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RAVEN & JOHNSON CHAPTER 5 CAMPBELL CHAPTER 8
Membrane Structure & Function
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Membrane StructureLipids (phospholipids) & proteins
They are amphipathic – both hydrophobic & hydrophilic
Fluid-mosaic model(Singer & Nicolson, 1972)
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FluidityHeld by weak hydrophobic interactions
Movement is lateral
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FluidityMembrane must be fluid to work properly
Cholesterol is wedged into phospholipid tails – restrains movement in warm temp (less fluid), but also lowers solidifying temp (more fluid)
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Fluidity
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MosaicEmbedded proteins (over 50 types) determine specific function of the membrane
Integral proteinsTransmembraneHydrophobic (-helical
amino acids) & hydrophilic regions
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MosaicPeripheral proteins
On the surface of membrane
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Protein Functions
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Cell RecognitionCrucialDetermined by surface molecules (carbohydrates) on plasma membrane
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Cell RecognitionOligosaccharides (polysaccharide fewer than 15 units) vary for different types of cells
Used as markers to distinguish
Example – A, B, AB, O
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Molecular MovementHydrophobic molecules move across easilyHydrocarbons, CO2, O2
Hydrophilic have trouble moving through hydrophobic core (ions, polar molecules, water, glucose, sugars)Need transport proteins (facilitated diffusion)
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Molecular MovementProteins are specific to molecule it transports
Function: move molecules across the membrane that would not be able to do so otherwise
Two types:
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Molecular Movement
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Molecular Movement
1) Channel Proteins Water or small ions If water, called aquaporins If needs electrical or chemical
stimulus to open & close, called gated channel
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Molecular Movement2) Other carrier proteins
Some transport proteins change shape to transport molecules
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Molecular MovementReview:Review: Passive transport (diffusion)Active transportConcentration gradientEquilibriumOsmosisHypertonic, Hypotonic, Isotonic ( & what happens to the cell)
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Molecular Movement
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Molecular Movement
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Molecular MovementAnimal cells need isotonic environment
If not, cells must adapt for osmoregulation (water balance)Ex – Paramecium’s contractile vacuole
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Active TransportAgainst concentration gradient – requires energy (ATP)
Ability to have cell with concentrations very different from surroundingsEx) Animal cell has much K+ and little Na+ compared to surroundings
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Active TransportAccomplished by membrane proteinsATP transfers P to transport protein, protein changes shape, molecule moves in or out
Ex) Sodium-Potassium pumpOne ATP pumps three Na+ out and two K+ in
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Active Transport
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CotransportCoupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient
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Exocytosis & Endocytosis
For macromoleculesExocytosis – cell secretes macromolecules by fusion of vesicles with plasma membrane
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Exocytosis & Endocytosis
Endocytosis – cells takes in macromolecules by forming new vesicles
Three types: Phagocytosis (particles) Pinocytosis (any liquid) Receptor-mediated endocytosis
(specific)
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Exocytosis & Endocytosis
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Exocytosis & Endocytosis
Receptor-mediated endocytosisProteins embedded in membrane –
specific receptor sites (area where they are – coated pits)
Extracellular substances (ligands) bind to sites – triggers vesicle formation
Process enables cell to acquire bulk quantities of specific substances (cholesterol)
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ReviewReview Website
(http://www.wiley.com/college/pratt/0471393878/student/animations/membrane_transport/index.html)