Galvanic Electrolytic Cells & Neurons

[scarterinscrubs]

I've read somewhere that the MCAT may use Galvanic and Electrolytic Cells to explain neuron resting potentials. Could someone explain how they could tie in these to topics?

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[DrMattOglesby]

actually thats a REALLY good idea!
i think ill base my neurophysio lecture tonight on that very topic!
or at least do my best to do so...
ill let you know how it goes and if i happen to find a good way of explaining it.

 

[jdla]

Can someone explain how the galvanic cell work?

 

[tncekm]

I've read somewhere that the MCAT may use Galvanic and Electrolytic Cells to explain neuron resting potentials. Could someone explain how they could tie in these to topics?

I'm definitely not seeing it. Both galvanic and electrolytic cells have one thing in common: a current, or in other words, a transfer of electrons through a difference in electric potential (Voltage = V = Δ&#934 either as a result of spontaneous redox reaction (galvanic aka voltaic) or to induce non-spontaneous redox reactions (electrolytic).

The resting potential and action potential do not involve the movement of electrons, nor does it directly influence redox chemistry, as far as I know. Its the movement of ions (or discharge) across a membrane which acts as a capacitor.

If someone can draw a relevant analogy between how a galvanic or electrolytic cell is analogous to an axon membrane during an action potential, I'd love to see it--that one has me stumped.

 

[BloodySurgeon]

There is a famous study about the potassium channels in squid neuron cell bodies and they discovered interesting info on the action potential. The guy won a Nobel prize for it so I think you should look it up. But basically the set up involves a voltage battery that is clamped at a specific voltage. And if the charge is not equal to the magnitude of an action potential then the battery will input charge equal to the differences of the voltage. now this current will go through an ammemeter and finally to the squid neuron. Now if the charge surrounding the neuron is positive, what will happen. If the charge surrounding the neuron, what will happen.

Hint: think of where the Na+ and K+ ions go when the medium is positive vs. negative? How do we have action potentials.

I don't remember the small details, but the study was something like this. It can be applicable to neuro/physics.

 

[scarterinscrubs]

I will look that up later tonight when I try to figure it out. I probably wont spend too much time on it because I need to finish reviewing all this stuff and start doing practices.

 

[BloodySurgeon]

I will look that up later tonight when I try to figure it out. I probably wont spend too much time on it because I need to finish reviewing all this stuff and start doing practices.

Actually don't even waste ur time... no point looking up these experiments b/c the probability it will show up is 1 in a million and it will not be applicable otherwise. Know the basics, if u see it on a test, they will give u enough info to figure it out. I just gave an example cuz u ask for one. Forget me saying look it up

 

[RSAgator]

Honestly, I don't really know how much of an analogy you can draw between the two. The only similarity I see is that it's an electrochemical interaction in which chemical energy (in the form of a concentration gradient) is converted into electrical energy. There is no redox pair, no phase separation, etc.

Maybe it can act as an electrolytic cell where the driving force is ATP (required for active transport against the concentration gradient). Also perhaps it's the Na+/K+ pump acting as the theoretical redox pair. This is all conjecture and I would imagine that if it were on a passage the question would be far clearer.



As far as how a galvanic cell works:

It's pretty simple but there are a few components to it so it's probably easier to understand if you have a visual aid. The galvanic cell converts chemical energy into electrical energy via a redox reaction. A basic galvanic cell consists of an anode where oxidation occurs and a cathode where reduction occurs.

The anode is a solid immersed in a solution of the ionic form of that solid (the cathode is the same but a different solid). If the solid is oxidized, it loses electrons and turns into a positively charged ion. Thus the metal on the cathode is oxidized to a metal ion which then floats into the solution. The electrons that are lost travel through a wire to the cathode where the metal on is reduced (gains electrons) and the ions in the solution become solid on the cathode.

