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Fluidity Held by weak hydrophobic interactions Movement is lateral
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RAVEN & JOHNSON CHAPTER 5 CAMPBELL CHAPTER 8
Membrane Structure & Function
Membrane StructureLipids (phospholipids) & proteins
They are amphipathic – both hydrophobic & hydrophilic
Fluid-mosaic model(Singer & Nicolson, 1972)
FluidityHeld by weak hydrophobic interactions
Movement is lateral
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)
Fluidity
MosaicEmbedded proteins (over 50 types) determine specific function of the membrane
Integral proteinsTransmembraneHydrophobic (-helical
amino acids) & hydrophilic regions
MosaicPeripheral proteins
On the surface of membrane
Protein Functions
Cell RecognitionCrucialDetermined by surface molecules (carbohydrates) on plasma membrane
Cell RecognitionOligosaccharides (polysaccharide fewer than 15 units) vary for different types of cells
Used as markers to distinguish
Example – A, B, AB, O
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)
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:
Molecular Movement
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
Molecular Movement2) Other carrier proteins
Some transport proteins change shape to transport molecules
Molecular MovementReview:Review: Passive transport (diffusion)Active transportConcentration gradientEquilibriumOsmosisHypertonic, Hypotonic, Isotonic ( & what happens to the cell)
Molecular Movement
Molecular Movement
Molecular MovementAnimal cells need isotonic environment
If not, cells must adapt for osmoregulation (water balance)Ex – Paramecium’s contractile vacuole
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
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
Active Transport
CotransportCoupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient
Exocytosis & Endocytosis
For macromoleculesExocytosis – cell secretes macromolecules by fusion of vesicles with plasma membrane
Exocytosis & Endocytosis
Endocytosis – cells takes in macromolecules by forming new vesicles
Three types: Phagocytosis (particles) Pinocytosis (any liquid) Receptor-mediated endocytosis
(specific)
Exocytosis & Endocytosis
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)
ReviewReview Website
(http://www.wiley.com/college/pratt/0471393878/student/animations/membrane_transport/index.html)