Allow for communication between cells.

Act to produce reaction w/in another cell.

Classified by receptor site.

Membrane-bound

Intracellular

Three actions of the alpha subunit

in a G Protein

1. Alter membrane permeability

2. Activate Intracellular mediators

3. Alter functional cellular pathways

• 1. Chemical binds with integral protein causing conformational change.
• 2. γ, β and α subunits separate.
• 3. α subunit is energized by binding GTP and dephosphorilating to GDP and P_i
• 4. α subunit leaves the membrane and works in cell or shifts over within the membrane.

• 1. Na+ channel is closed
• 2. Acetylcholine binds to receptor sites on channel.
• 3. Channel opens and Na+ diffuses through the open channel.

Cyclic guanine monophosphate (cGMP)

Cyclic adenosine monophosphate (cAMP)

Inositol triphosphate (IP₃)

Nitric oxide (NO)

Non-gated

Voltage-Gated

Ligand-Gated

Other-gated

Also known as leak channels.

Always open.

Permeable to specific ions.

Open or close in response to a ligand binding to a receptor.

Common in glands and nervous and muscle tissue.

Ex: acetylcholine-sodium ion channel.

Gated ion channels that respond to stimuli other than ligands or voltage changes.

Present in specialized electrically excitable tissues.

Depolarization

Repolarization

After Potential

Depolarization Phase

(Voltage Gated Ion Channels)

1. Voltage gated Na+ ion channels open. 
2. Na+ ions enter cell.
3. Voltage-gated K+ channels begin to open.
4. Na+ ions reduce electrical gradient between intracellular and extracellular fluid.

Repolarization Phase

(Voltage Gated Ion Channels)

1. Voltage-gated Na+ ion channels close.
2. Voltage-gated K+ ion channels remain open until RMP is re-established.
3. K+ ions move out of cell.

Charge is naturally negative on the inside of the cell membrane.

Between -70 and -90 mV

3 Na+ out

2 K+ in