This continues until all the potential is used up at which point the two halves are in equilibrium. The reason that two separate compounds create a potential difference is because different compounds have differing affinities for electrons (reduction potentials). Some like to give them away and some like to receive them. Put something that loves to donate electrons with something that loves to accept them and you create a strong potential difference or a strong driving force. I left out some details and simplified some things because it's all in a review book so I'd suggest you go over it and if you have any specific questions ask them.

 

[wannabedocta]

As a neuro major, I have never encountered any instance of the electrolytic or galvanic cells being used as an analogy for neurons. Neuron physiology is more closely related to physics topics like ohm's law, and electricity. Galvanic cells and Electrolytic cells are used for electroplating, one is spontaneous, the other is not.

 

[wannabedocta]

There is a famous study about the potassium channels in squid neuron cell bodies and they discovered interesting info on the action potential. The guy won a Nobel prize for it so I think you should look it up. But basically the set up involves a voltage battery that is clamped at a specific voltage. And if the charge is not equal to the magnitude of an action potential then the battery will input charge equal to the differences of the voltage. now this current will go through an ammemeter and finally to the squid neuron. Now if the charge surrounding the neuron is positive, what will happen. If the charge surrounding the neuron, what will happen.

Hint: think of where the Na+ and K+ ions go when the medium is positive vs. negative? How do we have action potentials.

I don't remember the small details, but the study was something like this. It can be applicable to neuro/physics.

Hodgkin/Huxley. Yea this experiment is based on ohm's law. V=IR, but i don't think it is related to electrolytic or galvanic cells.

 

[BloodySurgeon]

true it's testing ohms law not galvanic or electrolytic cell... it was the closest I could think of.

 

[Kaustikos]

So then I CAN'T hook the red and black to my ears and connect the red to my battery to jump start my car?

I knew I should've put false on that question for the mcat.

 

[fileserver]

"Galvanic and Electrolytic Cells to explain neuron resting potentials."

Totally a conjecture: (After reading BS's post) You can put either a galvanic or electrolytic cell directly to the neuron. In other words, one end of an electrode will be near the neuron so that the galvanic or electrolytic cell would influence the neuron potential (positive/negetive charge). Therefore, you can stimulate action potential of the neuron or hyperpolarized the neuron to prevent them from fire an action potential by varying which end of the electrode you put into the cell.

Edit: you can think of E of the galvanic cell as the concentration gradient of Na
electron moving toward the cathode as depolarization of the neuron
salt brigde as the repolarization of the neuron
how fast the current by the resistance of the wire => the speed of action potential on myelinate/unmyelinate

so on...

 

[Kaustikos]

"Galvanic and Electrolytic Cells to explain neuron resting potentials."

Totally a conjecture: (After reading BS's post) You can put either a galvanic or electrolytic cell directly to the neuron. In other words, one end of an electrode will be near the neuron so that the galvanic or electrolytic cell would influence the neuron potential (positive/negetive charge). Therefore, you can stimulate action potential of the neuron or hyperpolarized the neuron to prevent them from fire an action potential by varying which end of the electrode you put into the cell.

Edit: you can think of E of the galvanic cell as the concentration gradient of Na
electron moving toward the cathode as depolarization of the neuron
salt brigde as the repolarization of the neuron
how fast the current by the resistance of the wire => the speed of action potential on myelinate/unmyelinate

so on...

Yes, but that's just grasping at straws for an analogy that works well. I honestly don't know of any professors that actually go by galvanic cells. most speak in terms of resistance/voltage, at most.

The best analogy: Action Potential in terms of stacking dominoes
Stacking them is repolarization. now remember, that there is a refractory period (ie, don't knock those damn dominoes until I get to at least half-way done with stacking them! It's cooler!) and then the action potential is the tipping of dominoes - the one falling affecting the next. Guess what, you're the Potassium. Yes, YOU! You are the one responsible for bringing back the state to resting - so pick those dominoes up because we want to see that again.

