Chapter 3 The Plasma Membrane and Membrane Potentialbiology/Classes/255/Chapter3.pdf · 2009-10-05 · Membrane Potential VII edit. Pag 54-58, Pag 75-83 VI edit. ... Active transport

Chapter 3

The Plasma Membrane and

Membrane Potential

VII edit. Pag 54-58, Pag 75-83

VI edit. Pag 53-58, Pag 73-83

V edit. Pag 57-62, Pag 87-97

© Brooks/Cole - Thomson Learning

Cell Structure

Cells are the basic unit of structure and

function in living organisms

1) Plasma membrane

2) Nucleus

3) Cytoplasm

a) Endoplasmic

reticulum

b) Golgi complex

c) Mitochondria

d) Cytosol

e) …

Main cellular components:

-fluid lipid bilayer consisting of lipids and

proteins

-separates intracellular content from

external environment

-regulates the movement of nutrients

and substances in or out of the cell

(differential permeability)

What is the Plasma Membrane?

1) Phospholipids

2) Cholesterol

3) Proteins

4) Carbohydrates

What is composition of the Plasma

Membrane?

© Brooks/Cole - Thomson Learning

Phospholipid Bilayer

Cholesterol adds fluidity to the lipid

bilayer

© Brooks/Cole - Thomson Learning

Plasma Membrane

(Fluid Mosaic Model)

Proteins in the Plasma Membrane

1) Integral proteins: ion channels, receptors,

transporters or carriers

2) Peripheral proteins: enzymes

Ion Channels1) Leak channels

2) Voltage-activated ion channels

3) Ligand-gated ion channels

4) G-protein coupled receptors

1) Control the entry and exit of substances

into or out the cell

(transport & selective permeability)

2) Intercellular communication and signaling

3) Maintain a resting membrane potential

(RMP)

Functions of the Plasma Membrane

Separation of opposite charges (ions)

across the cell membrane:

membrane is polarized

What is the Membrane Potential?

It allows nerve/muscle cells to work:

generate and transmit electrical signals

What does the

Resting Membrane Potential

DO?

The intracellular and extracellular space of all

cells contain charged particles or ions: if

the number of negative and positive

charges is equal on both sides then the

membrane is electrically neutral

The intracellular and extracellular space of all

cells contain charged particles or ions: if

the number of negative and positive

charges is different on both sides then the

membrane is polarized

How is the Membrane Potential

generated?

Charged particles can not cross the cell

membrane easily -Why?

Ion channels allow some ions to

cross the cell membrane in a

selective manner

Notice: Some transporters or carriers also allow the selective

movement of ions across the cell membrane

When the membrane becomes permeable

to some ions but not others

a separation of charges occurs

Unequal distribution of ions across

the cell membrane generate the

Resting Membrane Potential (RMP)

Ion

Extracellularconcentration

(mM / L)

Intracellular

concentration(mM / L)

Relative

Permeability

K+ 5 150 50-75

Na+ 150 15 1

A- 0 65 0

Cl- ~100 ~7 ~10

Normal distribution of ions in a

neuron at rest

1) Unequal distribution of charged

particles or ions across the membrane

(electrochemical gradient)

2) Selective permeability to different ions

3) Active transport

How is the Resting Membrane

Potential generated?

I. How is the resting membrane potential

generated?

High concentration of negatively charged

ions inside the cell (generated by protein

groups) that can not cross the plasma

membrane

INOUT

II. How is the resting membrane potential

generated?

High concentration of positively charged

potassium (K+) ions inside the cell

that can cross the membrane easily

(by diffusion)

through leak potassium channels

INOUT

Why K+ ions do

not escape

from the

cytoplasm and

do not collapse

the unequal

distribution of

K+ if they can

cross the

membrane

easily?

INOUT

Potassium equilibrium potential

Nernst equation and K+ equilibrium

potential (at room temperature)

EK= 61 log Co/Ci ~ -90 mV

Co: extracellular K+ concentration

Ci: intracellular K+ concentration

INOUT

Potassium equilibrium potential

III. How is the resting membrane potential

generated?

High concentration of positively charged

sodium ions outside the cell

that cannot cross the membrane easily

WHY?

