kim folger's cell membrane diffusion 2008

8/8/2019 Kim Folger's Cell Membrane Diffusion 2008

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AP Biology 2007-2008

The Cell Membrane


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AP Biology

Phospholipids

Fatty acid

Phosphate

Phosphate headhydrophilic

Fatty acid tailshydrophobic

Arranged as a bilayer

Aaaah,one of those

structurefunctionexamples

repelled by water

attracted to water


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AP Biology

Arranged as a Phospholipid bilayer

polar hydrophilic

heads

nonpolar hydrophobic

tails

polar hydrophilic

heads

Serves as a cellular barrier / border

H2Osugar

lipids

salt

waste

impermeable to polar molecules


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AP Biology

Cell membrane defines cellCell membrane separates living cell fromaqueous environment

thin barrier = 8nm thick

Controls traffic in & out of the cellallows some substances to cross moreeasily than others

hydrophobic (nonpolar) vs. hydrophilic (polar)


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AP Biology

Permeability to polar molecules?Membrane becomes semi-permeable viaprotein channels

specific channels allow specific material

across cell membrane

inside cell

outside cell

sugar aaH2O

saltNH3


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AP Biology

Cell membrane is more than lipidsTransmembrane proteins embedded inphospholipid bilayer

create semi-permeabe channels

lipid bilayer membrane protein channelsin lipid bilyer membrane


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W hy areproteins the perfect

molecule to build structuresin the cell membrane?


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AP Biology

Classes of amino acidsWhat do these amino acids have in common?What do these amino acids have in common?

nonpolar & hydrophobicnonpolar & hydrophobic


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AP Biology

Classes of amino acidsWhat do these amino acids have in common?What do these amino acids have in common?

polar & hydrophilicpolar & hydrophilic

I like thepolar onesthe best!


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AP Biology

Proteins domains anchor molecule

Within membranenonpolar amino acids

hydrophobicanchors proteininto membrane

On outer surfaces of membrane in fluid

polar amino acidshydrophilicextend intoextracellular fluid &

into cytosol

Polar areasof protein

Nonpolar areas of protein


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AP Biology

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar (hydrophobic)E-helices in thecell membrane H+

Porin monomer

F-pleated sheets

Bacterialouter membrane

proton pump channelin photosynthetic bacteria

aquaporin =water channel in bacteria

function throughconformational change =protein changes shape

function throughconformational change =protein changes shape

Examples

H2O

H2O

H+

H+


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AP Biology

Many Functions of Membrane Proteins

Outside

Plasmamembrane

InsideTransporter Cell surface

receptor Enzymeactivity

Cell surfaceidentity marker Attachment to thecytoskeletonCell adhesion

Antigen

Channel


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AP Biology

Membrane ProteinsProteins determine membranes specific functions

cell membrane & organelle membranes each haveunique collections of proteins

Classes of membrane proteins:peripheral proteins

loosely bound to surface of membrane

ex: cell surface identity marker ( antigens )

integral proteinspenetrate lipid bilayer, usually across whole membranetransmembrane protein

ex: transport proteinschannels, permeases (pumps)


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AP Biology

Cell membrane must be more than lipids

In 1972, S.J. Singer & G. Nicolsonproposed that membrane proteins areinserted into the phospholipid bilayer

I ts like a fluidI ts like a mosaic

I ts theFluid Mosaic Model!


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AP Biology

Membrane is a collage of proteins & other moleculesembedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments of cytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model


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AP Biology

Membrane carbohydratesPlay a key role in cell-cell recognition

ability of a cell to distinguish one cellfrom another

antigensimportant in organ &tissue developmentbasis for rejection of foreign cells byimmune system


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AP Biology

Any Questions??


