change in cell volume

Chapter 4 19

Osmosis IOsmosis I

“Bound” water moleculesclustered around sugar:cannot fit through pore

“Free” watermolecule: can fitthrough pore

Chapter 4 20

Normal RBCs

Isotonic SolutionIsotonic Solution

The Effects of OsmosisThe Effects of Osmosis

Equal movement of waterinto and out of cells

Net movement ofwater out of cells Net movement of

water into cells

Shriveled RBCs

Swollen RBCs

Hypertonic SolutionHypertonic Solution Hypotonic SolutionHypotonic Solution

Chapter 4 21

Active TransportActive Transport

Requires energyRequires energy

Ex: Sodium/Potassium pumpEx: Sodium/Potassium pump

Energy from ATPEnergy from ATP

One phosphate per “fare”One phosphate per “fare”

Pumped by flip/flop changes in a membrane Pumped by flip/flop changes in a membrane proteinprotein

Chapter 4 22

Active Transport StepsActive Transport Steps

Inside CellInside CellInside CellInside Cell

Outside CellOutside CellOutside CellOutside Cell

EnergyEnergyProviderProviderEnergyEnergy

ProviderProviderSpent EnergySpent Energy

ProviderProviderSpent EnergySpent Energy

ProviderProvider

TransportedTransportedMoleculeMolecule

TransportedTransportedMoleculeMolecule

1 2 3 4

Chapter 4 23

Endo- & ExocytosisEndo- & Exocytosis

This is like cell eating & pooping.This is like cell eating & pooping.

Active processes for large groups of Active processes for large groups of molecules.molecules.

• Endocytosis Endocytosis - Engulfment- Engulfment

• Exocytosis Exocytosis - Expulsion- Expulsion

Chapter 4 24

EndocytosisEndocytosis

(extracellular fluid)

Pinocytosis

vesicle containingextracellularfluid)(cytoplasm)

12 33

cell

(extracellular fluid)

(cytoplasm)

1

233 44

coatedpit forming

Receptor-mediatedEndocytosis

Phagocytosis

coatedvesicle

nutrientsreceptors

food particle

pseudopodparticle enclosedin food vacuole

Chapter 4 25

Receptor-Receptor-mediated Endocytosis Imediated Endocytosis I

Begins with a shallowBegins with a shallowpit in plasma membranepit in plasma membrane

Pit is coated withPit is coated withproteinprotein

(cytoplasm)

(extracellular fluid)

coatedpit

proteincoating

extracellular particlesbound to receptors

plasma membrane

Pit deepensPit deepens

aa bb

0.1 Micrometer

Chapter 4 26

Receptor-Receptor-mediated Endocytosis IImediated Endocytosis II

Pit deepens further andPit deepens further andbegins to pinch offbegins to pinch off

Eventually becomes aEventually becomes acoated vesiclecoated vesicle

ddcc

0.1 Micrometer

coatedvesicle

Chapter 4 27

ExocytosisExocytosis

(cytoplasm)

1

VesicleVesicle

(extracellular fluid)

plasma membrane

2

SecretedSecretedMaterialMaterial

33

Chapter 4 28

Attributes of CellsAttributes of Cells

Cell sizeCell size

• 1–100 µm1–100 µm

•Why is there a limit to cell size?Why is there a limit to cell size?

– Surface-to-volume (S/V) ratioSurface-to-volume (S/V) ratio

– Distance from surface to centerDistance from surface to center

Chapter 4 29

Surface to Volume RatioSurface to Volume Ratio

331133S/V RatioS/V Ratio

113.1113.1113.1113.14.24.2VolumeVolume4/34/3rr33

339.4339.4113.1113.112.612.6SurfaceSurface44rr22

1.01.03.03.01.01.0RadiusRadius

GrowGrow BreakBreakapartapart

Chapter 4 30

CellCellCommunication StructuresCommunication Structures

DesmosomeDesmosomedesmosomedesmosome

Protein strandsholding cellstogether

Protein strandsholding cellstogether

Protein filamentsin cytoplasmProtein filamentsin cytoplasm

Small intestineSmall intestine Plasma membrane(edge view)Plasma membrane(edge view)

Cells liningsmall intestineCells liningsmall intestine

Tight JunctionTight Junction

Tight junctionsformed by strandsof protein

Tight junctionsformed by strandsof protein

Plasma membrane(edge view)Plasma membrane(edge view)

Cells liningbladderCells liningbladder

Tight junctionsseal membranesof adjacent cellstogether

Tight junctionsseal membranesof adjacent cellstogether

Chapter 4 31

CellCellAttachment StructuresAttachment Structures

Gap JunctionsGap Junctions

desmosomedesmosomeGap Junctions:pairs of channelsconnect insides ofadjacent cells

Gap Junctions:pairs of channelsconnect insides ofadjacent cells

LiverLiver Plasma membrane(edge view)Plasma membrane(edge view)

Liver cellsLiver cells

PlasmodesmataPlasmodesmata

Plasmodesmataconnect insidesof adjacent cells

Plasma membrane(edge view)

Root cells

Cell wall(edge view)

Middle lamella(edge view)

RootRoot

Chapter 4Chapter 4

The EndThe End