11.. Biology 102 Lecture 6: Cell...
Transcript of 11.. Biology 102 Lecture 6: Cell...
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Biology 102Biology 102
Lecture 6: Cell MembranesLecture 6: Cell Membranes
1.1. Physical isolationPhysical isolation
•• Separates inside of cell fromSeparates inside of cell from
extracellular fluidextracellular fluid
Cell Membrane FunctionsCell Membrane Functions
2. Regulates intracellular2. Regulates intracellular--extracellular exchangeextracellular exchange
•• Controls ions, nutrients, waste, and secretory Controls ions, nutrients, waste, and secretory product exchangeproduct exchange
Cell Membrane FunctionsCell Membrane Functions
3. Sensitivity to the extracellular environment3. Sensitivity to the extracellular environment
•• Receptors allow cell recognition/response to Receptors allow cell recognition/response to molecules in the environmentmolecules in the environment
Cell Membrane FunctionsCell Membrane Functions
4. Attachment (within cell)4. Attachment (within cell)
•• CytoskeletonCytoskeleton
•• Microfilaments (Microfilaments (actinactin))
•• Microtubules (Microtubules (tubulintubulin))
•• Intermediate filamentsIntermediate filaments
•• Animal cells onlyAnimal cells only
•• >60 proteins>60 proteins
Cell Membrane FunctionsCell Membrane Functions
5. Attachment (outside cell)5. Attachment (outside cell)
•• Cells don’t live floating in fluid Cells don’t live floating in fluid –– must attach to must attach to surface and to each othersurface and to each other
•• Molecules secreted by cellsMolecules secreted by cells
•• Provide structural and biochemical support, Provide structural and biochemical support,
stiffness, and elasticitystiffness, and elasticity
•• Examples: Examples: fibronectinfibronectin, , proteoglycanproteoglycan, , cadherinscadherins
Cell Membrane FunctionsCell Membrane Functions
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•• Membrane separates inside/outsideMembrane separates inside/outside
•• Inside cell = aqueousInside cell = aqueous
•• Outside cell = aqueousOutside cell = aqueous
•• Solutes are mostly polarSolutes are mostly polar
•• How do we keep molecules How do we keep molecules
where they need to be?where they need to be?
Cell Membrane StructureCell Membrane Structure
•• Solution: cell membrane must be fundamentally Solution: cell membrane must be fundamentally nonnon--polar, but able to exist in aqueous polar, but able to exist in aqueous
environmentenvironment
•• Molecule that makes this possible = Molecule that makes this possible = phospholipidphospholipid
Cell Membrane StructureCell Membrane Structure
•• Polar heads face both outside and insidePolar heads face both outside and inside
•• Hydrophilic heads face aqueous environmentsHydrophilic heads face aqueous environments
•• NonNon--polar tails protected in betweenpolar tails protected in between
Cell Membrane StructureCell Membrane Structure
•• Membrane must be very fluidMembrane must be very fluid
•• Fluidity adjusted by changing saturation of fatty Fluidity adjusted by changing saturation of fatty acid tailsacid tails
•• More saturated = less fluidMore saturated = less fluid
Cell Membrane StructureCell Membrane Structure
•• Problem: system is too effectiveProblem: system is too effective
•• Like a room with no doors and no windowsLike a room with no doors and no windows
•• How does anything get in or out?How does anything get in or out?