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Movement across the

Cell Membrane


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AP Biology

Diffusion2nd Law of Thermodynamicsgoverns biological systems

universe tends towards disorder (entropy)

Diffusion

movement from HIGHp

LOW concentration

Diffusion

movement from HIGHp

LOW concentration


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AP Biology

Simple DiffusionMove from HIGH to LOW concentration

passive transportno energy needed

diffusion osmosis

movement of water


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AP Biology

Facilitated DiffusionDiffusion through protein channels

channels move specific molecules acrosscell membraneno energy needed

The Bouncer

The Bouncer

open channel = fast transport

facilitated = with help

HIGH

LOW


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AP Biology

Active Transport

The Doorman

The Doorman

conformational change

Cells may need to move molecules against concentration gradient

conformational shape change transports solutefrom one side of membrane to other

protein

pump costs energy = ATP

ATP

LOW

HIGH


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AP Biology symportantiport

Active transportMany models & mechanisms

ATP ATP


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AP Biology

Getting through cell membranePassive Transport

Simple diffusiondiffusion of nonpolar, hydrophobic molecules

lipidsHIGH p LOW concentration gradient

Facilitated transportdiffusion of polar, hydrophilic moleculesthrough a protein channel

HIGH p LOW concentration gradient

Active transportdiffusion against concentration gradient

LOW p HIGH

uses a protein pumprequires ATP

ATP


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AP Biology

Transport summarysimplediffusion

facilitateddiffusion

activetransport

ATP


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AP Biology

How about large molecules?Moving large molecules into & out of cell

through vesicles & vacuolesendocytosis

phagocytosis =

cellular eatingpinocytosis = cellular drinking

exocytosis

exocytosis


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AP Biology

Endocytosis

phagocytosis

pinocytosis

receptor-mediatedendocytosis

fuse withlysosome for digestion

non-specificprocess

triggered bymolecular signal


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The Special Case of Water

Movement of water acrossthe cell membrane


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AP Biology

Osmosis is just diffusion of water Water is very important to life,so we talk about water separatelyDiffusion of water fromHIGH co n ce ntrati o n of water toLOW co n ce ntrati o n of water

across asemi-permeablemembrane


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AP Biology

Concentration of water Direction of osmosis is determined bycomparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water


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AP Biology freshwater balanced saltwater

Managing water balanceCell survival depends on balancingwater uptake & loss


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AP Biology

Managing water balanceHypotonic

a cell in fresh water high concentration of water around cell

problem : cell gains water,

swells & can burstexample : P aram ec ium

ex: water continually entersP aram ec ium cell

solution : contractile vacuolepumps water out of cellATP

plant cellsturgid = fullcell wall protects from bursting

freshwater

ATP

1

N o problem,here

KABOOM!


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AP Biology

Pumping water outContractile vacuole in P aram ec ium

ATP


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AP Biology

Managing water balanceHypertonic

a cell in salt water low concentration of water around cell

problem : cell loses water &can dieexample : shellfishsolution : take up water or pump out salt

plant cellsplasmolysis = wiltcan recover

saltwater

2

I willsurvive!

I m shrinking,I m shrinking!


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AP Biology

Managing water balanceIsotonic

animal cell immersed inmild salt solution

no difference in concentration of

water between cell & environmentproblem : none

no net movement of water flows across membrane equally, inboth directions

cell in equilibriumvolume of cell is stable

example :blood cells in blood plasma

slightly salty IV solution in hospital

balanced

3

I couldbe better

Thatsperfect!


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AP Biology

AquaporinsWater moves rapidly into & out of cells

evidence that there were water channelsprotein channels allowing flow of water

across cell membrane

1991 | 2003

Peter AgreJohn Hopkins

Roderick MacKinnonRockefeller


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AP Biology

Cell (compared to beaker) p hypertonic or hypotonicBeaker (compared to cell) p hypertonic or hypotonic

Which way does the water flow? p in or out of cell

.05 M .03 M

Do you understand Osmosis


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AP Biology

Any Questions??


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Ghosts of Lectures Past(storage)


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AP Biology

Diffusion through phospholipid bilayer

What molecules can get through directly?fats & other lipids

inside cell

outside cell

lipidsalt

aa H2Osugar

NH3

What molecules canNOT get throughdirectly?

polar moleculesH2O

ions (charged)salts, ammonia

large moleculesstarches, proteins


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AP Biology

Membrane fat composition variesFat composition affects flexibility

membrane must be fluid & flexibleabout as fluid as thick salad oil

% unsaturated fatty acids in phospholipidskeep membrane less viscouscold-adapted organisms, like winter wheat

increase % in autumn

cholesterol in membrane


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AP Biology

Diffusion across cell membraneCell membrane is the boundary betweeninside & outside

separates cell from its environment

INfood

carbohydratessugars, proteinsamino acidslipidssalts, O 2, H2O

OUTwaste

ammoniasaltsCO 2H2Oproducts

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

kim folger's cell membrane diffusion 2008

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