a, b, c, d all move solutes by diffusion down concentration gradient

Final mechanism can work against gradient

e. Active transport

XXX XXXXX

e. Active transport

XXX XXXXX

e. Active transport

XXX XXXXX

Pump Protein

e. Active transport

XXX XXXXX

e. Active transport

XXX XXXXX

e. Active transport

XXX XXXXX

ADP + Pi

e. Active transport

XXX XXXXX

e. Active transport

XXXXXXXXX

Concentrates against gradient

Ion pumps

Uniporter (one solute one way):

I- pump in thyroid

Coupled transporters (two solutes)

Symporter (same direction):

Antiporter (opposite directions)

Na+/K+ ATPase in mitochondria

3. Cells can control solute distribution across their membranes by controlling:

a. Synthesis of integral proteins

b. Activity of integral proteins

c. E supply for pumps

Therefore, expect that solutes would be unequally distributed across membranes

4. Actual ion distributions

Squid Axon (mM):

ION [CYTOPLASM] [ECF]

Na+ 50 460

K+ 400 10

Cl- 40 540

Ca++ <1 10

A- 350 <1

Organic anions with multiple - charges

COO- on proteins, sulfates, phosphates, etc....

5. Reasons for unequal distribution

a. Metabolic production of organic anions

A- produced by biosynthetic machinery inside the cell

b. Membrane permeability

impermeable to A-

moderate Cl- permeability

30-50X more permeable to K+ than Na+

Given a and b, system passively comes to unequal ion distribution

Diffusion of ions governed not only by their concentration gradients, but also their electrical gradients

Permeable uncharged solutes will come to equilibrium across membranes if no other forces acting

1 M sucrose

1 M sucrose 0.5 M sucrose

0.5 M sucrose

Permeable charged solutes will not come to concentration equilibrium across

membrane if other charged impermeable solutes are present

Impermeable

K+ Cl-

Permeable

K+ Cl-

Cl-Na+

K+ Cl-

Cl-Na+

K+ Cl-

At equilibrium: chemical force driving K+ out

Cl-Na+

K+ Cl-

is exactly balanced by the electrical force (electromotive force) holding K+ in

Cl-Na+

K+ Cl-

Result: an unequal ion distribution which will be maintained passively

Cl-Na+

K+ Cl-

Result: an unequal ion distribution which will be maintained passively

“Donnan Equilibrium”

Donnan Equilibrium resembles situation in real cell, with one exception:

cell is not maintained passively

Poison real cell and unequal distribution eventually goes away

c. Cells work via pumps to maintain unequal ion distribution

Na+ “leaks” in down chemical and electrical gradients

A-Na+Na+/K+ ATPase

If Na+ allowed to build up, inside becomes + , drives K+ out, and lose unequal distribution

Therefore, cells use combination of active and passive mechanisms to maintain unequal ion distributions

REASON?

B. Membrane Potentials

1. Significance of unequal distributions

Whenever an ion is unequally distributed across a membrane, it endows the membrane with an electrical potential

“membrane potential” (EM or VM)

2. Membrane potential measurement

a. Voltmeter

Inside is -80 mV

a. Voltmeter

b. Calculate with Nernst equation

EM = RT x ln[ion]outside

FZ ln[ion]inside

R = gas constant, T = abs. temperature

F = Faraday constant, Z = valance

Magnitude of the voltage due to 1 unequally distributed ion is directly proportional to the magnitude of its unequal distribution

BUT: can't use it for a real cell

only valid for 1 ion

only valid for freely permeable ions

Can use it to calculate voltage due to any one freely permeable ion in a mixture

e.g. K+ = -91 mV

Na+ = +65 mV

c. Alternative: GOLDMAN EQUATION

accounts for multiple ions

accounts for permeability of each

multiplies [ion] ratios X permeability constant for each ion, then sums

up all to get total membrane EM

d. CONCLUSION:

In ion mixture, each ion contributes to the overall EM in proportion to its permeability

Most permeable ions contribute the most charge

Which ion is most permeable?

real cell: inside is -80 mV = resting EM

cell is “negatively polarized”

EM is due almost exclusively to the unequal distribution of K+

Changes in [K+] alter EM easily

Changes in [Na+] do not alter EM

All cells have resting potential due to ion distributions

Some cells can use this electrical potential to transmit information

C. Nervous System Components

1. Glial cells: supportive

diverse functions

support

insulation

protection

communication

up to 90% of nervous system by weight

2. Neurons

soma: nucleus, usual organelles

dendrites: receptive, input

axon: transmission (microm to m)

axon terminals: synapse, output

3. Integrated Function of Neurons

Generate and conduct electrical signals for communication or coordination

a. Propagation of electrical signals along individual cells (wires)

b. Communication of electrical information between cells

c. Model system for study:

Squid giant axon (J.Z. Young)

c. Model system for study:

Squid giant axon (J.Z. Young)

D. Electrical Characteristics of Neurons

1. Intracelluar Recording:Hodgkin and Huxley

recording electrode

coupled with stimulating electrode

recording electrode

64Can change EM by adding charge

recording electrode

65Can change EM by adding charge

recording electrode

STIMULUS

STIMULUSmV

STIMULUS

RESPONSE OF CELL

STIMULUS

RESPONSE OF CELL

STIMULUS

RESPONSE OF CELL

Add negative charge,

EM gets more negative0

HYPERPOLARIZATION

-80EM moves away from 0 HYPERPOLARIZATION

Add positive charge,

EM gets more positive0

EM gets more positive

DEPOLARIZATION

EM gets more positive

DEPOLARIZATION

EM moves towards 0

2. Passive responses

a. Magnitude directly proportional to amount of current

Increase current: increase magnitude of passive depolarization

b. Magnitude inversely proportional to distance from stimulus

Die out locally

3. At some point, small increase in applied current triggers a membrane depolarization much greater than the stimulus current

Active response

ACTION POTENTIAL

Characteristics of Action Potentials:

a. Minimum stimulus necessary to elicit

“threshold” current raises membrane to threshold potential

b. Once stimulated, all-or-none event

c. Propagated over long distances without decrement

4. Voltage changes during action potentials

Time (msecs)

mVolts

Time (msecs)

mVolts

Time (msecs)0 1 2 3 4

mVolts

1. Resting membrane before arrival

mVolts

2. Depolarization to 0 mV

mVolts

2. Depolarization to 0 mV

hyperpolarizing overshoot

mVolts

3. Repolarization back to -80 mV

mVolts

4. Hyperpolarizing afterpotential

mVolts

5. Return to resting

a, b, c, d all move solutes by diffusion down concentration gradient

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