sfb1006 & sfp1005 l2 internal communication - cell membrane and rmp

8/2/2019 Sfb1006 & Sfp1005 L2 Internal Communication - Cell Membrane and RMP

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Communication & control

The cell membrane & cell excitability

Lecture 3

Marieb & Hoehn (2009) Ch 3 & Ch 11

Sherwood (2009) Ch 3


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Lecture Objectivesby the end of the lecture you should be able to

1. List the functions of proteins in the plasma membrane

2. Describe the process of facilitated diffusion & active transport using specificexamples

3. Describe the process of exocytosis & endocytosis using specific examples

4. Define resting membrane potential (RMP) & give a typical value using theappropriate units

5. Explain the 3 reasons why a separation of charge exists across the cellmembrane

6. Calculate the equilibrium potential for Na+, K+ & Cl-

7. Describe how the movement of K+, Na+ & the NaK ATPase pump contribute tothe RMP


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Plasma Membrane StructureFluid lipid bilayer embedded with proteins

Construction of a cell membrane - http://www.wisc-online.com/objects/index.asp?objID=AP1101
http://www.wisc-online.com/objects/index.asp?objID=AP1101http://www.wisc-online.com/objects/index.asp?objID=AP1101http://www.wisc-online.com/objects/index.asp?objID=AP1101http://www.wisc-online.com/objects/index.asp?objID=AP1101


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Plasma Membrane Structure Phospholipids

Polar end is hydrophilic; nonpolar end is hydrophobic Carbohydrates

Small amount on outer surface only Cholesterol

Contributes to fluidity & stability of cell membrane

Proteins Attached to or inserted within lipid bilayer Functions of membrane proteins

Form water-filled channels across lipid bilayer Serve as carrier molecules

Serve as docking-marker acceptors Membrane-bound enzymes Receptor sites Cell adhesion molecules (CAMs) Proteins on surface are important in cells ability to recognize

self & in cell-to-cell interaction


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Membrane Transport

Cell membrane is selectively permeable

Two properties influence whether substances can

permeate the cell membrane without assistance

Relative solubility of the particle in lipid

Size of the particle


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Membrane Transport

Unassisted membrane transport, i.e. molecules canpenetrate through the plasma membrane on theirown

Diffusion

Osmosis

Assisted membrane transport or carrier-mediatedtransport

Facilitated transport

Active transport

Vesicular transport


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Diffusion Uniform spreading of molecules due to their random

motion

They move from area of high concentration to area of lowconcentration

Ions are influenced by concentration & charge, hence theymove down the concentration & electrical gradient, calledan electrochemical gradient

Process is crucial to survival of every cell

Plays important role in Exchange of O2 & CO2 between blood & air in lungs Movement of substances across kidney tubules Movement of ions during the action potential


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Osmosis

Net diffusion of waterdown its ownconcentration gradient

Important in

the movement of fluidacross secretoryepithelia e.g. lungepithelium & sweat

gland Cell volume

regulation


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Carrier-mediated transport

Accomplished by membrane carrier protein

Can be active transport or passive transport(facilitative diffusion)

Characteristics that determine the kind and amountof material that can be transferred across themembrane

Specificity

Saturation

Competition


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Comparison of simple diffusion & facilitativediffusion

Rate oftransport ofmoleculeinto cell

Concentration of transported molecules in ECF

Low ------------------------------------------------------ > High

Simplediffusion downconcentrationgradient

Transportmaximumor T m

Facilitated diffusion(carrier-mediatedtransport downconcentration gradient)

Substances move from ahigher concentration to a

lower concentration

Requires protein carriermolecule

Means by which glucoseis transported into cells


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Active transport

Moves a substance against its concentration gradient

Requires a protein carrier molecule

Primary active transport

Requires direct use of ATP, e.g. Ca2+ ATPase pump & NaKATPase pump

Secondary active transport

Driven by an ion concentration gradient established by a primaryactive transport system, e.g. Na+-Ca2+ exchanger or Na+-glucosetransporter in the GIT

http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#
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Vesicular Transport - Active

Material is moved into or out of the cell wrapped in membrane

Two types of vesicular transport

Endocytosis

Process by which substances move into cell

Pinocytosis (cell drinking) nonselective uptake of ECF Receptor-mediated selective uptake of a large molecule, e.g.

insulin and iron

Phagocytosis selective uptake of multimolecular particle, e.g.neutrophils (leukocytes) phagocytose bacterial particles

Exocytosis

Provides mechanism for secreting large polar molecules, insulin& catecholamines

http://highered.mcgraw-

hill.com/sites/0072437316/student_view0/chapter6/animations.html#
http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.htmlhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.htmlhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.htmlhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.htmlhttp://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html


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Measuring the Resting Membrane Potential (RMP)


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Resting Membrane Potential (RMP)

A separation of charge exists across themembrane of all cells due to

an unequal distribution of key ions

the selective permeability of the membrane the Na+K+ ATPase pump

pumps 2K+ into the cell for every 3Na+ out of the cell

A typical value of RMP is -70mV withreference to inside the cell


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Concentrations & permeability of ions responsiblefor the RMP in a typical mammalian cell

Concentration(mM)

RelativePermeability

Ion Extracellular Intracellular

Na+

155 15 1

K+

5 160 50-75

A-

0 65 0

Cl-

70 5 50


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ICF

ECF

K+

A-

Distribution of K+ ions

K+

Concentration gradient

Electrical gradient

-

-

-

-

-

-

-

+

+

+

+

+

+

+


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-

-

-

-

-

-

-

Na+

Distribution of Na+ ions

ICF

ECF

Cl-

Na+

Concentration gradient

Electrical gradient

+

+

+

+

+

+

+


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Equilibrium Potential for an ion Equilibrium is reached when there is no net flux of an ion

The voltage measured across the membrane at this point iscalled the equilibrium potential for that ion

It can be calculated using the Nernst equation;

E= 61 log Co where:CI

E= equilibrium potential for ion in mV

61 is a constant incorporating the universal gas constant (R), absolutetemperature (T), the ions valence (z), an electrical constant Faraday (F)and the conversion of the natural logarithm (ln) to the logarithm to thebase 10 (log); 61 = RT/zF

Co is the extracellular concentration of ion(mM)

Ci is the intracellular concentration of ion(mM)


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Calculations

Calculate the EK, ENa and ECl1. EK =

2. ENa =

3. ECl =

What direction is the driving force for K+, Na+ and Cl- ifthe RMP is -70mV?

1. Driving force for K+

2. Driving force for Na+

3. Driving force for Cl-


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ECF

K+

Effect of concurrent K+ & Na+ movement on RMP

K+

EK ~-90mV

Na+Na+

ENa ~60mV

2 K+

3 Na+

RMP =-70mV


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Action Potential in a single axon

-90

0

-70

-50

+30

1 2

threshold

Approaches EK

ApproachesENa

Why is

the RMPcloser toEK thanENa?


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Summary

A typical value for the RMP is -70mV with reference to the ICF

Both K+ and Na+ ions play an important role in the establishmentof the RMP

The equilibrium potential for a particular ion is the voltage at

which there is no net movement of that ion

The RMP is closer to EK, rather then ENa, because the restingmembrane is more permeable to K+

At RMP neither K+ or Na+ ions are at equilibrium, hence passivediffusion of these ions is prevented by the action of NaK ATPase

sfb1006 & sfp1005 l2 internal communication - cell membrane and rmp

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