AP Biology
CHAPTER 7 MEMBRANE STUCTURE
AND FUNCTION
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Study guide #20-27
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Diffusion 2nd Law of Thermodynamics
Governs biological systems!Universe tends to disorder!
Diffusion –movement from high>low concentration of THAT SUBSTANCE!
diffusion
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Diffusion of 2 Solutes
Each substance diffuses down its own concentration gradient, independent of concentration
gradient of any other substance.
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Diffusion•PASSIVE TRANSPORT
•No energy required
•So – How does that happen?
Brownian Motion
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Osmosis is the diffusion of water across a membrane 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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Osmosis 250 times the volume of cell/second! Osmosis = NET movement of water across a
selectively permeable membrane driven by a difference in solute concentration on either side of the memrane.
“Free” water moves Less solute = more free water Water flows from low solute [ ] to high solute [ ] Until equilibrium
Slide 2
osmosis
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Osmosis
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What determines when and what direction water will move?
3 molecules of albumin 15 molecules of glucose
66,000 mw 180 mw
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Isotonic? Hypertonic? Hypotonic?
A & B are isotonic
A & B are hypertonic to C
C is hypotonic to A and B
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Osmotic Pressure Pressure generated by diffusion of water across
a membrane When pressure is equal water flow will stop Called “hydrostatic pressure” – water-stopping
pressure Osmolarity- [ ] in terms of # of particles in a
volume of liquid 1 osmolar soln = 1 M of osmoltically active
particles per liter.
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Red Blood Cells in NaCl solutions100mOs 500 mOs
hypotonic
isotonic
hypertonic
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Osmosis problems
Hydrostatic generator
Osmosis problems
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EndDiffusionOsmosis
Cell membrane
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The Cell Membrane
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Overview Cell membrane separates living cell from
nonliving surroundings thin barrier = 8nm thick
Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily
than others hydrophobic vs hydrophilic
Made of phospholipids, proteins & other macromolecules
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Phospholipids
Fatty acid
Phosphate
Fatty acid tails hydrophobic
Phosphate group head hydrophilic
Arranged as a bilayer
Aaaah, one of those
structure–functionexamples
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Phospholipid bilayer
polarhydrophilic
heads
nonpolarhydrophobic
tails
polarhydrophilic
heads
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Two generations of membrane models
How do we know?
phospholipid bilayer -1920’s;
1930-60’s; models from EM views;
??-Not as hydrophyllic as pure PL’s?
D & D - Hydrophyllic Protein sandwich
??- all membranes identical???
??-amphipathic proteins? Solubile in H2O
hydrophobic region in aqueous?
1972; Singer, Nicholson
Dispersion model
Hydrophyllic regions in aqueous
Hydrophobic in hydrophobic PL
FLUID MOSAIC MODEL!
Freeze Fracture evidence
Permanent model????
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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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Evidence for the drifting of membrane proteins
Other Evidence: microsurgery on cells
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Membranes are fluid- like salad oil!
•held in place weak hydrophobic interactions•PL’S drift laterally •rarely flip-flop between layers
•larger proteins move slower•some proteins guided by cytoskeleton “motors”•some proteins anchored by cytoskeleton
Fluid Membrane
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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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The fluidity of membranes
Maintains and increases/decreases
fluidity
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Fluidity influenced by temperature
cool - more solid- PL’s closely packedif rich in unsaturated fatty acids - more fluid than those rich in saturated fatty acids - kinks prevent tight packingcholesterol steroid- wedged between PL’s of animal cells•warm -limits mvmnt. of PL’s, reduces fluidity•cool - maintains fluidity, prevents tight packing-EX: salmon
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•Must be fluidto work w/ enzymes & be permeable
•Cells alter lipid makeup to adjust for temp. changes
EX: cold organisms ( winter wheat, salmon, bears) increase % of unsat PL’s in autumn - prevents solidifying membranes
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Membrane fat composition varies Fat composition affects flexibility
membrane must be fluid & flexible about as fluid as thick salad oil
% unsaturated fatty acids in phospholipids keep membrane less viscous cold-adapted organisms, like winter wheat
increase % in autumn cholesterol in membrane
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Why areproteins the perfect
molecule to build structures in the cell membrane?
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Classes of amino acidsWhat do these amino acids have in common?
nonpolar & hydrophobic
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Classes of amino acidsWhat do these amino acids have in common?
polar & hydrophilic
I like thepolar onesthe best!