 

[Rofeh20]

I don't recall the particulars of the analogy, but the gist is that a neuron functions like an RC circuit. If you're really interested I would recommend checking the Purves Neurobiology textbook.

 

[BerkReviewTeach]

I've read somewhere that the MCAT may use Galvanic and Electrolytic Cells to explain neuron resting potentials. Could someone explain how they could tie in these to topics?

The analogy you are speaking about probably stems from the duplicity of the Nernst equation.

It can be used to determing the emf of an electrochemical cell by summing the standard emf and the 0.059/n log Q, where Q for a typical simple galvanic cell is [anode cation]/[cathode cation]. There is a minus sign before the 0.059 part.

It can also be used to determine an action potential, although there is no electrochemical reaction going on, so the action potential (in millivolts) is found using only emf = 60 log [out]/[in]. Someone can (should) correct me if there is a sign I'm forgeting, because biological sciences is not my area of teaching.

I think the analogy between electrochemical cells and neurons is the rational behind using the Nernst equation for both.

And has several have pointed out, there is a correlation of circuitry (V = IR) to neurons, although from my perspective it's more a case of neurobiology being just another example of physics in action.

 

[jdla]

What is the difference between a galvanic cell and a electrolytic cells?

 

[jdla]

Honestly, I don't really know how much of an analogy you can draw between the two. The only similarity I see is that it's an electrochemical interaction in which chemical energy (in the form of a concentration gradient) is converted into electrical energy. There is no redox pair, no phase separation, etc.

Maybe it can act as an electrolytic cell where the driving force is ATP (required for active transport against the concentration gradient). Also perhaps it's the Na+/K+ pump acting as the theoretical redox pair. This is all conjecture and I would imagine that if it were on a passage the question would be far clearer.



As far as how a galvanic cell works:

It's pretty simple but there are a few components to it so it's probably easier to understand if you have a visual aid. The galvanic cell converts chemical energy into electrical energy via a redox reaction. A basic galvanic cell consists of an anode where oxidation occurs and a cathode where reduction occurs.

The anode is a solid immersed in a solution of the ionic form of that solid (the cathode is the same but a different solid). If the solid is oxidized, it loses electrons and turns into a positively charged ion. Thus the metal on the cathode is oxidized to a metal ion which then floats into the solution. The electrons that are lost travel through a wire to the cathode where the metal on is reduced (gains electrons) and the ions in the solution become SOLID on the cathode.

This continues until all the potential is used up at which point the two halves are in equilibrium. The reason that two separate compounds create a potential difference is because different compounds have differing affinities for electrons (reduction potentials). Some like to give them away and some like to receive them. Put something that loves to donate electrons with something that loves to accept them and you create a strong potential difference or a strong driving force. I left out some details and simplified some things because it's all in a review book so I'd suggest you go over it and if you have any specific questions ask them.

Why does the ions on the cathode turn solid?

 

[tf2medic]

Why does the ions on the cathode turn solid?

who cares! just know that it does! i just took my real MCAT and it definitely had a galvanic cell on it. EK told me that cathode is ALWAYS reduction and that's all i knew....and it was all i needed on the test!!

 

[wannabedocta]

Why does the ions on the cathode turn solid?

because it is gaining electrons and hence being oxidized

X^2+(aq) + 2e- --> X (s)

 

[wannabedocta]

who cares! just know that it does! i just took my real MCAT and it definitely had a galvanic cell on it. EK told me that cathode is ALWAYS reduction and that's all i knew....and it was all i needed on the test!!

Is that true for all cathodes, i.e. cathodes in cathode ray tubes, etc.??

 

[jdla]

because it is gaining electrons and hence being oxidized

X^2+(aq) + 2e- --> X (s)

I know but I still don't understand.

 

[iA-MD2013]

Is that true for all cathodes, i.e. cathodes in cathode ray tubes, etc.??

Cathode is always reduction and Anode oxidation...right? The charge is what changes.

 

[tncekm]

Is that true for all cathodes, i.e. cathodes in cathode ray tubes, etc.??