There are few leak sodium channels in the

membrane

Ion

Extracellularconcentration

(mM / L)

Intracellular

concentration(mM / L)

Relative

Permeability

K+ 5 150 50-75

Na+ 150 15 1

A- 0 65 0

Cl- ~100 ~7 ~10

OUT IN

Sodium equilibrium potential

Nernst equation and the Na+ equilibrium

potential (at room temperature)

ENa= 61 log Co/Ci ~ +60 mV

Co: extracellular Na+ concentration

Ci: intracellular Na+ concentration

Ion

Extracellularconcentration

(mM / L)

Intracellular

concentration(mM / L)

Relative

Permeability

K+ 5 150 50-75

Na+ 150 15 1

A- 0 65 0

Cl- ~100 ~7 ~10

The greater the permeability of an ion,

the greater the contribution of the ion to

the resting membrane potential

Resting Membrane Potential

In nerve cells the RMP is -70 mV

OUT IN

If the RMP is determined by the K+

permeability and the EK=-90 mV, why is the

RMP=-70 mV and not -90 mV?

Resting Membrane Potential

INOUT

Do negatively charged proteins

have an equilibrium potential? Why?

OUT IN

Depolarization and hyperpolarization

of the resting membrane potential

© Brooks/Cole - Thomson Learning

EK

ENa+60

-70

-90

0

IV. Role of active transport in the generation

and maintenance of the resting membrane

potential

Sodium-Potassium pump (Na+/K+ ATPase)

OUT IN

Sodium-Potassium Pump

http://www.brookscole.com/chemistry_d/templates/student_resources/shared_r

esources/animations/ion_pump/ionpump.html

Sodium-Potassium Pump (Na+/K+ ATPase)

-electrogenic active transporter (?)

-pumps out 3 Na+ for every 2 K+ pumped in

-pump activity is regulated by negative

feedback

-generate ~20% of the RMP

OUT

IN

What will happen if you inhibit

the Na+/K+ pump?

Digitalis-like compounds like digitoxin and

ouabain can block the Na+/K+ pump and cause

membrane depolarization of neuronal and

heart tissue: used to stimulate heart beats

after cardiac failure

purple foxglove, digitalis purpurea.

How do chloride ions Cl- move

across the cell membrane?

Ion

Extracellular concentration

(mM / L)

Intracellular

concentration(mM / L)

Relative

Permeability

K+ 5 150 50-75

Na+ 150 15 1

A- 0 65 0

Cl- ~100 ~7 ~10

Why is there more Cl- outside

than inside the cell?

What is the chloride equilibrium potential?

Nernst equation and the Cl- equilibrium

potential (at room temperature)

ECl= -61 log Co/Ci ~ -70 mV

Co: extracellular Cl- concentration

Ci: intracellular Cl- concentration

Will Cl- move in or out of the cell at rest?

OUT IN

The membrane potential determines the

movement of Cl- ions in or out of the cell

(passive distribution)

OUT IN

In what direction Cl- ions move if…

-membrane potential = -90 mV?

-membrane potential = -50 mV?

Ion

Extracellular conc

(mM / L)

Intracellular

conc(mM / L)

Relative

Permeability

Equilibrium

Potential

(mV)

K+ 5 150 50-75 -90

Na+ 150 15 1 +60

A- 0 65 0 none

Cl- ~100 ~7 ~10 -70

http://www.lifesci.ucsb.edu/~mcdougal/neurobehavior/modules_homework/lect2.dcr

http://www.sumanasinc.com/webcontent/anisamples/neurobiology/signaling.html

What is the contribution of all ions to the

resting membrane potential?

Goldman equation (at room temperature):

EV= 61 log [(PNa+ Co+ PK+Co + PCl-Ci) /

(PNa+Ci+ PK+Ci + PCl-Co)]

EV ~ -70 mV

Where: P= Permeability for Na+, K+, and Cl- ions

Co: extracellular ion concentration

Ci: intracellular ion concentration

Function of the Resting Membrane

Potential?

1) Generation of electrical signals (action

potentials and synaptic potentials) in excitable

cells: neurons and muscle cells

2) Secretion of hormones and other substances

from endocrine/glandular tissue

3) Maintenance of a constant milieu for cell

function

Conclusions:

1) The resting membrane potential (RMP) is

determined by: an unequal distribution of

charged particles across the membrane (K+, Na+,

A-), differential permeability of the membrane to

ions, and the work of the Na+/K+ pump

2) The RMP is close to the K+ equilibrium potential

because K+ ions are more permeable at rest

(through leak K+ channels)

3) Cl- ions are distributed passively across the

membrane, Cl- ions made the bulk of negatively

charged particles in the extracellular space