Membrane permeability

Define:

Extracellular

Intracellular

Intercellular

Diffusion & Flux

A Unidirectional flux

D_mK

P = ----------------

c

Where, D_m = diffusion coefficient

K = partition coefficient of substance

(lipid-water diffusion property)

c = thickness of membrane

Permeability (P)

n rate at which a substance passively

penetrates a membrane

n P can be altered by hormones, temperature, electrical charge of

membrane and other factors. All these changes in P result in changes in the physical properties of the membrane.

Reaching equilibrium

Two factors:

1. ionic concentration

2. charge of particles (or electrical field across the membrane)

electrochemical gradient; sum of above forces, i.e.,

concentration & electrical gradients.

Equilibrium potential

The potential (voltage) difference over a membrane which exactly counteracts the electrochemical gradient, so as to prevent a net tranmembrane flux.

Or: the potential at which an ion is in electrochemical equilibrium.

n Depends on the ion concentration on each side of the membrane. (also Temperature)

[O]

Eq Potential = .058 log₁₀ ---------

[I]

.058 V = 58 mV

Donnan Equilibrium

n The equilibrium that results when a membrane is freely permeable to water and electrolytes but totally impermeable to at least one species of ion confined to one compartment.

[K⁺]_I [C1-]_II

--------- = ----------

[K⁺]_II [C1-]_I

Assumptions:

1. electroneutrality (I vs. II)

2. ions move in pairs, + with –

3. at equilibrium, ion flux in one direction equals flux in the other direction.

Start Equilibrium

	® Time

K

I II I II

A

KA Start Equilibrium

I II B I II

4-15 (A) When KCI is added to compartment I of a container divided by a permeable membrane, K⁺ and CI^- diffuse across the membrane until the concentrations are equal on either side.

(B) If the potassium salt of an impermeant anion is added to compartment I, some K⁺ and CI^- diffuse into compartment II until electrochemical equilibrium is reestablished. It should be noted that these chambers (unlike the living cell) are not distensible.

[Na⁺]_o = 120

[K⁺]_o = 2.5

[Ca²⁺]o < 2.0

[Ci^-]_o = 120

Ion in the myoplasm. Because the list of ions is incomplete, the totals do not balanced out perfectly. [A-], represents the molar equivalent negative charges carried by various impermeant anions.

Representative concentrations of

Common Ions on the inside and outside

of a vertebrate skeletal Muscle cell.

The concentrations are in millimoles per

liter. The concentration given for intra-

cellular ca 2+ is for the free, unbound,

and unsequestered.