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Chap 7…
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The plasma membrane is the boundary that separates the living cell from its surroundings
The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others
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Fig. 7-1
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Phospholipids are the most abundant lipid in the plasma membrane
Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions
The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
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Membranes have been chemically analyzed and found to be made of proteins and lipids
Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer
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Fig. 7-2
Hydrophilichead
WATER
Hydrophobictail
WATER
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In 1935, Hugh Davson and James Danielli proposed a sandwich model in which the phospholipid bilayer lies between two layers of globular proteins
Later studies found problems with this model, particularly the placement of membrane proteins, which have hydrophilic and hydrophobic regions
In 1972, J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water
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Fig. 7-3
Phospholipidbilayer
Hydrophobic regionsof protein
Hydrophilicregions of protein
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Freeze-fracture studies of the plasma membrane supported the fluid mosaic model
Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer
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Fig. 7-4
TECHNIQUE
Extracellularlayer
KnifeProteins Inside of extracellular layer
RESULTS
Inside of cytoplasmic layer
Cytoplasmic layerPlasma membrane
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Phospholipids in the plasma membrane can move within the bilayer
Most of the lipids, and some proteins, drift laterally
Rarely does a molecule flip-flop transversely across the membrane
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Fig. 7-5
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
(a) Movement of phospholipids
(b) Membrane fluidity
Fluid Viscous
Unsaturated hydrocarbontails with kinks
Saturated hydro-carbon tails
(c) Cholesterol within the animal cell membrane
Cholesterol
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Fig. 7-5a
(a) Movement of phospholipids
Lateral movement(107 times per second)
Flip-flop( once per month)
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Fig. 7-6
RESULTS
Membrane proteins
Mouse cellHuman cell
Hybrid cell
Mixed proteinsafter 1 hour
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As temperatures cool, membranes switch from a fluid state to a solid state
The temperature at which a membrane solidifies depends on the types of lipids
Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids
Membranes must be fluid to work properly; they are usually about as fluid as salad oil
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Fig. 7-5b
(b) Membrane fluidity
Fluid
Unsaturated hydrocarbontails with kinks
Viscous
Saturated hydro-carbon tails
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The steroid cholesterol has different effects on membrane fluidity at different temperatures
At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids
At cool temperatures, it maintains fluidity by preventing tight packing
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Fig. 7-5c
Cholesterol
(c) Cholesterol within the animal cell membrane
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A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Proteins determine most of the membrane’s specific functions
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Fig. 7-7
Fibers ofextracellularmatrix (ECM)
Glyco-protein
Microfilamentsof cytoskeleton
Cholesterol
Peripheralproteins
Integralprotein
CYTOPLASMIC SIDEOF MEMBRANE
GlycolipidEXTRACELLULARSIDE OFMEMBRANE
Carbohydrate
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Peripheral proteins are bound to the surface of the membrane
Integral proteins penetrate the hydrophobic core
Integral proteins that span the membrane are called transmembrane proteins
The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices
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Fig. 7-8
N-terminus
C-terminus
HelixCYTOPLASMICSIDE
EXTRACELLULARSIDE
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Six major functions of membrane proteins:TransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular
matrix (ECM)
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Fig. 7-9
(a) Transport
ATP
(b) Enzymatic activity
Enzymes
(c) Signal transduction
Signal transduction
Signaling molecule
Receptor
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
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Fig. 7-9ac
(a) Transport (b) Enzymatic activity (c) Signal transduction
ATP
Enzymes
Signal transduction
Signaling molecule
Receptor
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Fig. 7-9df
(d) Cell-cell recognition
Glyco-protein
(e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)
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Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane
Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)
Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual
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Membranes have distinct inside and outside faces
The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
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Fig. 7-10
ER1
Transmembraneglycoproteins
Secretoryprotein
Glycolipid
2Golgiapparatus
Vesicle
3
4
Secretedprotein
Transmembraneglycoprotein
Plasma membrane:
Cytoplasmic face
Extracellular face
Membrane glycolipid
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A cell must exchange materials with its surroundings, a process controlled by the plasma membrane
Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
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Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly
Polar molecules, such as sugars, do not cross the membrane easily
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Transport proteins allow passage of hydrophilic substances across the membrane
Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel
Channel proteins called aquaporins facilitate the passage of water
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Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane
A transport protein is specific for the substance it moves
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Diffusion is the tendency for molecules to spread out evenly into the available space
Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction
At dynamic equilibrium, as many molecules cross one way as cross in the other direction
Animation: Membrane SelectivityAnimation: Membrane Selectivity Animation: DiffusionAnimation: Diffusion
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Fig. 7-11Molecules of dye Membrane (cross section)
WATER
Net diffusion Net diffusion Equilibrium
(a) Diffusion of one solute
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
(b) Diffusion of two solutes
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Molecules of dye
Fig. 7-11a
Membrane (cross section)
WATER
Net diffusion Net diffusion
(a) Diffusion of one solute
Equilibrium
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Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another
No work must be done to move substances down the concentration gradient
The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen
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(b) Diffusion of two solutes
Fig. 7-11b
Net diffusion
Net diffusion
Net diffusion
Net diffusion
Equilibrium
Equilibrium
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Osmosis is the diffusion of water across a selectively permeable membrane
Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration
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Lowerconcentrationof solute (sugar)
Fig. 7-12
H2O
Higher concentrationof sugar
Selectivelypermeablemembrane
Same concentrationof sugar
Osmosis
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Tonicity is the ability of a 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
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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
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Hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments
The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump
Video: Video: ChlamydomonasChlamydomonas Video: Video: ParameciumParamecium Vacuole Vacuole
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Fig. 7-14
Filling vacuole 50 µm
(a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm.
