to this point we've been talking about movement of uncharged molecules across membranes and the direction that these uncharged molecules are going to move will be determined by the concentration gradient across the membrane as well as the permeability of the membrane to the particular solutes or molecules we're talking about in this lecture we're going to be considering the movement of ions across the membrane ions are charged particles so therefore we have to consider the impact of the electrical forces and how that may affect the movement of molecules across the membrane now at its most fundamental level we this is nothing more than considering how magnets would interact with each other so when for example if we have two magnets that have the same polarity we place the same positive ends toward each other or the same negative ends toward each other we all know from experience that though that causes those magnets to tend to repel each other on the other hand if we have oppositely charged polarity of magnets and we place those ends those will tend to attract so building off of that is how we'll see how the electrical forces may impact the movement of ions now we need to develop an understanding of the different forces or gradients that are involved as we talk about ions so first off ions are also chemicals so imagine this is a cell and the yellow circle represents the relative concentration of a particular ion in this case we're showing that it's more concentrated outside the cell than inside the cell so one way that we can describe a gradient that's affecting the movement of this ion is by what we term the chemical gradient so the chemical or concentration gradient is one thing that we have to consider now in this slide we're demonstrating cells that we are saying in the green cell has a positive environment and the red cell has a negative environment inside so if we take a positively charged particle and place it inside the cell inside the positively charged cell or we take a negatively charged ion and place it inside of the negatively charged cell what we'd expect to see is that the electrical forces are going to tend to push that substance out of the cell just like if we had two magnets with the same polarity with the ends going toward each other they would tend to repel each other now if we had a positively charged ion outside of the cell with or I'm sorry a negatively charged ion outside of the cell with a positive environment then there would be an electrical force that would tend to draw that ion in likewise if we're dealing with a cell with a negative environment and we have a positively charged substance on the outside there would be an electrical force which tends to drop that ion in so what we have to do is to consider both of these forces both of the chemical force and the electrical force that's going to give us what we call the electrochemical gradient so let's look at the diagram on the left here we have a large red cell so this would be a negatively charged environment and we're dealing with an cation which is positively charged and on the left side it's more concentrated inside the cell than outside of the cell so on this example on the left we first would identify the direction of the chemical or concentration gradient and we would say it's going outward now I'm just arbitrarily giving a particular size of this Arrow indicating the magnitude of that charge of that Force now because we're dealing with a positively charged ion in a negatively charged environment as far as the cell then we're going to have an electrical force that's going to be tending to pull that positively charged ion into the cell or at least work to prevent it from leaving this would be the electrical gradient or the electrical force now to determine which direction this ion is going to move we add these two forces together we take the net effect so if both of the size of these arrows represents the relative magnitude of these forces the net effect would be going outward and we would call that the electrochemical gradient for this particular ion we switch things around on the right side so here we would draw the chemical gradient as moving in we're dealing with oppositely charged substances so the electrical gradient would be directed inward as well when we combine these two forces we will have an electroclimical gradient which is very strongwardly directed inward so it's important to understand we need to break down these two different types of forces or gradients we can look at the chemical gradient and that's going to be strictly dependent upon the concentration on one side of the membrane versus the other the direction of the electrical gradient is going to be determined by like charges repel opposite charges attract we then we'll combine those two gradients and come up with our electrochemical gradient which will be the force It ultimately determines which direction an ion is going to move now in this cell I've drawn both positively and negatively charged ions so the positively charged ions are cations and the negatively charged ions are anions in this picture I have equal number of cations and anions present therefore as we have it drawn here this cell would be electrically neutral and that's demonstrated on our small little altometer here now let's say that the cell is permeable to the cation well in that case the cell or the cation we tend to want to move out of the cell because of the relative concentration gradient now realize what happens when that cation leaves the cell is we basically have set up a situation now where we no longer have an equal number of positive and negative charges inside the cell we now have more negative charges on the inside and more positive charges on the outside so our voltmeter would change and we would now see it shifting to the negative number so this is a what we call a diffusion potential potential just simply refers to the electrical force that was created as a result of ions diffusing across the membrane so a diffusion potential is the electrical potential created from ions moving across a membrane