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Membrane potentials - part 1

Find out how a cell that is permeable to one ion can become charged (either positive or negative) if there is permeability and a concentration gradient. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

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• You say that () that the resting potential is at -92. mV
in my book and my teacher both said it is at -70 mV, who is right?
• In the video, he says that the equilibrium potential for K+ is -92mV, which means that there's an outward gradient for potassium ions. The membrane potential is a little bit different: it accounts for the permeability of the membrane to the relevant ions, and includes the equilibrium potentials for sodium and chloride ions. It varies depending on cell type, but the resting membrane potential is usually around -70mV.
• @ when you draw those channels for the K+ ions.
When they are in the cell you, say they are attracted to the anions and that's the way they are hold up in a cell.
How then they break the force of attraction between them (K+ and anions) when they move out through the channels?
I mean they do need some energy to break the force, where do they get the energy? Can you explain in some other video or give me reference to this doubt?
• The K+ are moving along the concentration gradient from inside the cell (cytosol) to the space surrounding the cell (extracellular space), because (as he correctly mentioned in the video) the ratio of K+ in- and outside the cell is 150:5! Since the K+ Channels are specific to K+ Ions the Anions are held up within the cell.

The energy for the K+ to "break" their bond with the Anions is provided by the concentration gradient provided by the Na+/ K+ ATpase which is the pump he drew in the beginning of the video. This pump uses a LOT of energy in most cells in our body (usually about 1/3 of all the energy that is used in our cells!)

• at : how does the k+ channels of the cell determine if it is a potassium ion knocking on its door and not some other ion?
• The Ion channels of our cells are made up of proteins. In the case of the K+ channels the proteins within the channel twist and form in a way that only allows K+ ions to pass!

To further explain: The channels vary both in size and their electrical charge, depending on what ion they need to let through the membrane.

In the case of the K+ channels the channel must be just broad enough to let K+ (and no other ions!) through and it's electrical charge must be just right for the K+ not to be either repelled or "stuck" in the channel because the attraction is too big for it to pass through.

• At , Rishi points out that anions exist within the cells for the potassium cations, etc. to sit with, what are these anions and why do they pre-exist in the cell? Also why do they stay within the cell when the potassium cations cross the channels due to the concentration gradient? Is it because no protein transports are available for this to happen or something else that I missed? Thanks
• How does the sodium get in and the K get out? I don't understand the equilibrium potential. When would there be an EP? Could you help me understand this concept?
• Is membrane potential the same as voltage and electric potential difference, but in this case, applied to membrane as a system?
• Yes, it's just a potential difference measured in volts.
• at , Rishi is basically saying that - regarding the actual number of ions we are considering - few movements of ions in and out don't really change the concentrations.

However, in the "correction to Sodium-potassium pump" video (health and medicine > Advanced nervous system physiology > Nervous system introduction), Sal mentions the importance of the Sodium-potassium pump in maintaining the concentration gradient. Why is it so, if - as it said in this video - the change in concentration is neglectable? Does it have something to do with the number of cells we are talking about?
• Are the concentrations of K+ inside (150mMol/L) and outside (5mMol/L) the cell as shown in this video constant?