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Chapter 7: Membrane Structure and Function
Overview: Life at the Edge
Plasma membrane: boundary that separates the living cell from its surroundings
Exhibits selective permeability allows some
substances to cross it more easily than others
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Concept 7.1: Cellular membranes are fluid mosaics of lipids and
proteinsPhospholipids: most abundant lipid in the plasma
membraneamphipathic molecules: contain hydrophobic and
hydrophilic regionsFluid mosaic model: membrane is a fluid structure
with a “mosaic” of various proteins embedded in it
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
1935: Hugh Davson & James Danielli proposed sandwich model phospholipid bilayer lies between two
layers of globular proteinsPROBLEM: Placement of membrane
proteins which have hydrophilic and hydrophobic regions
1972: J. Singer & G. Nicolson proposed membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water Freeze-fracture studies of the plasma membrane
supported the fluid mosaic model Freeze-fracture: specialized preparation technique
that splits membrane along the middle of the phospholipid bilayer
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
History of the Membrane
Fig. 7-4
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
The Fluidity of Membranes Phospholipids move within the
bilayer Most lipids, and some proteins, drift
laterally Rarely molecule flip-flop
transversely across membrane Temperatures cool membranes
switch from fluid to solid state Temperature at which
membrane solidifies = types of lipids
Unsaturated fatty acids = more fluid
Saturated fatty acids = more solid Membranes must be fluid to work
properly usually about as fluid as salad oil
Fig. 7-5a
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
Fig. 7-5b
(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
Fig. 7-6
RESULTS
Membrane proteins
Mouse cellHuman cell
Hybrid cell
Mixed proteinsafter 1 hour
Steroid cholesterol effects membrane fluidity @ different temperatureswarm temperatures (such as 37°C) cholesterol
restrains movement of phospholipidscool temperatures maintains fluidity by preventing
tight packing
The Fluidity of Membranes
Membrane Proteins and Their Functions
Collage of different proteins embedded in the fluid matrix of the lipid bilayer
Proteins membrane’s specific functionsPeripheral proteins: bound to the surface of the
membraneIntegral proteins:
penetrate hydrophobic core Transmembrane proteins:
Integral proteins that span the membrane
Hydrophobic regions consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-7
Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognition Intercellular joiningAttachment to the
cytoskeleton and extracellular matrix (ECM)
Membrane Functions
Fig. 7-9ac
(a) Transport (b) Enzymatic activity (c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor
Fig. 7-9df
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
The Role of Membrane Carbohydrates in Cell-Cell Recognition
Cells recognize each other by binding to surface molecules, often carbohydrates, on plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on external side of plasma membrane vary among species, individuals, and even cell types in an individual
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Synthesis and Sidedness of Membranes
Membranes have distinct inside and outside faces
Asymmetrical distribution of proteins, lipids, and carbohydrates in plasma membrane determined when membrane is built by ER and Golgi apparatus
Concept 7.2: Membrane structure results in selective permeability
Cell must exchange materials with its surroundings controlled by the plasma membrane
Plasma membranes = selectively permeable regulating the
cell’s molecular traffic
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Permeability of the Lipid BilayerHydrophobic/nonpolar molecules (like
hydrocarbons) can dissolve in lipid bilayer and pass through membrane rapidly
Polar molecules (like sugars) do not cross the membrane easily
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Transport ProteinsTransport proteins: allow passage of hydrophilic
substances across the membraneChannel proteins: have a hydrophilic channel that
certain molecules or ions can use as tunnelAquaporins: channel proteins that facilitate
passage of waterCarrier proteins: bind to molecules and change
shape to shuttle them across the membranespecific for
the substance it moves
Concept 7.3: Passive transport is diffusion of a substance across a membrane with no
energy investmentDiffusion: tendency for molecules to spread out
evenly into the available spaceEach molecule moves randomly HOWEVER diffusion
of molecules may exhibit net movement in one direction
Dynamic equilibrium molecules crossing one direction = other direction
Substances diffuse down concentration gradient the difference in concentration of a substance from
one area to another
Passive transport: requires no energy from the cell The diffusion of a substance across biological
membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Passive Transport
Effects of Osmosis on Water Balance
Osmosis: diffusion of water across a selectively permeable membrane
Water diffuses across membrane from lower [solute] higher [solute]
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Water Balance of Cells Without Walls
Tonicity: ability of solution to cause a cell to gain or lose water
Isotonic solution: Solute concentration is the same as that inside the cell no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell cell gains water
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-13
Hypotonic solution
(a) Animal cell
(b) Plant cell
H2O
Lysed
H2O
Turgid (normal)
H2O
H2O
H2O
H2O
Normal
Isotonic solution
Flaccid
H2O
H2O
Shriveled
Plasmolyzed
Hypertonic solution
Hypertonic OR hypotonic environments create osmotic problems for organismsWhy? What are some problems?
