Confused by bioelectrical phenomenon

[Smachine]

Hey, first post in this forum!

Ive been reading a lot of books related to biology and the more I read the more confusing the subject of electricity in living organisms gets. For example, a lot of books introduce membrane potentials as something that can be measured using two microelectrodes and voltmeter, placing one in the intracellular and one in the extracellular fluid. But how does it really work like that? When I normally think about a voltmeter, I imagine it as ammeter with a component of high resistance and there is a current flow that is converted into a value of voltage, but when the voltage is caused by small separation of ions I don't see how it really works then. I mean, the situation is similar to capacitor, and the plasma membrane works as dielectric, but If I would connect both ends of capacitor with a resistive path current proportional to the voltage created between the plates would flow and it could be measured, but I don't get how it would be done when the charge separation comes from ions not electrons.
Even more confusing seems the electric mechanism of defense of some fish, like electric ray.
If I imagine that the fish has two points in the body with potential difference between them and current can flow from one to the other. Why would the fish discharge when those two points come in contact with the victim, but not while it flows freely around?

I hope I made some sens with this, because english is my second language.

 

[mazinse]

doesnt it work either way, if the ions are positive, the electrons in the voltmeter copper circuit still get transferred because of the difference in potential.

 

[Andy Resnick]

You forget that ions do indeed pass through the membrane (through channel proteins and transporter proteins). There is a real, measurable current through epithelial monolayers due to a net transport of sodium ions, for example.

I used to remember how electric eels worked... there are specialized organs that hold the charge and specific discharge routes, that's all I can remember.

 

[Smachine]

You forget that ions do indeed pass through the membrane (through channel proteins and transporter proteins). There is a real, measurable current through epithelial monolayers due to a net transport of sodium ions, for example.

I used to remember how electric eels worked... there are specialized organs that hold the charge and specific discharge routes, that's all I can remember.

Thanks for replays guys. Yes, i know about voltage gated/ligand gated ion channels and Na⁺/K⁺ pump. And I understand how they work together to create both resting and action potentials. What I don't understand is how you can detect changes in membrane potential when those channels open and there is an influx of Na⁺ ions and potential turns positive. Also how the voltage gated channels really work. How I would imagine the situation is that there is an imbalance of ions and the most energetically effective way for them is to collect as close as possible to the membrane. And the electric field from this imbalance would be directed perpendicular to the membrane. Now if the membrane channel proteins have some charged moving parts they could respond to this field force and at certain voltages block or allow the ability of ions to flow trough. Please correct me if I am assuming something wrong.

 

[alxm]

Thanks for replays guys. Yes, i know about voltage gated/ligand gated ion channels and Na⁺/K⁺ pump. And I understand how they work together to create both resting and action potentials. What I don't understand is how you can detect changes in membrane potential when those channels open and there is an influx of Na⁺ ions and potential turns positive.

Electrochemical potential can be measured very accurately, the real problem of measuring this stuff is how get it it all set up with one electrode on each side of the membrane. It's tricky but they can do it. There was a big debate that went on for over 10 years (between Mitchell and Wikström, IIRC) on whether Cytochrome C Oxidase really moved protons across the membrane or not, which is indicative of the difficulties involved in the experiments.

Also how the voltage gated channels really work. How I would imagine the situation is that there is an imbalance of ions and the most energetically effective way for them is to collect as close as possible to the membrane. And the electric field from this imbalance would be directed perpendicular to the membrane.

Well, if or when there is (e.g. the inner mitochondrial membrane), they don't actually gather near the membrane, because the field is not that strong. Nevertheless, there's indeed a potential there.

Now if the membrane channel proteins have some charged moving parts they could respond to this field force and at certain voltages block or allow the ability of ions to flow trough. Please correct me if I am assuming something wrong.

Well, many ion channels only allow motion in one direction, and are usually selective for a particular ion as well. MacKinnon got the Nobel prize for largely figuring out how K-ion channels do this. (I only remember parts of the story, so I'm not going to attempt to explain it, I'd probably get something wrong) He shared with Peter Agre, who studied water channels (aquaporin), which is also an interesting story - because letting H₂O through without letting H₃O⁺ through isn't trivial. It's enough to have a chain of hydrogen-bonded waters across the membrane and protons would get through via a grotthuss mechanism. Aquaporin 'solves' this by having a largely hydrophobic water channel (discouraging charged ions from getting in), and in the middle there's a residue, an asparagine IIRC, that causes the chain of water molecules to switch orientation, breaking the hydrogen-bond network.

As for Cytochrome c Oxidase - nobody really knows how it stops protons from falling back through after it's 'pumped' them up. The structure and reaction mechanism is more or less known, but how the actual 'pumping' happens isn't, either. If you can figure it out, there may be a Nobel prize in waiting for you! :)

 

[pgardn]

You forget that ions do indeed pass through the membrane (through channel proteins and transporter proteins). There is a real, measurable current through epithelial monolayers due to a net transport of sodium ions, for example.

I used to remember how electric eels worked... there are specialized organs that hold the charge and specific discharge routes, that's all I can remember.

I think I remember reading that most of these animals able to use a lot of current as a defense mechanism actually had stacked plates of either cartilage or some sort of protein like keratin with a goo in between so they basically have a voltaic stack or battery. I found this fascinating.