Elizabeth S.

asked • 10/16/17

Active ion channels

I'm wondering about active versus passive ion channels. I know that gated ion channels are considered active, but why? Since ions still flow with their concentration gradient and not against it. Mostly referring to the sodium and potassium gated ion channels in neurons.

Elizabeth S.

So with regards to ion channels being active does not necessarily mean the use of ATP?
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10/17/17

J.R. S.

tutor
They do not use ATP as in Na/K ATPase (sodium potassium pump) for the flow of ions down their concentration gradient, but the ATPase is used to pump the ions back across the membrane against the gradient.  As an example, if an agonist acts at the nicotinic acetylcholine receptor (an active ion channel), it will stabilize it in the open state and Na+ will enter the cell, thus depolarizing the cell.  This does not require ATP.  But later, in order to pump the Na out of the cell and return it to normal, ATP may be used, but it is pumped out by a different mechanism/channel. Make sense?
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10/17/17

Elizabeth S.

Yes that makes sense, so the active gated ion channel that allows them to come in does not use ATP, but the sodium potassium pump does use ATP. I still don’t know why they are called active channels, since the sodium PUMP is the one using ATP, not the Ligand Or voltage gated channel, though. activation Or inactivation of a gate doesn’t take energy, does it?
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10/17/17

J.R. S.

tutor
Well, it doesn't take energy in the sense that we usually think of energy, and it doesn't use ATP as the energy source. But it is called "active" only to distinguish it from something that is passive.  In the passive case, there is no "stimulus" or messenger.  I can see where your confusion may arise since most often when we speak of an "active" process we think of ATP involvement. I guess they are "active" because they move ions and they aren't passive.  How's that for a cop out?
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10/17/17

1 Expert Answer

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J.R. S. answered • 10/17/17

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Ph.D. in Biochemistry--University Professor--Chemistry Tutor
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The active ones are "activated" by a messenger, such as a neurotransmitter.  Of course, not all neurotransmitters are ionotropic as some are metabotropic and do not function via ion channels.  However, back to ion channels.  The passive channels just operate all the time via diffusion and do not need to be activated by a messenger.
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