Contracting vacuole
(b) When full, the vacuole and canals contract, expelling fluid from the cell.
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Cell walls help maintain water balanceA plant cell in a hypotonic solution swells until
the wall opposes uptake; the cell is now turgid (firm)
If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt
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Video: PlasmolysisVideo: Plasmolysis
Video: Turgid Video: Turgid ElodeaElodea
Animation: OsmosisAnimation: Osmosis
In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis
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In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane
Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane
Channel proteins includeAquaporins, for facilitated diffusion of waterIon channels that open or close in response to a
stimulus (gated channels)
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Fig. 7-15
EXTRACELLULAR FLUID
Channel protein
(a) A channel protein
Solute CYTOPLASM
Solute Carrier protein
(b) A carrier protein
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Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane
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Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria
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Facilitated diffusion is still passive because the solute moves down its concentration gradient
Some transport proteins, however, can move solutes against their concentration gradients
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Active transport moves substances against their concentration gradient
Active transport requires energy, usually in the form of ATP
Active transport is performed by specific proteins embedded in the membranes
Animation: Active TransportAnimation: Active Transport
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Active transport allows cells to maintain concentration gradients that differ from their surroundings
The sodium-potassium pump is one type of active transport system
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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
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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
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Na+ binding stimulatesphosphorylation by ATP.
Fig. 7-16-2
Na+
Na+
Na+
ATP P
ADP
2
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Fig. 7-16-3
Phosphorylation causesthe protein to change itsshape. Na+ is expelled tothe outside.
Na+
P
Na+ Na+
3
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Fig. 7-16-4
K+ binds on theextracellular side andtriggers release of thephosphate group.
P P
K+
K+
4
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Fig. 7-16-5
Loss of the phosphaterestores the protein’s originalshape.
K+
K+
5
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Fig. 7-16-6
K+ is released, and thecycle repeats.
K+
K+
6
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Fig. 7-17Passive transport
Diffusion Facilitated diffusion
Active transport
ATP
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Membrane potential is the voltage difference across a membrane
Voltage is created by differences in the distribution of positive and negative ions
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Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane:A chemical force (the ion’s concentration
gradient)An electrical force (the effect of the membrane
potential on the ion’s movement)
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An electrogenic pump is a transport protein that generates voltage across a membrane
The sodium-potassium pump is the major electrogenic pump of animal cells
The main electrogenic pump of plants, fungi, and bacteria is a proton pump
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Fig. 7-18
EXTRACELLULARFLUID
H+
H+
H+
H+
Proton pump
+
+
+
H+
H+
+
+
H+
–
–
–
–
ATP
CYTOPLASM
–
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Cotransport occurs when active transport of a solute indirectly drives transport of another solute
Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell
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Fig. 7-19
Proton pump
–
–
–
–
–
–
+
+
+
+
+
+
ATP
H+
H+
H+
H+
H+
H+
H+
H+
Diffusionof H+
Sucrose-H+
cotransporter
Sucrose
Sucrose
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Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins
Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles
Bulk transport requires energy
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In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents
Many secretory cells use exocytosis to export their products
Animation: ExocytosisAnimation: Exocytosis
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In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane
Endocytosis is a reversal of exocytosis, involving different proteins
There are three types of endocytosis:Phagocytosis (“cellular eating”)Pinocytosis (“cellular drinking”)Receptor-mediated endocytosis
Animation: Exocytosis and Endocytosis IntroductionAnimation: Exocytosis and Endocytosis Introduction
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In phagocytosis a cell engulfs a particle in a vacuole
The vacuole fuses with a lysosome to digest the particle
Animation: PhagocytosisAnimation: Phagocytosis
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Fig. 7-20PHAGOCYTOSIS
EXTRACELLULARFLUID
CYTOPLASM
Pseudopodium
“Food”orother particle
Foodvacuole
PINOCYTOSIS
1 µm
Pseudopodiumof amoeba
Bacterium
Food vacuole
An amoeba engulfing a bacteriumvia phagocytosis (TEM)
Plasmamembrane
Vesicle
0.5 µm
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)
RECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein
Coatedvesicle
Coatedpit
Ligand
Coatprotein
Plasmamembrane
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)
0.25 µm
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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
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In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles
Animation: PinocytosisAnimation: Pinocytosis
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Fig. 7-20b
PINOCYTOSIS
Plasmamembrane
Vesicle
0.5 µm
Pinocytosis vesiclesforming (arrows) ina cell lining a smallblood vessel (TEM)
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In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation
A ligand is any molecule that binds specifically to a receptor site of another molecule
Animation: Receptor-Mediated EndocytosisAnimation: Receptor-Mediated Endocytosis
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Fig. 7-20cRECEPTOR-MEDIATED ENDOCYTOSIS
Receptor Coat protein
Coatedpit
Ligand
Coatprotein
Plasmamembrane
0.25 µm
Coatedvesicle
A coated pitand a coatedvesicle formedduringreceptor-mediatedendocytosis(TEMs)
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Fig. 7-UN1
Passive transport:Facilitated diffusion
Channelprotein
Carrierprotein
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Fig. 7-UN2
Active transport:
ATP
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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
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Fig. 7-UN4
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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
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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
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