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The Cell MembraneCell Physiology
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Overview
Cell membrane separates living cell fromnonliving surroundingsu thin barrier = 8nm thick
Controls traffic in & out of the cellu selectively permeableu allows some substances to cross more
easily than others hydrophobic vs hydrophilic
Made ofphospholipids, proteins & othermacromolecules
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Phospholipids
Fatty acid
Phosphate
Fatty acid tailsu hydrophobic
Phosphate group headu
hydrophilic Arranged as a bilayer
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Phospholipid bilayer
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
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More than lipids
In 1972, S.J. Singer & G. Nicolsonproposed that membrane proteins areinserted into the phospholipid bilayer
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Membrane fat composition varies
Fat composition affects flexibilityu membrane must be fluid & flexible
about as fluid as thick salad oil
u % unsaturated fatty acids inphospholipids keep membrane less viscous
u cholesterol in membrane
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Membrane Proteins
Proteins determine membranes specific functionsu cell membrane & organelle membranes each
have unique collections of proteins
Membrane proteins:u peripheral proteins
loosely bound to surface of membrane
cell surface identity marker (antigens)
u integral proteins
penetrate lipid bilayer, usually across wholemembrane
transmembrane protein
transport proteins
w channels, permeases (pumps)
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Proteins domains anchor molecule
Within membraneu nonpolaramino
acids hydrophobic
anchors proteinintomembrane
On outer surfaces of
membraneu polaramino acids
hydrophilic
extend into
extracellular
Polar areasof protein
Nonpolar areas of protein
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Many Functions of Membrane Proteins
Outside
Plasmamembrane
Inside Transporter Cell surfacereceptor
Enzymeactivity
Cell surfaceidentity marker Attachment to thecytoskeletonCell adhesion
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Membrane carbohydrates
Play a key role in cell-cell recognitionu ability of a cell to distinguish one cell
from another antigens
u important in organ &tissue development
u basis for rejection of
foreign cells byimmune system
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Movement across the
Cell Membrane
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Diffusion
2nd Law of Thermodynamicsgoverns biological systemsu universe tends towards disorder
(entropy)
Diffusionu
movement from high
low concentration
Diffusionu
movement from high
low concentration
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Diffusion
Move from HIGH to LOW concentrationu passive transport
u no energy needed
diffusion osmosis
movement of water
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Diffusion across cell membrane
Cell membrane is the boundary betweeninside & outsideu separates cell from its environment
INfood
carbohydratessugars, proteinsamino acidslipidssalts, O2,H2O
OUTwaste
ammoniasaltsCO2H2O
products
cell needs materials in & products or waste out
IN
OUT
Can it be an impenetrable boundary? NO!
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Diffusion through phospholipid bilayer
What molecules can get through directly?u fats & other lipids
inside cell
outside cell
lip
id
salt
aa
H
2
O
sugar
What molecules can
NOT get throughdirectly?
upolar molecules
H2O
uionssalts, ammonia
ularge moleculesstarches,
proteins
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Channels through cell membrane
Membrane becomes semi-permeablewith protein channelsu specific channels allow specific
material across cell membrane
inside cell
outside cell
suga
r
aa
H
2
O
sal
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Facilitated Diffusion
Diffusion through protein channelsu channels move specific molecules across
cell membrane
u no energy needed
The BouncerThe Bouncer
open channel = fast transport
facilitated = with help
high
low
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Active Transport
The DoormanThe Doorman
conformationalchange
Cells may need to move molecules againstconcentration gradientu shape change transports solute from
one side of membrane to otheru protein pumpu costs energy = ATP
ATP
low
high
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Active transport
Many models & mechanisms
ATP
ATP
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Getting through cell membrane
Passive Transportu Simple diffusion
diffusion of nonpolar, hydrophobic moleculesw lipids
w high low concentration gradient
u Facilitated transport diffusion of polar, hydrophilic molecules
through a protein channelw high low concentration gradient
Active transportu diffusion againstconcentration gradient
low high
u uses a protein pumpu requires ATP
ATP
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Transport summary
simplediffusion
facilitateddiffusion
activetransport
ATP
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How about large molecules?
Moving large molecules into & out of cellu through vesicles & vacuoles
u endocytosis phagocytosis = cellular eating
pinocytosis = cellular drinking
u exocytosis
exocytosis
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Endocytosis
phagocytosis
pinocytosis
receptor-mediatedendocytosis
fuse withlysosome fordigestion
non-specificprocess
triggered bymolecularsignal
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The Special Case of Water
Movement of water acrossthe cell membrane
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Osmosis is diffusion of water
Water is very important to life,so we talk about water separately
Diffusion of water fromhigh concentration of water tolow concentration of wateru across a
semi-permeable
membrane
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Concentration of water
Direction of osmosis is determined bycomparing total solute concentrations
u Hypertonic - more solute, less water
u Hypotonic - less solute, more wateru Isotonic - equal solute, equal water
hypotonic hypertonic
water
net movement of water
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Managing water balance
Isotonicu animal cell immersed in
mild salt solution
example:
blood cells in bloodplasma
problem: none
wno net movement
of watern flows across
membraneequally, inbothdirections
wvolume of cell is balanced
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Managing water balance
Hypotonicu a cell in fresh water
example: Paramecium
problem: gains water,swells & can burst
wwater continually enters
Paramecium cell
solution: contractile vacuole
wpumps water out of cell
w ATP
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Managing water balance
Hypertonicua cell in salt water
example: shellfish
problem: lose water & diesolution: take up water or
pump out salt
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Aquaporins
Water moves rapidly into & out of cellsu evidence that there were water
channels
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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Cell (compared to beaker) hypertonic or hypotonic
Beaker(compared to cell) hypertonic or hypotonic
Which way does the water flow? in or out of cell
.05 M .03 M
Osmosis