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Topic1–CellBiology1.4MembraneTransport
Essen:alQues:on:Membranescontrolthecomposi1onofcellsbyac1veandpassivetransport
Phospholipidbilayerisselec:velypermeablecontrolled entry/exit of molecules
WhatcontrolshowsubstancesMovethruthemembrane?• Sizeofthemolecule(selec:vepermeability)• Chargeofthemolecule
Whatmoveseasilythruthemembrane?(diffusion)
• Water• Carbondioxide
• Oxygen
WhatdoesNOTmoveeasilythruthemembrane? (facilitateddiffusion&/orac5vetransport)• Largemoleculessuchasglucoseandproteins• moleculesoratomswithcharges,suchasions(sodium,potassium,
calcium)
EmbeddedProteins ChannelProteins
• Formsmallopeningsformoleculestodiffusethrough
CarrierProteins• Bindingsiteonproteinsurfaceselectscertain
moleculesandallowsthemtopassthrough
ReceptorProteins• Moleculartriggersthatsetoffcellresponses(release
ofhormones)
CellRecogni:onProteins• IDtagstoiden:fycertaincellstothebody’simmune
system
Enzyma:cProteins• Carryoutmetabolicreac:ons
Solu5onsconsistofsolutesdissolvedinasolventConcentration is a measure of the amount of solute in the solution
Modified/adapted from i-biology.net
PassiveTransport• Does NOT require chemical energy from the cell • Depends on permeability of the cell
• Consider characteristics of the membrane (lipids, proteins, etc)
Three main types: 1. Diffusion 2. Facilitated transport 3. Osmosis
1.SimpleDiffusion
• MovementofmoleculesfromanareaofHIGHconcentra:on(moresolute)toLOWconcentra:on(lesssolute)
• Thesemoleculesfollowtheirconcentra:ongradient• CellsareabletodisposeofO2,CO2,andH2Owithout
energy
n http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
Namethreefactors,otherthantemperature,thataffecttherateofdiffusionofasoluteacrossamembrane.
Concentra:onGradient SurfaceArea
LengthofDiffusionPath
Several factors determine rate of diffusion: • Number of molecules in cell (mol) • Permeability of a particular molecule • Surface area (SA) • Width of cell membrane • Concentration of diffusing molecule (mole/cm3) • Temperature (rate ↑’s as temp ↑’s)
Maximize SA for Absorption: • alveoli in lungs (grapelike sacs)
• membrane folds in mitochondria and in cristae of chloroplasts
• root hairs for water and mineral uptake
• villi for absorption of digested food in S. Intestine
Reduce length of diffusion path: • Folded membranes increase SA:VOL ratio
• more membrane in smaller volume = shorter distance across for molecules to diffuse
• membranes are very thin (7-10nm)
2. FacilitatedDiffusion/Transport
• Movementofamoleculefromanareaof[HIGH]to[lower]withtheaidofproteins(carrierorchannelproteins)
• Channels/Carriersworkasafacilita:ngpathwaysforlargermoleculesorchargedions(aminoacids,glucose,Na+andK+)
• Highlyspecificproteinsforeachmolecule
3. Osmosis
• Diffusion of WATER through a semi-permeable membrane
• Water moves from high H2O potential and low [solute] to low H2O potential and hi [solute]
Given: A - 100% water B - 90% water
Question:
What direction does the water flow? Water flows from A --> B
Given: A - 1% glucose B - 20% glucose
Question:
What direction does the water flow? What direction does the glucose flow?
Given: A - 1% glucose B - 20% glucose
Question:
What direction does the water flow? Water flows from A --> B What direction does the glucose flow? Glucose flows from B --> A
Types of Solutions: 1. Hypertonic • Hyper = more ; tonic = solute • Thus, a cell in a solute rich environment
• Water flows outward from region of high [water] to low [water] (or low to high [solute])
• Cell undergoes plasmolysis (fluid loss) and crenate (shrivel)
2. Hypotonic • Hypo = less ; tonic = solute • Thus, a cell in a solute poor environment
• Water flows inward from region of high [water] to low [water] • Cell undergoes cytolysis (cell breaks) and bursts
3. Isotonic • Iso = equal ; tonic = solute • Thus, a [solute] both equal within and outside the cell
• Water flows in and out of cell with net zero movement • Equilibrium
n http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Ac5veTransport• Requireschemicalenergy(ATP)• Involvesmovementofmolecules
againstaconcentra:ongradientusingmembraneproteinpumps
Endocytosis and Exocytosis
Enter cell process exit cell process
Exocytosis – export of macromolecules from the cell Endocytosis – the import of macromolecules
ENDOCYTOSIS (endo=inside; cyte=cell) Phagocytosis – the ingestion of solid molecules
Pinocytosis – the ingestion of liquids and solutes
Endocytosis: Phagocytosis • Ingest large particles
Endocytosis: Pinocytosis • engulf very small particles or liquids • “sipping”
EXOCYTOSIS (exo=outside; cyte=cell) • Export of a secretory vesicle that will migrate to
membrane’s inner surface and fuse with membrane • Contents are forced out of the cell • Opposite of endocytosis
• Examples: peptide hormones, steroid hormones, wastes
1. Protein is synthesised and present in the rER. Then protein is moved through the rER and modified.
2. A spherical vesicle is formed form the end of the rER with the protein inside.
3. The vesicle migrates to the Golgi and the membranes fuse. The protein is released into lumen of the Golgi.
6. The vesicle migrates to the plasma membrane fuses and secretes content its contents out of the cell. A process called exocytosis
5. A new vesicle is formed from Golgi membrane which then breaks away.
4. Golgi modif ies the protein further by adding lipid or polysaccharides
How Do Membranes Fuse? 1.Vesicleapproachestheplasmamembrane.Allmembranesaremadeupofphospholipidssotheysharethesameproper:es
2.Themembranebeginstofuse.Rememberthefluidityoftheplasmamembrane–phospholipidscanflowaroundeachother
3.Foramoment,thereisasinglephospholipidbilayeratthepointofcontact=temporaryintermediatestructure”
4.Themembraneporeopens,allowingitscontentstopassthrough.No:cethatthereisneveranunbrokensec:onofthemembranethroughtheen:reprocess.
Exocytosis-vesiclemembranefuseswiththeplasmamembrane.Endocytosis-vesicleformedbytheinfoldingoftheplasmamembrane
Ineachofthecasesabovethemembranesareabletoformandbreakwithoutlossofthecon1nuityoftheplasmamembranes.
Thephospholipidmoleculesareconstantlychangingplacesinthehorizontalplanecrea:ngthefluidpropertyofthemembrane.Exchangeinthever:calplanedoesnotoccur.Thismaintainstheintegrityofthemembrane.