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Membrane Proteins Proteins determine membrane’s specific functions
cell membrane & organelle membranes each have unique collections of proteins
Membrane proteins: peripheral proteins
loosely bound to surface of membrane cell surface identity marker (antigens)
integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins
channels, permeases (pumps)
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2 major types of membrane proteins.Peripheral proteins - not embedded; bound
to inner/outer surface; may be connected to integral proteinsIntegral proteins - penetrate bilayer; - Transmembrane protein
•hydrophobic regions of nonpolar aa’s in contact with bilayer core; often alpha helices•hydrophilic regions of aa’s in contact w/ environment – beta pleated
- Unilateral – partially through; coupled
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Protein’s domain anchor molecule Within membrane
nonpolar amino acids hydrophobic anchors protein
into membrane On outer surfaces of
membrane 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)-helices in thecell membrane H+
Porin monomer-pleated sheets
Bacterialoutermembrane
proton pump channel in photosynthetic bacteria
water channel in bacteria
function through conformational change = shape change
Examples
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Many Functions of Membrane Proteins
Outside
Plasmamembrane
InsideTransporter Cell surface
receptorEnzymeactivity
Cell surface identity marker
Attachment to thecytoskeleton
Cell adhesion
UniportSymportAntiport
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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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Some proteins reinforce shape of cell•cytoplasmic side, •some connect to cytoskeleton
• exterior side, •some attach to fibers of ECM
•ECM = extracellular matrix
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Sidedness of the plasma membrane
What makes the
endomembrane system
work?
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embranes are bifacial and sided•may differ in lipid composition•proteins have a direction orientation•outer surface has carbo’s•asymmetry begins w/ synthesis in ER•proteins in plasma membrane provide a variety of major cell functions
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The detailed structure of an animal cell’s plasma membrane, in cross section
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Cell-cell recognition = distinguish one type of neighboring cell from another
important in cell sorting•organization into tissues and organs during development
basis for rejection of foreign cells by immune system
•key in on surface molecules, often carbs
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Carbo’s - usually branched oligosaccharides = < 15
monomers•may be covalently bonded to •lipids= glycolipids •proteins = glycoproteins
•external OS’s vary from species to species, individual to individual, and even from cell type to cell type w/in same individual - identification
marks each cell type as distinct•blood types (A, B, AB, O)-RBCs
•Glycocalyx of animal cells – fuzzy!
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End membranes
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Movement across the Cell Membrane
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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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Diffusion Move from HIGH to LOW concentration
“passive transport” no energy needed
diffusion osmosis
movement of water
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Diffusion across cell membrane Cell membrane is the boundary between
inside & outside… separates cell from its environment
INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O
OUTwasteammoniasaltsCO2
H2O 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?
fats & other lipids
inside cell
outside cell
lipidsalt
aa H2Osugar
NH3
What molecules can NOT get through directly?
polar molecules H2O
ions salts, ammonia
large molecules starches, proteins
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Channels through cell membrane Membrane becomes semi-permeable
with protein channels specific channels allow specific material
across cell membrane
inside cell
outside cell
sugaraaH2O
saltNH3
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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 shape change transports solute from
one side of membrane to other protein “pump” “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 Transport
Simple diffusion diffusion of nonpolar, hydrophobic molecules
lipids high low concentration gradient
Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel
high low concentration gradient Active transport
diffusion against concentration gradient low high
uses a protein pump requires ATP
ATP
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Transport summarysimplediffusion
facilitateddiffusion
activetransport
ATP
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How 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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The Special Case of Water
Movement of water across the 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 from
high concentration of water to low concentration of water across a
semi-permeable membrane
AP Biology
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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Managing water balance Cell survival depends on balancing
water uptake & loss
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Managing water balance Isotonic
animal cell immersed in mild salt solution example:
blood cells in blood plasma problem: none
no net movement of water flows across membrane
equally, in both directionsvolume of cell is stable
balanced
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Managing water balance Hypotonic
a cell in fresh water example: Paramecium problem: gains water,
swells & can burst water continually enters
Paramecium cell solution: contractile vacuole
pumps water out of cell ATP
plant cells turgid
freshwater
ATP
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Water regulation Contractile vacuole in Paramecium
ATP
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Managing water balance Hypertonic
a cell in salt water example: shellfish problem: lose water & die solution: take up water or
pump out salt plant cells
plasmolysis = wilt
saltwater
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Aquaporins Water moves rapidly into & out of cells
evidence that there were water channels
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
Osmosis…
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Any Questions??