Yeah, cathode always reduces and anode always oxidizes... or as Jordan and John say: An Ox, Red Cat!

The one difference is that in a galvanic cell the anode is negatively charged and the cathode is positively charged where as with the electrolytic cell the flow is backward so the positive and negative signs are switched.

Think of the negative side as the side the electrons are originating from and the positive side as the side the electrons are flowing to.

 

[jdla]

who cares! just know that it does! i just took my real MCAT and it definitely had a galvanic cell on it. EK told me that cathode is ALWAYS reduction and that's all i knew....and it was all i needed on the test!!

lol thanks.

 

[jdla]

How does the salt bridge minimize the potential difference in a galvanic cell?

 

[wannabedocta]

How does the salt bridge minimize the potential difference in a galvanic cell?

I think it prevents charge separation so that transfer of electrons from the anode to the cathode in a galvanic cell can ensue to its maximum

 

[iA-MD2013]

I think it prevents charge separation so that transfer of electrons from the anode to the cathode in a galvanic cell can ensue to its maximum

exactly. If it weren't for the salt bridge, a capacitor would form where the solution with the anode is negatively charged while the solution with the cathode is positively charged.

 

[tncekm]

From what I know, the salt bridge is just the electrical conductor that connects the two separated reactions prohibiting the build up of charge.

Maybe I don't understand the question well, or your wording isn't clicking with my brain, but I'm note sure if it minimizes the potential difference in the galvanic cell--to me, that means lowering voltage.

I think without the salt bridge the two half reactions would reach an electronic equilibrium quickly where the build up of negative charge on the cathode side would prevent the oxidation at the anode side.

edit: looks like two ppl beat me to it

 

[iA-MD2013]

From what I know, the salt bridge is just the electrical conductor that connects the two separated reactions prohibiting the build up of charge.

Maybe I don't understand the question well, or your wording isn't clicking with my brain, but I'm note sure if it minimizes the potential difference in the galvanic cell--to me, that means lowering voltage.

I think without the salt bridge the two half reactions would reach an electronic equilibrium quickly where the build up of negative charge on the cathode side would prevent the oxidation at the anode side.

edit: looks like two ppl beat me to it

haha yeah...except i think i switched the charges. A positive charge buildup on the anode side and a negative charge on the cathode...right?

 

[jdla]

From what I know, the salt bridge is just the electrical conductor that connects the two separated reactions prohibiting the build up of charge.

Maybe I don't understand the question well, or your wording isn't clicking with my brain, but I'm note sure if it minimizes the potential difference in the galvanic cell--to me, that means lowering voltage.

I think without the salt bridge the two half reactions would reach an electronic equilibrium quickly where the build up of negative charge on the cathode side would prevent the oxidation at the anode side.

edit: looks like two ppl beat me to it

How does it prohibit the build up of charges?

 

[tncekm]

haha yeah...except i think i switched the charges. A positive charge buildup on the anode side and a negative charge on the cathode...right?

Actually, I'm not sure we'd even get a capacitor? I was thinking of it as more of losing the potential difference which was driving the reaction? Like, eventually the charge would be pretty evenly distributed?

Now I'm confused

I'm gonna dig into it later when I get back from volunteer work

 

[tncekm]

How does it prohibit the build up of charges?

Here is the wikipedia answer (hopefully it helps, I'm still a bit confused):

As electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. With the charge difference balanced, electrons can flow once again, and the reduction and oxidation reactions can proceed. In general, keeping the two cells separate is preferable from the point of view of eliminating variables from an experiment. When no direct contact between electrolytes is allowed, there is no need to make allowance for possible interactions between ionic species.
http://en.wikipedia.org/wiki/Salt_bridge

 

[iA-MD2013]

Actually, I'm not sure we'd even get a capacitor? I was thinking of it as more of losing the potential difference which was driving the reaction? Like, eventually the charge would be pretty evenly distributed?

