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 25Receptor-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 26Receptor-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 30CellCellCommunication 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 31CellCellAttachment 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