Cell Membrane StructureCell Membrane Structure
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•• Solution: membranes not 100% phospholipidSolution: membranes not 100% phospholipid
•• PhospholipidPhospholipid
•• CholesterolCholesterol
•• (Carbohydrate)(Carbohydrate)
•• ProteinsProteins
Cell Membrane StructureCell Membrane Structure
•• 50% dry weight of membrane50% dry weight of membrane
•• Adds stiffnessAdds stiffness
•• Straightens Straightens phospholipidphospholipid tailstails
•• Prevents small polar molecules from passing Prevents small polar molecules from passing through membranethrough membrane
CholesterolCholesterol
•• Carbohydrate functionsCarbohydrate functions
•• Membrane lubrication and protectionMembrane lubrication and protection
•• Hygroscopic (attract water)Hygroscopic (attract water)
Membrane CarbohydratesMembrane Carbohydrates
•• Carbohydrate functionsCarbohydrate functions
•• Carbohydrate chains on Carbohydrate chains on glycoproteinsglycoproteins provide provide “fingerprint”“fingerprint”
•• Cell can recognize self Cell can recognize self vsvs nonnon--selfself
•• Genetically determinedGenetically determined
Membrane Protein FunctionsMembrane Protein Functions
•• Proteins = major functional componentProteins = major functional component
Membrane ProteinsMembrane Proteins
•• Two configurationsTwo configurations
•• IntegralIntegral
•• Span entire width Span entire width
of membraneof membrane
•• Part of membranePart of membrane
structurestructure
•• PeripheralPeripheral
•• Bind to inner or Bind to inner or
outer surfacesouter surfaces
•• Distinct from membraneDistinct from membrane
Membrane ProteinsMembrane Proteins
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•• TransportersTransporters
•• ChannelsChannels
•• Integral proteinsIntegral proteins
•• Central poreCentral pore
•• Permits entry of small molecules, waterPermits entry of small molecules, water
Membrane Protein FunctionsMembrane Protein Functions
•• TransportersTransporters
•• CarriersCarriers
•• Bind solutes, transport across membraneBind solutes, transport across membrane
•• Protein changes shapeProtein changes shape
•• May or may not require ATPMay or may not require ATP
Membrane Protein FunctionsMembrane Protein Functions
•• EnzymesEnzymes
•• Can be integral or peripheralCan be integral or peripheral
•• Catalyze reactions inside and outside cellCatalyze reactions inside and outside cell
•• Example: membraneExample: membrane--associated enzymes in associated enzymes in small intestine break down small intestine break down dipeptidesdipeptides into into
amino acidsamino acids
Membrane Protein FunctionsMembrane Protein Functions
•• ReceptorsReceptors
•• Bind to external chemicals to regulate Bind to external chemicals to regulate processes within cellprocesses within cell
•• ShapeShape--specificspecific
•• Not necessary forNot necessary for
steroid hormonessteroid hormones
•• Example: insulinExample: insulin
(peptide hormone)(peptide hormone)
Membrane Protein FunctionsMembrane Protein Functions
•• AnchorsAnchors
•• IntercellularIntercellular
•• Attach cells to each otherAttach cells to each other
Membrane Protein FunctionsMembrane Protein Functions
•• AnchorsAnchors
•• Cytoskeleton and/or extracellular matrixCytoskeleton and/or extracellular matrix
•• Hold membrane to cytoskeletonHold membrane to cytoskeleton
•• Hold cell to matrixHold cell to matrix
Membrane Protein FunctionsMembrane Protein Functions
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•• Materials must constantly cross the cell Materials must constantly cross the cell membranemembrane
•• ExampleExample
•• Food energy molecules inFood energy molecules in
•• Waste molecules outWaste molecules out
•• Some things can cross membranes freely; others Some things can cross membranes freely; others cannotcannot
•• Cells have developed a number of ways around Cells have developed a number of ways around
thisthis
Cell MembranesCell Membranes
•• Cell membrane is semiCell membrane is semi--permeablepermeable
•• Some things can cross, others can notSome things can cross, others can not
•• Primary influences:Primary influences:
•• SizeSize
•• PolarityPolarity
•• ChargeCharge
Cell MembranesCell Membranes
•• Some solutes can cross the cell membrane along Some solutes can cross the cell membrane along a diffusion gradienta diffusion gradient
•• Will equalize concentrations on both sidesWill equalize concentrations on both sides
•• Costs no cellular energy = Costs no cellular energy = passive transportpassive transport
DiffusionDiffusion
•• Several types of diffusionSeveral types of diffusion
•• Simple diffusionSimple diffusion
•• Substance flows Substance flows
freely across membranefreely across membrane
•• Mostly small, nonMostly small, non--polarpolar
moleculesmolecules
•• Examples: OExamples: O22, CO, CO22
DiffusionDiffusion
•• Facilitated diffusionFacilitated diffusion
•• Membrane protein facilitates movement Membrane protein facilitates movement across membraneacross membrane
•• May be a channelMay be a channel
•• Based on size,Based on size,
chargecharge
•• Example: ionsExample: ions
DiffusionDiffusion
•• Facilitated diffusionFacilitated diffusion
•• May be carrierMay be carrier
•• Highly specificHighly specific
•• Based on ShapeBased on Shape
•• Example: glucoseExample: glucose
•• DOES THIS REQUIRE ENERGY??DOES THIS REQUIRE ENERGY??