Now I'm confused

I'm gonna dig into it later when I get back from volunteer work

Why wouldn't you get a capacitor...It's just an equal charge separation. But, you're right, you would lose potential difference. You're producing a positive charge on the anode side and you're losing positive charge (producing negative charge) on the cathode side, so you formed a capacitor!
But, I'm confused now too...So I could be completely wrong.

 

[jdla]

Here is the wikipedia answer (hopefully it helps, I'm still a bit confused):

As electrons leave one half of a galvanic cell and flow to the other, a difference in charge is established. If no salt bridge was used, this charge difference would prevent further flow of electrons. A salt bridge allows the flow of ions to maintain a balance in charge between the oxidation and reduction vessels while keeping the contents of each separate. With the charge difference balanced, electrons can flow once again, and the reduction and oxidation reactions can proceed. In general, keeping the two cells separate is preferable from the point of view of eliminating variables from an experiment. When no direct contact between electrolytes is allowed, there is no need to make allowance for possible interactions between ionic species.
http://en.wikipedia.org/wiki/Salt_bridge

I know that the salt bridge balances the charge difference.

Now my question is:

What is the cause of the charge difference?

 

[wannabedocta]

Yea, you'd get a capacitor in the absence of a salt bridge. TPR says pg 464 on my 2006 review book

 

[iA-MD2013]

Yea, you'd get a capacitor in the absence of a salt bridge. TPR says pg 464 on my 2006 review book

Awesome...thanks!

 

[wannabedocta]

I know that the salt bridge balances the charge difference.

Now my question is:

What is the cause of the charge difference?

The redox reaction itself is the cause of charge separation. The transfer of electrons from the anode to the cathode causes the resultant anions, cations to build up. I think this would be more clear if you had a diagram, otherwise it seems like this argument is a vicious circle.

 

[jdla]

The redox reaction itself is the cause of charge separation. The transfer of electrons from the anode to the cathode causes the resultant anions, cations to build up. I think this would be more clear if you had a diagram, otherwise it seems like this argument is a vicious circle.

I think I understand better now.

 

[jdla]

Does anyone understand on AAMC 3 Item 48?

 

[BloodySurgeon]

Does anyone understand on AAMC 3 Item 48?

Yes.

Zn2+ + 2 e- ---> Zn E=–0.763 V

Which means that it is a bad oxidizing agent (or Zn2+ doesnt want to be reduced)

If we reverse the equation:

Zn -----> Zn2+ + 2 e-

The standard potential for the reaction will be E= +0.763 V

Which is saying that Zn is a good reducing agent or likes to be oxidized.

Now if you react Zn w/ HCl:

Zn + HCl ---> H2 + ZnCl2 (by knowing that Zn has an oxidation number of +2)

Well an outside knowledge is useful for this problem. The only way to make H2 gas is to reduce an acid with a reducing agent. We learned earlier that Zn is a good reducing agent and therefore this reaction will take place.

 

[tncekm]

Why wouldn't you get a capacitor...It's just an equal charge separation. But, you're right, you would lose potential difference. You're producing a positive charge on the anode side and you're losing positive charge (producing negative charge) on the cathode side, so you formed a capacitor!
But, I'm confused now too...So I could be completely wrong.

Okay, I just busted out an old G. Chem book and it confirmed which side was pos/neg. There is charge build up (which can be discharged when its connected by a salt bridge, thus it is a capacitor) and the negative charge would build up on the side to which the electrons were flowing to, and the positive side would be where the electrons were flowing from.

So, anode loses electrons and becomes positive as a result, and the cathode gains electrons and becomes negative as a result.

 

[iA-MD2013]

Okay, I just busted out an old G. Chem book and it confirmed which side was pos/neg. There is charge build up (which can be discharged when its connected by a salt bridge, thus it is a capacitor) and the negative charge would build up on the side to which the electrons were flowing to, and the positive side would be where the electrons were flowing from.

