[Music] hi it's Paul Anderson most people are familiar with the chemical gradient as seen in diffusion so if I add some red food coloring to this warm water the molecules are going to quickly distribute throughout that whole container what's going on at the molecular level well those red food coloring molecules have a certain amount of kinetic energy they're bouncing off of each other due to brownie in motion and they're going to move along their chemical gradient or their concentration gradient from an area of high concentration to an area of low concentration this is how diffusion works this is how oxygen gets into your cells and carbon dioxide gets out and it's real visual we can see what's going on but this is only one half of a more complex gradient called the electrochemical gradient and to understand complex biological systems like what causes a resting potential in your cells or how excitable cells like the neuron can fire off an action potential we really have to understand this electrochemical gradient what's going on start with a simulation this is a pH simulation I'll put a link in the description down below what we've got is some green food coloring on the bottom and we're going to put it in motion there's 50 molecules on the bottom now we're going to allow it to bounce around I'm going to pause for a second and we're going to open up these channels these channels allow that green food coloring to to move through so you should make a prediction of what you think is going to happen on this simulation if we let it go so let's watch what happens and you probably got this right so there molecules have a certain amount of kinetic energy this random brownie in motion is going to move them up from the bottom to the top and from the top to the bottom so we started with 50 and zero and now we have rely 25 on either side now they can still move back and forth but that chemical gradient that concentration gradient has gotten much much smaller and so when we started by opening up that channel that chemical gradient continues to shrink as they move that random walk from the bottom to the top that's diffusion and it's really visual you can see it but let's make this simulation a little bit more complex imagine I put some potassium chloride on the bottom and we dissolve it in water so again this is an ionic compound it's breaking apart into its ions we've got the potassium ions which have a positive charge and the chloride ions which have a negative charge and I put 50 of each on the bottom we start them moving and now I'm going to open up some channels that only allow potassium to move through so the only thing that can move through here is going to be the potassium ions the chloride ions can't move through again we're putting 50 of each on the bottom so predict what you think is going to happen if we let this run for a while hopefully you've made a prediction and let's see what happens so this is probably what you thought was going to happen we're getting some of that potassium to go through and so you probably got half of this problem right so there are 50 chloride on the bottom zero on the top because the chloride can't come through but it's not 25 and 25 when it comes to the potassium ions it's more like 37 to 13 so it's almost like there's a magical Force that's pushing them in the other direction from the top to the bottom and that is the electrical gradient and and it's counteracting that chemical gradient and it's the combination of those two that is the electrochemical gradient let's zoom into that membrane and I'll show you what's going on so once a pottassium goes through once this potassium ion goes through again it can go through this channel it's got a charge and as it moves through we have an increase in the positive charge outside the membrane and then now we have a relative negative charge on the inside of the membrane because we've lost that potassium ion each time a potassium ion goes through we're building up a charge we've got a positive charge on the on the outside of this membrane and a negative charge on the inside now if we think about these ions for a second these are all positive charges or like charges and you probably know that like charges repel in other words they don't like spending time around each other and so there's going to be a force that's pushing them in the other direction and also since they have a positive charge they're going to be attracted to the relative negative charge that's on the bottom and so what happens over time is we have a balance now of the chemical gradient that's pushing it in the from the bottom to the top this potassium and then we have this electric gradient that's moving in the other direction the combination of those two is an electrochemical gradient and it's reached a equilibrium or a potential we could put use a volt meter and we could we could measure the charge on either side of that and since we have a separation of the charge now we've got a potential now we've got a voltage that we could actually measure and so what's causing that resting potential in cells is simply the permeability of different ions moving across that membrane now we can look at the concentration inside and outside and use the nerst equation to figure out what that voltage is actually going to be and so the University of Arizona has put together a little simulator I'll put a link to that down below as well and you could run uh the typical potassium levels in a generic human cell and it'll show you what's going on again there's going to be more potassium on the inside of the cell than the outside we could do the same thing with other big ions like sodium we'll have more sodium on the outside and we could get a potential or a voltage there we could do the same thing with chloride ions as well or we could run the ner or the Goldman equation rather with all of these ions and it's going to show us what the voltage of that generic cell is going to be so what are we seeing we're seeing the molecules we're seeing that chemical gradient but what I want you to remember is there's also that electrical gradient the one that we can't see that's based on the charge and to really understand complex biological systems you have to understand both parts of that gradient that's electrochemical gradient and I hope that was helpful [Music]