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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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AP Biology 2007-2008
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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AP Biology 2007-2008
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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AP Biology 2007-2008
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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AP Biology 2007-2008
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!