chapter 3 the plasma membrane and membrane potential explain how cell membrane constituents function...
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Chapter 3The plasma membrane and membrane potential
• Explain how cell membrane constituents function in creating membrane potentials. This will be measured by quiz and exam scores.
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Review
• Membrane structure and composition
• Cell to cell adhesions
• Membrane transport
New• Membrane potentials
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Plasma Membrane
• Forms outer boundary of every cell
• Controls movement of molecules between the cell and its environment
• Joins cells to form tissues and organs
• Plays important role in the ability of a cell to respond to changes in the cell’s environment
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Plasma Membrane Structure• Fluid lipid bilayer embedded with proteins
– Most abundant lipids are phospholipids
• Also has small amount of carbohydrates
– On outer surface only
• Cholesterol
– Tucked between phospholipid molecules
– Contributes to fluidity and stability of cell membrane
• Proteins
– Attached to or inserted within lipid bilayer
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Gated channelprotein
Lipidbilayer
Cholesterolmolecule
Leak channelprotein
ICF
Cell adhesion molecule (linking microtubule tomembrane)
Carrierprotein
Microfilamentof cytoskeleton
Phospholipidmolecule
Carbohydratechain
ECF
Appearanceusing an electronmicroscope
Dark line
Light space
Dark lineIntegralproteins
Receptorprotein
Receptorprotein
Proteins
Glycolipid Glycolipid
Fig. 3-3, p. 59
Plasma Membrane Structure
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Cell-To-Cell Adhesions– Extracellular matrix
• Serves as biological “glue”
• Major types of protein fibers interwoven in matrix– Collagen, elastin, fibronectin
– CAMs in cells’ plasma membranes
– Specialized cell junctions• Desmosomes
• Tight junctions (impermeable junctions)
• Gap junctions (communicating junctions
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Specialized Cell Junctions
Gap junctions• Small connecting tunnels formed by
connexons• Especially abundant in cardiac and smooth
muscle• In nonmuscle tissues permit unrestricted
passage of small nutrient molecules between cells
• Also serve as method for direct transfer of small signaling molecules from one cell to the next
Desmosomes• Act like “spot rivets” that anchor two
closely adjacent nontouching cells• Most abundant in tissues that are
subject to considerable stretching
Tight junctions• Firmly bond adjacent cells together• Seal off the passageway between the
two cells• Found primarily in sheets of epithelial
tissue• Prevent undesirable leaks within
epithelial sheets• C
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Lumen (contains undigested foodand potent digestive enzymes)
SELECTIVE PASSAGETHROUGH CELLS
Luminalmembrane
NO PASSAGEBETWEEN CELLS
Bloodvessel
Epithelialcell liningintestine
Basolateralmembrane
Cell 2
Cell 1
Lateralmembrane
Tightjunction
Fig. 3-5a, p. 63
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Membrane Transport
• Unassisted membrane transport
– Diffusion
– Osmosis
• Assisted membrane transport
– Carrier-mediated transport
– Facilitated transport
– Active transport
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If a substance canpermeate the membrane
If the membrane isimpermeable to a substance
Membrane
(a) Diffusion occurs (b) No diffusion occurs
KEY
= Penetrating solute
= Nonpenetrating solute
Fig. 3-8, p. 66
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Area A Area AArea B Area B
Diffusion from area Ato area B
Diffusion from area Ato area B
Diffusion from area Bto area A
Diffusion from area Bto area A
Net diffusion
(a) Diffusion (b) Equilibrium
No net diffusion
KEY
Differences in arrow length, thickness, and direction represent the relative magnitude of molecular movement in a given direction.
Net diffusion = Diffusion from area A to area B minus diffusion from area B to area A
= Solute molecule
Fig. 3-7, p. 65
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Membrane Transport
Factors affecting rate of diffusion collectively make up Fick’s law of diffusion:
• Magnitude (or steepness) of the concentration gradient
• Permeability of the membrane to the substance– Charge?