Osmoregulation: control of water balancenecessary adaptation for
life in such environmentsProtist Paramecium is
hypertonic to its pond water environment has a contractile vacuole that acts as pump
Effects of Osmosis on Living Cells
Water Balance of Cells with Walls
Cell walls help maintain water balancePlant cell in… hypotonic solution swells until wall opposes
uptake cell now turgid (firm)isotonic no net movement of water into the cell
cell becomes flaccid (limp) plant may wilthypertonic environment plant cells lose water
membrane pulls away from the wall (usually lethal effect) called plasmolysis
Facilitated Diffusion: Passive Transport Aided by ProteinsFacilitated diffusion: transport proteins speed up
passive movement of molecules across plasma membrane
Channel proteins: provide corridors that allow specific molecule/ion to cross membrane
Channel proteins includeAquaporins, for
facilitated diffusion of water
Ion channels that open or close in response to a stimulus (gated channels)
Carrier proteins undergo subtle change in shape that translocates solute-binding site across the membrane
Some diseases caused by malfunctions in specific transport systems Ex: kidney disease, cystinuria
Facilitated Diffusion: Passive Transport Aided by Proteins
Concept 7.4: Active transport uses energy to move solutes against their gradients
Facilitated diffusion still passive because solute moves down its concentration gradient
Some transport proteins can move solutes against their concentration gradients
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Need for Energy in Active TransportActive transport: moves substances against their
concentration gradientrequires energy, usually in the form of ATPperformed by specific proteins embedded in the
membranesallows cells to
maintain concentration gradients that differ from their surroundings
Ex: sodium-potassium pump
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-16-1
EXTRACELLULAR
FLUID [Na+] high [K+] low
Na+
Na+
Na+ [Na+] low[K+] high CYTOPLASM
Cytoplasmic Na+ binds tothe sodium-potassium pump. 1
Na+ binding stimulatesphosphorylation by ATP.
Fig. 7-16-2
Na+
Na+
Na+
ATP P
ADP
2
Fig. 7-16-3
Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.
Na+
P
Na+ Na+
3
Fig. 7-16-4
K+ binds on theextracellular side andtriggers release of thephosphate group.
P P
K+
K+
4
Fig. 7-16-5
Loss of the phosphaterestores the protein’s originalshape.
K+
K+
5
Fig. 7-16-6
K+ is released, and thecycle repeats.
K+
K+
6
2
EXTRACELLULAR
FLUID [Na+] high [K+] low
[Na+] low
[K+] high
Na+
Na+
Na+
Na+
Na+
Na+
CYTOPLASM ATP
ADP P
Na+ Na+
Na+
P 3
K+
K+ 6
K+
K+
5 4
K+
K+
P P
1
Fig. 7-16-7
Fig. 7-17Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
How Ion Pumps Maintain Membrane Potential
Membrane potential: voltage difference across a membranecreated by differences in distribution of positive/negative
ionsElectrochemical gradient: two combined forces that
drive diffusion of ions across membrane:Chemical force: the ion’s concentration gradientElectrical force: the effect of the membrane potential on
the ion’s movementElectrogenic pump: transport protein that generates
voltage across membraneSodium-potassium pump: major electrogenic pump of
animal cellsProton pump: main electrogenic pump of plants, fungi,
and bacteria is a
Fig. 7-18
EXTRACELLULARFLUID
H+
H+
H+
H+
Proton pump
+
+
+
H+
H+
+
+
H+
–
–
–
–
ATP
CYTOPLASM
–
Cotransport: Coupled Transport by a Membrane Protein
Cotransport: active transport of a solute indirectly drives transport of another solute
Plants use gradient of hydrogen ionsgenerated by proton pumps to drive active transport of nutrients into the cell
Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and
endocytosis
Small molecules & water enter/leave cell through lipid bilayer or transport proteins
Large molecules like polysaccharides and proteins cross membrane in bulk via vesicles*requires energy*
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Exocytosis
Exocytosis: transport vesicles migrate to the membrane, fuse with it, and release their contentssecretory cells use exocytosis to export their products
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Endocytosis Endocytosis: cell takes in macromolecules by forming vesicles
from plasma membrane reversal of exocytosis, involving
different proteins Three types of endocytosis:
Phagocytosis: cell engulfs a particle in a vacuole then fuses with a lysosome to digest the particle (“cellular eating”)
Pinocytosis: molecules are taken up when extracellular fluid is “gulped” into tiny vesicles (“cellular drinking”)
Receptor-mediated endocytosis: binding of ligands to receptors triggers vesicle formationLigand: any molecule that binds
specifically to receptor site of another molecule
Fig. 7-20a
PHAGOCYTOSIS
CYTOPLASM EXTRACELLULARFLUID
Pseudopodium
“Food” orother particle
Foodvacuole Food vacuole
Bacterium
An amoeba engulfing a bacteriumvia phagocytosis (TEM)
Pseudopodiumof amoeba
1 µm
Fig. 7-20b
PINOCYTOSIS
Plasmamembrane
Vesicle
0.5 µm
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)
Fig. 7-20cRECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein
Coatedpit
Ligand
Coatprotein
Plasmamembrane
0.25 µm
Coatedvesicle
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)
You should now be able to:
1. Define the following terms: amphipathic molecules, aquaporins, diffusion
2. Explain how membrane fluidity is influenced by temperature and membrane composition
3. Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
4. Explain how transport proteins facilitate diffusion
5. Explain how an electrogenic pump creates voltage across a membrane, and name two electrogenic pumps
6. Explain how large molecules are transported across a cell membrane
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 7-UN1
Passive transport:Facilitated diffusion
Channelprotein
Carrierprotein
Fig. 7-UN2
Active transport:
ATP
Fig. 7-UN3
Environment:0.01 M sucrose
0.01 M glucose
0.01 M fructose
“Cell”
0.03 M sucrose
0.02 M glucose
Fig. 7-UN4