The Cell Membrane
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Transcript of The Cell Membrane
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The Cell Membrane
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Phospholipids
Fatty acid
Phosphate• Phosphate head – hydrophilic
• Fatty acid tails– hydrophobic
• Arranged as a bilayer
Aaaah, one of those
structure–functionexamples
“repelled by water”
“attracted to water”
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Arranged as a Phospholipid bilayer
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
• Serves as a cellular barrier / border
H2Osugar
lipids
salt
waste
impermeable to polar molecules
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Cell membrane defines cell• Cell membrane separates cell from aqueous
environment– Thin = 8nm thick
• Controls transport in & out of the cell– Some substances cross more easily than others• hydrophobic (nonpolar) vs. hydrophilic (polar) • Small vs. big.
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Cell membrane must be more than lipids…
• In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer
It’s like a fluid…It’s like a mosaic…
It’s the Fluid Mosaic Model!
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Permeability to polar molecules?• Membrane becomes semi-permeable via
protein channels – specific channels allow specific material across
cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
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Cell membrane is more than lipids…• Transmembrane proteins embedded in
phospholipid bilayer–create semi-permeable channels
lipid bilayermembrane
protein channelsin lipid bilyer membrane
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2007-2008
Why areproteins the perfect
molecule to build structures in the cell membrane?
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Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
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Classes of amino acids
What do these amino acids have in common?
polar & hydrophilic
I like thepolar onesthe best!
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Protein domains anchor molecule• Within membrane– nonpolar amino acids • hydrophobic • anchors protein
into membrane• On outer surfaces of
membrane in fluid– polar amino acids • hydrophilic• extend into
extracellular fluid & into cytosol
Polar areasof protein
Nonpolar areas of protein
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NH2
H+
COOH
Cytoplasm
Retinalchromophore
Nonpolar(hydrophobic)a-helices in thecell membrane H+
Porin monomerb-pleated sheets
Bacterialoutermembrane
proton pump channel in photosynthetic bacteria
aquaporin = water channel in bacteria
function through conformational change = protein changes shape
Examples
H2O
H2O
H+
H+
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Many Functions of Membrane ProteinsOutside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion“Antigen”
“Channel”
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Membrane Proteins• Proteins determine membrane’s specific functions– Every membrane in a cell has a unique collection of proteins
• Classes of membrane proteins:– peripheral proteins
• loosely bound to surface of membrane• ex: cell surface identity marker (antigens)
– integral proteins • penetrate lipid bilayer, across whole membrane • “transmembrane” protein• ex: transport proteins
– channels, permeases (pumps)
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Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer
Extracellular fluid
Cholesterol
Cytoplasm
Glycolipid
Transmembraneproteins
Filaments ofcytoskeleton
Peripheralprotein
Glycoprotein
Phospholipids
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Membrane carbohydrates • Play a key role in cell-cell recognition– ability of a cell to distinguish one cell from
another• antigens
– important in organ & tissue development
– basis for rejection of foreign cells by immune system
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Cell Membranes are not static!!
• Watch this
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Any Questions??
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2007-2008
Movement across the Cell Membrane(Ch. 7)
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Diffusion• 2nd Law of Thermodynamics
governs biological systems– universe tends towards disorder (entropy)
Diffusion movement from HIGH LOW concentration
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Simple Diffusion
• Move from HIGH to LOW concentration– “passive transport”– no energy needed
diffusion osmosis
movement of water
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Facilitated Diffusion• Diffusion through protein channels– channels move specific molecules across
cell membrane– no energy needed
“The Bouncer”
open channel = fast transportfacilitated = with help
HIGH
LOW
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Active Transport
“The Doorman”
conformational change
• Cells may need to move molecules against concentration gradient– conformational shape change transports solute
from one side of membrane to other – protein “pump”– “costs” energy = ATP
ATP
LOW
HIGH
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Getting through cell membrane• Passive Transport– Simple diffusion• nonpolar, hydrophobic molecules–HIGH LOW concentration gradient
– Facilitated transport• polar, hydrophilic molecules• through a protein channel–HIGH LOW concentration gradient
• Active transport– against concentration gradient• LOW HIGH
– uses a protein pump (requires ATP)ATP
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Transport summary
simplediffusion
facilitateddiffusion
activetransport
ATP
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What about large molecules?• Moving large molecules into & out of cell– through vesicles & vacuoles– endocytosis• phagocytosis = “cellular eating”• pinocytosis = “cellular drinking”
– exocytosis
exocytosis
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Endocytosis
phagocytosis
pinocytosis
receptor-mediated endocytosis
fuse with lysosome for digestion
non-specificprocess
triggered bymolecular signal
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About Osmosis
• Water is very important to life, so we talk about water separately
• Diffusion of water from HIGH concentration of water to LOW concentration of water– across a
semi-permeable membrane
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Concentration of water
• Direction of osmosis is determined by comparing 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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freshwater balanced saltwater
Managing water balance• Cell survival depends on balancing water
uptake & loss
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Managing water balance• Hypotonic– a cell in fresh water– high concentration of water around cell• problem: cell gains water, swells & can
burst • example: Paramecium • solution: contractile vacuole – pumps water out of cell– Uses ATP
– plant cells• turgid = full• cell wall protects from bursting
freshwater
ATP
1
No problem,here
KABOOM!
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Pumping water out• Contractile vacuole in Paramecium
ATP
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Managing water balance• Hypertonic– a cell in salt water– low concentration of water
around cell• problem: cell loses water &
can die • example: shellfish• solution: take up water or
pump out salt– plant cells• plasmolysis = wilt• can recover
saltwater
2
I willsurvive!
I’m shrinking,I’m shrinking!
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Managing water balance• Isotonic– animal cell immersed in mild salt solution– no difference in concentration of water
between cell & environment• problem: none–no net movement of water–cell in equilibrium –volume of cell is stable
• example: blood cells in blood plasma–slightly salty IV solution in hospital
balanced
3
I couldbe better…
That’sperfect!
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Aquaporins• Water moves rapidly into & out of cells– evidence that there were water channels• protein channels allowing flow of water across cell
membrane
1991 | 2003
Peter AgreJohn Hopkins
Roderick MacKinnonRockefeller
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Cell (compared to beaker) hypertonic or hypotonicBeaker (compared to cell) hypertonic or hypotonicWhich way does the water flow? in or out of cell
.05 M .03 M
Do you understand Osmosis…
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Any Questions??