11.. biology 102 lecture 6: cell...

10/5/2015

1

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


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


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


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


10/5/2015

2

•• 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


•• 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


•• 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


•• 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?


10/5/2015

3

•• Solution: membranes not 100% phospholipidSolution: membranes not 100% phospholipid

•• PhospholipidPhospholipid

•• CholesterolCholesterol

•• (Carbohydrate)(Carbohydrate)

•• ProteinsProteins


•• 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

10/5/2015

4

•• TransportersTransporters

•• ChannelsChannels

•• Integral proteinsIntegral proteins

•• Central poreCentral pore

•• Permits entry of small molecules, waterPermits entry of small molecules, water


•• 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


•• 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


•• 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)


•• AnchorsAnchors

•• IntercellularIntercellular

•• Attach cells to each otherAttach cells to each other


•• AnchorsAnchors

•• Cytoskeleton and/or extracellular matrixCytoskeleton and/or extracellular matrix

•• Hold membrane to cytoskeletonHold membrane to cytoskeleton

•• Hold cell to matrixHold cell to matrix


10/5/2015

5

•• 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

10/5/2015

6

•• 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

10/5/2015

7

•• 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


•• 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


•• 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


•• PhagocytosisPhagocytosis –– “cell eating”“cell eating”

•• Very large objects such as bacteriaVery large objects such as bacteria


10/5/2015

8

•• Process is very effective… but sometimes it goes Process is very effective… but sometimes it goes wrongwrong

PhagocytosisPhagocytosis


•• PinocytosisPinocytosis –– “cell drinking”“cell drinking”

•• Small, random samples of extracellular fluidSmall, random samples of extracellular fluid


•• 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

11.. biology 102 lecture 6: cell...

Documents