Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: Dudewtf on 18/08/2016 21:00:03
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I am reviewing for an exam and there is one question that I don't quite get. I translated it into English below
"In an electrically excitable cell with a resting potential of -70mV the following conditions are present (temperature is 37°C):
Intracellulär Extracellular
K+ 160nM 5mM
Na+ 20nM 140mM
Cl- 10nM 120mM
Ca2+ 100mM 1mM
1. When the cell depolarises [Ca2+] rises to 15uM, while the membrane potential reaches +61mV. Under these conditions , would Ca2+ move into or out of the cell?
2. Which way would Na+ move?
3. Which way would K+ move if Na+ were allowed to dominate?
I think applying the Nernst equation is part of the solution, but I'm obviously missing something important. I especially can't get question number 3. I would really appreciate your help!
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There are other considerations.
If you placed a lump of sugar in a glass of water, the sugar will dissolve and then diffuse in an attempt to create a uniform solution. This is driven by entropy and the second law; maximize entropy. The ions, in your example, are segregated at different concentration on each side of the membrane. This creates an concentration gradient and entropy potential for each ion. The entropy potential acts, side-by-side, with the charge potentials.
If you do just a charge balance, you will not expect sodium to spontaneously lead and go inside the cell, since there is already too much positive inside the cell. However, the sodium ions have a concentration gradient from outside to inside, so there is nevertheless a potential to flow inside, driven by entropy. The charge and entropy potential act in opposite directions, with the net total, the net push to act.
There is one more layer of potential. Potassium ions are called chaotropes, which means they create chaos/disorder in water. While sodium ions and calcium ions are kosmotropes and will create order in water. The chloride ions are in the middle, being of low impact on water. See the diagram below;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww1.lsbu.ac.uk%2Fwater%2Fimages%2Fchaotopes.gif&hash=c7d2c6917b5daff668d31f60848ec7a3)
All the ions around the membrane, need to move through water. Therefore, the state of the water; induced by the ions, will add a third leg of potential. Ordered water has lower structural entropy, than disordered water, which defines higher entropy. The net affect is the water on the inside has higher entropy while the outside water is lower entropy. The higher internal water entropy is needed to loosen and potentiate the cellular water for the enzymes. The water outside is more ordered by the sodium. What that order water does is create the analogy is higher surface tension. This makes the outside membrane amiable; lure, for organic to collect on the membrane; food attractant.
If potassium ions were to flow outside, they will carry their chaotropic nature and can help the external water gain entropy. There is a push by the water, on potassium ions, to go outside to cell to help the water gain entropy. The needs of the water allows the potassium ions to most feely flow across the membrane water, since it maximizes the second law for the water. Potassium has three legs of potential.
However, the movement of ions is regulated by protein channels. The net affect is one can start the discharge of potential using a less likely spontaneous ion, like calcium. Early cells, before all the bells and whistles, would be water and potassium driven. They would still make use of all the same ions, but with the channels less specific, potassium would lead the swapping; more cell division and less rest time.
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There are other considerations.
If you placed a lump of sugar in a glass of water, the sugar will dissolve and then diffuse in an attempt to create a uniform solution. This is driven by entropy and the second law; maximize entropy. The ions, in your example, are segregated at different concentration on each side of the membrane. This creates an concentration gradient and entropy potential for each ion. The entropy potential acts, side-by-side, with the charge potentials.
If you do just a charge balance, you will not expect sodium to spontaneously lead and go inside the cell, since there is already too much positive inside the cell. However, the sodium ions have a concentration gradient from outside to inside, so there is nevertheless a potential to flow inside, driven by entropy. The charge and entropy potential act in opposite directions, with the net total, the net push to act.
There is one more layer of potential. Potassium ions are called chaotropes, which means they create chaos/disorder in water. While sodium ions and calcium ions are kosmotropes and will create order in water. The chloride ions are in the middle, being of low impact on water. See the diagram below;
(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fwww1.lsbu.ac.uk%2Fwater%2Fimages%2Fchaotopes.gif&hash=c7d2c6917b5daff668d31f60848ec7a3)
All the ions around the membrane, need to move through water. Therefore, the state of the water; induced by the ions, will add a third leg of potential. Ordered water has lower structural entropy, than disordered water, which defines higher entropy. The net affect is the water on the inside has higher entropy while the outside water is lower entropy. The higher internal water entropy is needed to loosen and potentiate the cellular water for the enzymes. The water outside is more ordered by the sodium. What that order water does is create the analogy is higher surface tension. This makes the outside membrane amiable; lure, for organic to collect on the membrane; food attractant.
If potassium ions were to flow outside, they will carry their chaotropic nature and can help the external water gain entropy. There is a push by the water, on potassium ions, to go outside to cell to help the water gain entropy. The needs of the water allows the potassium ions to most feely flow across the membrane water, since it maximizes the second law for the water. Potassium has three legs of potential.
However, the movement of ions is regulated by protein channels. The net affect is one can start the discharge of potential using a less likely spontaneous ion, like calcium. Early cells, before all the bells and whistles, would be water and potassium driven. They would still make use of all the same ions, but with the channels less specific, potassium would lead the swapping; more cell division and less rest time.
Thank you for bringing the science.
Excellent answer.
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First of all, thank you very much for the explanation! However, it was somewhat obaove my level, I study medicinen and not chemustry, and barely Touch subjects like this.
Unfortunately I still don't quite understand the question. Could you perhaps explain it to me?
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However, the movement of ions is regulated by protein channels.
To be specific, the flow of ions is regulated by the voltage across the membrane protein i.e. the movement of the voltage sensor initiates a conformational change in the conducting pathway.