So, anode loses electrons and becomes positive as a result, and the cathode gains electrons and becomes negative as a result.

ok...that makes sense. Thanks tncekm! It's kinda confusing keeping track of the charges. I'm going to make sure that I have it all memorized rather than trying to figure it out during the real thing.

 

[unsung]

How does it prohibit the build up of charges?

Okay, correct me if I'm wrong guys... but I just think of the salt bridge in a galvanic cell as the inside of a battery: in an electrical circuit, as electrons flow from the anode of the battery through the circuit, back to the cathode of the battery, some kind of chemical process in the battery is working to maintain the potential difference.

I.e., if there wasn't a chemical process going on in the battery, the cathode of the battery would quickly become negative with all the electrons being pushed towards it by the process at the anode that's pushing electrons away from it. So... there would no longer be a potential difference in the circuit.

Same thing with the salt bridge in the galvanic circuit- it's like the chemical process that takes place in a battery that helps maintain the potential difference across the anode/cathode.

There's a little problem with taking that analogy too far, as the salt bridge isn't the source of the electrons- the metal at the anode is- whereas in an electric circuit, the anode of the battery is more like the metal at the anode...

Does that analogy work ?? I'm still trying to figure out a good way to think about this...

 

[jdla]

You are saying that the salt bridge maintains the potential difference between the cathode and anode so the the electrons can flow to the cathode.

Okay, correct me if I'm wrong guys... but I just think of the salt bridge in a galvanic cell as the inside of a battery: in an electrical circuit, as electrons flow from the anode of the battery through the circuit, back to the cathode of the battery, some kind of chemical process in the battery is working to maintain the potential difference.

I.e., if there wasn't a chemical process going on in the battery, the cathode of the battery would quickly become negative with all the electrons being pushed towards it by the process at the anode that's pushing electrons away from it. So... there would no longer be a potential difference in the circuit.

Same thing with the salt bridge in the galvanic circuit- it's like the chemical process that takes place in a battery that helps maintain the potential difference across the anode/cathode.

There's a little problem with taking that analogy too far, as the salt bridge isn't the source of the electrons- the metal at the anode is- whereas in an electric circuit, the anode of the battery is more like the metal at the anode...

Does that analogy work ?? I'm still trying to figure out a good way to think about this...

 

[jdla]

Yes.

Zn2+ + 2 e- ---> Zn E=–0.763 V

Which means that it is a bad oxidizing agent (or Zn2+ doesnt want to be reduced)

If we reverse the equation:

Zn -----> Zn2+ + 2 e-

The standard potential for the reaction will be E= +0.763 V

Which is saying that Zn is a good reducing agent or likes to be oxidized.

Now if you react Zn w/ HCl:

Zn + HCl ---> H2 + ZnCl2 (by knowing that Zn has an oxidation number of +2)

Well an outside knowledge is useful for this problem. The only way to make H2 gas is to reduce an acid with a reducing agent. We learned earlier that Zn is a good reducing agent and therefore this reaction will take place.

Why is Zn2+ a bad oxidizing agent?

 

[tncekm]

Because the process of gaining electrons, for Zn, is non spontaneous at standard conditions.

The E value for the reduction of Zn (meaning its acting as an oxidant) is -0.763V.

From the equation:

&#916;G&#176;=-nFE&#176; we can see that at standard conditions this reaction is non spontaneous because E&#176; < 0 means that &#916;G&#176; > 0.

 

[jdla]

Because the process of gaining electrons, for Zn, is non spontaneous at standard conditions.

The E value for the reduction of Zn (meaning its acting as an oxidant) is -0.763V.

From the equation:

&#916;G°=-nFE° we can see that at standard conditions this reaction is non spontaneous because E° < 0 means that &#916;G° > 0.

Okay thanks.

 

[Kaustikos]

You are saying that the salt bridge maintains the potential difference between the cathode and anode so the the electrons can flow to the cathode.

Yes, precisely.