• Surface area of the membrane across which diffusion is taking place
• Molecular weight of the substance
• Distance through which diffusion takes place
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Membrane Transport
• Osmosis
– Net diffusion of
water down its
own concentration
gradient
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100% water concentration 0% solute concentration
90% water concentration10% solute concentration
(a) Pure water (b) Solution
KEY
= Water molecule
= Solute moleculeFig. 3-9, p. 67
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Normal cell volumeIntracellular fluid 300 mOsm/L
nonpenetrating solutes
300 mOsm/Lnonpenetrating solutes
200 mOsm/Lnonpenetrating solutes
400 mOsm/Lnonpenetrating solutes
H2O H2O
No net movement of water; no change in cell volume.
Water diffuses intocells; cells swell.
Water diffuses out ofcells; cells shrink.
(a) Isotonicconditions
(b) Hypotonicconditions
(c) Hypertonicconditions Fig. 3-13, p. 71
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Membrane TransportUnassisted membrane transportAssisted membrane transport• Carrier-mediated transport
– Accomplished by membrane carrier flipping its shape
– Can be active or passive– Characteristics that determine the kind and
amount of material that can be transferred across the membrane
• Specificity• Saturation• Competition
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Membrane Transport
Types of assisted membrane transport• Facilitated diffusion
• Active transport
• Vesicular transport
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Direction oftransport
Transportedsolute is released
and carrier proteinreturns to
conformation instep 1.
Plasmamembrane
Carrier protein takesconformation in which solutebinding site is exposed toregion of higher concentration.
Carrier protein changesconformation so that bindingsite is exposed to region oflower concentration.
Solutemolecule binds to carrier protein.
(Low)
(High)
ConcentrationgradientECF
ICF
Binding siteCarrier protein
Solute moleculeto be transported
1
2
3
4
Fig. 3-14, p. 72
Facilitated diffusion• Substances move from a higher
concentration to a lower concentration
• Requires carrier molecule• Means by which glucose is
transported into cells
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Membrane Transport
Active transport• Moves a substance against its concentration
gradient• Requires a carrier molecule• Primary active transport
– Requires direct use of ATP• Secondary active transport
– Driven by an ion concentration gradient established by a primary active transport system
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K+ concentrationgradient
2
High-affinity bindingsite for K+
Low-affinity bindingsite for Na+
3 Na+
Direction ofNa+ transport
3
2 K+
4
5
Direction ofK+ transport
Na+ concentrationgradient
2 K+
6
Stepped Art
Low-affinity bindingsite for K+
High-affinity bindingsite for Na+
Na+–K+ pump
3 Na+
Low K+High Na+
ECF
ICFHigh K+Low Na+
1
Plasmamembrane
Fig. 3-16, p. 75
Active TransportSodium Potassium PumpWhen open to the ECF, the carrier drops off Na+ on its high-concentration side and picks up K+ from its low-concentration side
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Active Transport• Moves a substance against its
concentration gradient.
• Primary active transport:
– Requires direct use of ATP
• Secondary active transport:
– Driven by an ion concentration gradient established by a primary active transport
– Two types, symport and antiport
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Transportedsolute in highconcentration
(a) Symport
Driving ionin highconcentration
Transportedsolute in lowconcentration
Driving ionin lowconcentration
Fig. 3-17a, p. 77
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Transportedsolute in lowconcentration
(b) Antiport
Driving ionin highconcentration
Transportedsolute in highconcentration
Driving ionin lowconcentration
Fig. 3-17b, p. 77
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Secondary Activetransport
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Active Transport• Moves a substance against its
concentration gradient.
• Primary active transport:
– Requires direct use of ATP
• Secondary active transport:
– Driven by an ion concentration gradient established by a primary active transport
– Two types, symport and antiport
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Carrier-mediated Transport Characteristics
• Specificity: Each carrier transports a specific substance or a few closely related compounds.
• Saturation: A limited number of carrier binding sites are available.
• Transport maximum (Tm): The amount of a substance transported in a given time.
• Competition: Several closely related compounds may compete for transport on the same carrier.
