Changes in electrical gradients

• Electrical disequilibrium• Consequences of electrical disequilibrium• Resting membrane potential• Equilibrium potential• Membrane depolarization and

hyperpolarization

Cell in the body are:

• In chemical disequilibrium• In osmotic equilibrium• In electrical disequilibrium – few extra

negative ions inside cells and their matching positive ions are outside

Na+

Cl-

Organic anions

K+

Na+

Cl-

OrganicAnions

K+

Distribution of main ions

Na+

Cl-

Organic anions

K+

Na+

Cl-

OrganicAnions

K+

ATPase

3 Na+

2 K+

Electrical disequilibrium across the cell membrane membrane potential difference

Anionic proteins are trappedInside the cell

How does electrical charge separation occur?

There are more positive charges outside and more negative charges inside

The cell membrane Is an insulator

Na+

Cl-

Organic anions

K+

Na+

Cl-

OrganicAnions

K+

Electrochemical gradient is a combination of the electrical and chemical gradients

Electrochemical gradient

• Electrical gradients and chemical gradients across the cell membrane

• Electrical force moves K+ into the cell (cell has more neg. charges)

• Chemical gradient favors K+ to leave the cell (K+ concentration is low outside)

• These forces reach a steady state

Membrane Resting Potential

• The voltage difference across the cell membrane when there is an electrochemical gradient at a steady state

• There is a voltage difference between the inside and the outside (potential difference)

The value for the resting membrane potential

Membrane Potential

• Vm is the membrane potential (millivolts)• Resting membrane potential for nerves

and muscles is -40 mV to -90 mV• The resting membrane potential is

determined by K+

K+ channels are open during the resting membrane potential.

If K+ channels are open.

Equilibrium Potential

• The membrane potential when the channels for a particular ion are open is called the equilibrium potential for that particular ion.

• At EK+ the rate of ions moving in due to the electrical gradient equals the rate of ions leaving because of the concentration gradient.

• EK+ is close to the resting membrane potential

Factors that are important for the equilibrium potential for an ion:

• Only channels for that ion are open• The charge of the ion• Concentration of the ion inside the cell• Concentration of the ion outside the cell

At the equilibrium potential for Na+

Artificial cell, Na+ is leaving because the inside became + after the inwardMovement of Na+

Currents during resting membrane potential

K+ outward current is much stronger than Na+ inward current. Lots of K+ channels are open, few Na+ channels are open at rest.

Currents during resting membrane potential

K+ outward current is much stronger than Na+ inward current. Lots of K+ channels are open, few Na+ channels are open at rest.

The value for the resting membrane potential

Membrane potential changes when channels open or close.

Changes in membrane potential

• Resting membrane is polarized• Depolarization positive charges move

in membrane potential moves toward 00

-70mV

time

Membrane potential changes when channels open or close.

Changes in membrane potential

• Repolarization membrane potential returns to polarized state (+ charges leave cell)

• Hyperpolarizationmembrane potential becomes more negative than at rest (extra + charges leave the cell)

During changes in membrane potential

• Very few ions move to cause changes in membrane potential.

Large molecules can cross in vesicles.

• Cell expends metabolic energy

Phagocytosis – cell engulfs aparticle into a vesicle

Vesicular traffic across cell membranes

• Endocytosis– Pinocytosis, cell engulfs extracellular fluid– Receptor-mediated endocytosis

Receptor mediated endocytosis

LDL (which is a cholesterol carrier) is a ligand that enters by receptor mediated endocytosis

Exocytosis

• Some molecules leave a cell by exocytosis• E.g. proteins leave cells by exocytosis

Integrated membrane activity during insulin secretion

Resting membrane potential

Integrated membrane activity during insulin secretion