DiffusionDiffusion
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•• Both channels and carriers can be regulatedBoth channels and carriers can be regulated
•• Turned on and offTurned on and off
•• Number on membrane changedNumber on membrane changed
Facilitated DiffusionFacilitated Diffusion
•• Diffusion of water molecules onlyDiffusion of water molecules only
•• Occurs when…Occurs when…
1. Concentration gradient 1. Concentration gradient
existsexists
2. Membrane is not 2. Membrane is not
permeable to solute(s)permeable to solute(s)
3. Membrane is permeable 3. Membrane is permeable
to waterto water
OsmosisOsmosis
•• Takes place through theTakes place through the
membrane proteinmembrane protein
aquaporinaquaporin
•• A type of facilitatedA type of facilitated
diffusiondiffusion
OsmosisOsmosis
•• Review: several types of passive transportReview: several types of passive transport
•• Simple diffusionSimple diffusion
•• ChannelChannel--mediated diffusionmediated diffusion
•• CarrierCarrier--mediated diffusionmediated diffusion
•• OsmosisOsmosis
Transport Across Cell MembranesTransport Across Cell Membranes
•• What happens when these methods don’t work?What happens when these methods don’t work?
•• What if we need to move solutes AGAINST a What if we need to move solutes AGAINST a concentration gradient?concentration gradient?
•• Requires energyRequires energy
Transport Across Cell MembranesTransport Across Cell Membranes
••
•• Moves substances Moves substances
through carrier proteinsthrough carrier proteins
specific to solutespecific to solute
•• Energy provided by ATPEnergy provided by ATP
•• Works AGAINST Works AGAINST
concentration gradientconcentration gradient
Transport of Small SolutesTransport of Small Solutes
Active transportActive transport
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•• Active transportActive transport
•• Used to…Used to…
•• Concentrate substances in the cell or in Concentrate substances in the cell or in
organellesorganelles
•• Get rid of unneeded thingsGet rid of unneeded things
•• Create gradients that drive other processesCreate gradients that drive other processes
•• Example: sodiumExample: sodium--potassium pumppotassium pump
Transport of Small SolutesTransport of Small Solutes
•• FiltrationFiltration
•• Hydrostatic pressure forces water and solute Hydrostatic pressure forces water and solute across a porous membraneacross a porous membrane
•• Process based on pore size of membraneProcess based on pore size of membrane
•• Example: cardiovascular systemExample: cardiovascular system
Transport of Small SolutesTransport of Small Solutes
•• Some substances are too large to pass through a Some substances are too large to pass through a membrane protein or poremembrane protein or pore
•• Examples: whole proteins, bulk transportExamples: whole proteins, bulk transport
•• Must be moved by vesicular transportMust be moved by vesicular transport
•• Membrane reorganizes to move substances Membrane reorganizes to move substances
through the membrane in vesiclesthrough the membrane in vesicles
•• REQUIRES ATPREQUIRES ATP
Transport of Large SolutesTransport of Large Solutes
•• Substances moving into the cell = Substances moving into the cell = endocytosisendocytosis
Transport of Large SolutesTransport of Large Solutes
•• Several types of Several types of endocytosisendocytosis
•• ReceptorReceptor--mediated mediated endocytosisendocytosis
•• Very specific substancesVery specific substances
EndocytosisEndocytosis
•• Several types of Several types of endocytosisendocytosis
•• PhagocytosisPhagocytosis –– “cell eating”“cell eating”
•• Very large objects such as bacteriaVery large objects such as bacteria
EndocytosisEndocytosis
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•• Process is very effective… but sometimes it goes Process is very effective… but sometimes it goes wrongwrong
PhagocytosisPhagocytosis
•• Several types of Several types of endocytosisendocytosis
•• PinocytosisPinocytosis –– “cell drinking”“cell drinking”
•• Small, random samples of extracellular fluidSmall, random samples of extracellular fluid
EndocytosisEndocytosis
•• Essentially the opposite of Essentially the opposite of endocytosisendocytosis
•• Substances moved through the membrane to the Substances moved through the membrane to the outside of the celloutside of the cell
•• Example: insulin, neurotransmittersExample: insulin, neurotransmitters
ExocytosisExocytosis EnergyEnergy--Dependent Membrane TransportDependent Membrane Transport
•• Recall: all of these types of transport require Recall: all of these types of transport require energyenergy
•• ATPATP
•• Pressure gradientPressure gradient
•• Solute gradientSolute gradient