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Simple diffusiondown concentrationgradient
Carrier-mediatedtransport downconcentration gradient(facilitated diffusion)
Rat
e o
f tr
ansp
ort
of
mo
lecu
le i
nto
cel
l
Low High
Concentration of transported molecules in ECFFig. 3-15, p. 73
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Membrane Transport• Vesicular transport
– Material is moved into or out of the cell wrapped in membrane
– Active method of membrane transport– Two types of vesicular transport
• Endocytosis– Process by which substances move into cell– Pinocytosis – nonselective uptake of ECF– Phagocytosis – selective uptake of multimolecular
particle
• Exocytosis – Provides mechanism for secreting large polar molecules– Enables cell to add specific components to membrane
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Table 3-2b p80
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What is an excitable cell?
Membranes and their potentials are what make cells excitable.
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Membrane Potential
• Plasma membrane of all living cells has a membrane potential (polarized electrically)
• Separation of opposite charges across plasma membrane
• Due to differences in concentration and permeability of key ions
• Separated charges create the ability to do work like electrons in a battery.
• millivolt- 1/1000 volt
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Basic Physics
• Brownian motion
• Electrons protons neutrons
• Ohms law E=I*P
• Opposites attract, likes repel (hydrophobic/hydrophyllic)
• Potential and kinetic energy
• Velocity and force, F= MA
– Larger mass requires more force to move
– Objects in motion stay in motion unless there is friction and drag.
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Basic measurements
• Volt – unit of charge
– mv – 1/1000 volt
– Car battery =12V, Cell = -70 mv
• Watt = unit of power
– Kw = 1000 watts, light bulb = 60 wattsShearon Harris = 900 MW
• Ampere
– Unit of current• ma = 1/1000 ampere
• http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html
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Membrane Potential
Which has the greatest membrane potential?
B>A B<C
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Membrane Potential
• Nerve and muscle cells– Excitable cells– Have ability to produce rapid, transient
changes in their membrane potential when excited
• Resting membrane potential– Constant membrane potential present in cells
of nonexcitable tissues and those of excitable tissues when they are at rest
– Na+, K+, A-
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Membrane Potential
• Effect of sodium-potassium pump on membrane potential
– Makes only a small direct contribution to membrane potential through its unequal transport of positive ions
– The movement of ions and the large negatively charged proteins (A-) generate the potential difference.
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High-affinity bindingsite for K+
Low-affinity bindingsite for Na+
3 Na+
K+ concentrationgradient
Direction ofNa+ transport
2 K+
Direction ofK+ transport
Na+ concentrationgradient
Low-affinity bindingsite for K+
High-affinity bindingsite for Na+
Na+–K+ pump
2 K+
3 Na+
Low K+High Na+
ECF
ICF
High K+Low Na+
1
2
3
4
5
6
Fig. 3-16, p. 75
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ICF
ECF
(Passive)Na+–K+
pump (Active)
(Active)(Passive)K+ channelNa+ channel
Fig. 3-23, p. 79
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Table 3-3 p82
• Rp + -70mv• Variable from one cell to another• Poison eliminates this potential• Generated by the imbalance of ions in the intracellular and extracellular spaces.
Nernst Equation E=(61) log Co/Ci
60mv
-90mv
-70mv
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Nernst equation
• E=(61) log Co/Ci
• For Potassium Ek=(61) log 5mM/150mM
• For sodium ENa=(61) log 150mM/15mM
• Co concentration in the ECF• Ci concentration in the ICF• Used to calculate the contribution of ions to the resting potential of -70mv
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Resting potential
• EK = -90mv
• ENa = 60mv
• ECl = -70mv
• K and Na drive Cl gradient
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Usefulness?
• Neurons and muscle fibers can alter membrane potential to send signals and create motion.
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Plasma membrane
ECF ICF
Concentrationgradient for K+
Electricalgradient for K+
EK+ = –90 mVFig. 3-20, p. 83
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Plasma membrane
ECF ICF
Concentrationgradient for Na+
Electricalgradient for Na+
mostly
ENa+ = +60 mV
ECFanions,
Fig. 3-21, p. 84
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Plasma membrane
ECF ICF
and associated
Resting membrane potential = –70 mV
Relatively small net diffusion of Na+ inward neutralizessome of thepotential created by K+ alone
No diffusion of A– across membrane
Relatively large net diffusion of K+ outward establishesan EK+ of –90 mV
Fig. 3-22, p. 85
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ANIMATION: Resting Potential