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# Permeability and membrane potentials

Find out why a cell that is permeable to multiple ions has a membrane potential that is influenced by the ion with the highest permeability. Rishi is a pediatric infectious disease physician and works at Khan Academy. Created by Rishi Desai.

## Want to join the conversation?

• Honestly feels like this video's target audience are high school kids not college graduates preparing for the MCAT.

12 minute video, I don't want to skip around to avoid skipping something vital... and 10 minutes ended up being redundant information.

Save yourself 12 minutes:
1) Most cells in the body aren't just permeable to one ion
2) To Calculate the membrane potential of the cell that's permeable to multiple ions, you take the % movement of each ion, multiply each by their respective membrane potential, and add them all up together.
• I didn't fully understood permenbility, can somebody explain it to me?
• Permeability in this context refers to how easily a molecule can pass through the cell membrane between the internal cytoplasm and external interstital fluid. Several things dictate how permeable the membrane is to a substance, such as the substances size (is it small enough to easily pass between the phospholipid bilayer- e.g. water), or is it lipid-soluble (as the membrane consists of lipids, they will often resist larger molecules that are lipid-insoluble). permeability to Ions as in this lesson is largely governed by intrinsic membrane proteins (carrier or channel proteins) that can ferry the ions to and fro whilst circumventing the phospholipids. these can allow them to pass down their concentration gradient (facilitated diffusion) or 'pump' them against the gradient to artificially influence it (active transport).
• Why are we picking percentages out of the air for this? isn't there a biologically determined amount for each ion or does the percentage change all the time? It's not clear from this lesson.
• My understanding is that the permeability for each ion would indeed be biologically determined, but that it would change based on the cell type; different functions, different needs for ions, different permeability to get around those needs. I think the main take away by having the numbers be random is that, in the case of an MCAT question, values will be provided. You just have to know what to do with those values :)
(1 vote)
• What exactly is membrane potential? Can someone please explain this to me simply?
I'm guessing it's the difference in the charge between a cell and its outside environment, but I'm not sure.
• you're right. it's the difference of charges. that's what makes the cell positive or negative. remember that the cell has a negative charge most of the time and that opposites attract. therefore, a positive cell will attract anions and remove cations and vise versa.
• Why would K+ have a negative membrane potnetial? Do other cations?
(1 vote)
• It's a property of K+. It has a ek (equilibrium reduction potential) of -96mV. That is to say in addition to balancing charges, potassium will rest (find equilbrium) on a membrane with the energy potential of -96mV (balancing electrical and concentration gradients). Sodium and calcium are both positive. This value is dependent on the Nernst Equation (http://en.wikipedia.org/wiki/Nernst_equation).
• At how does the permeability of the cell change?
• by opening of certain channels allowing entry of ions. The opening of these channels is triggered either mechanically (Heat, amount of cholestrol in the cell membrane) or by a neurotransmitter or by hormone stimulating opening of the channels
• I enjoyed this series Rishi, thank you.
• Can someone explain to me how does hypokalaemia and hyperkalaemia affect the membrane potential, threshold, heart rate , SV?
• These two videos have been professionally produced and answer your questions. The movement of ions is complex and interrelated with other ions and hormones.