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

FacilitatedTransport-McGraw-Hill

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?

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

Comparediffusionandosmosis.

Comparediffusionandosmosis.

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.