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Current time:0:00Total duration:5:17

Electrochemical gradients and secondary active transport

AP.BIO:
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ENE‑2.G.2 (EK)

Video transcript

in the video on the sodium potassium pump we talk about how it helps this helps a cell establish its resting membrane potential and it does that by pumping actively pumping three sodium ions out for every two potassium ions it pumps in and that by itself that ratio of three to two by itself doesn't establish the full resting membrane potential but then the potassium ions are allowed to start to start diffusing down their concentration gradient from the inside back to the outside and of course there's a balancing force there or a balancing factor there and that's the charge because if the outside is more positive than the inside a positively charged ion which the potassium ions are well it's something they're not going to want to go up here so much because of their charge it's more positive here than it is over here they had actually want to go back but their concentration gradient they're going to be bumping into the bottom of this channel more than the top and so you're going to have a balance they're going to start diffusing through but you're not going to have equal concentrations because the charge is going to keep them back here but what about the sodium ions the sodium ions are getting more and more concentrated up here and up here is getting more and more positive if the sodium ions were left to their own devices if there was no membrane over here they would naturally if we just looked at the concentration gradient they would naturally want to diffuse down we have a high concentration over here we have a low concentration over there so if there was no membrane then they would just naturally diffuse from high to low that's their concentration gradient and also if there was no membrane we've already talked about it being much more positive on this side than it is on this side or you could say we have a positive potential difference between here and here so the positively charged ions like the sodium's up here would want to go down because of their charge and so there's two reasons why they would want to go from this side of the membrane to that side of the membrane their concentration gradient and the and their charge the electric potential there's this potential energy of them wanting to get away from all the positive charges and so that combined motivation for the sodium ions to go in that direction we call that the electrochemical gradient electro Oh electro chemical chemical gradient and I'm already set at once but I'll say it again there it's a combination of the electric gradient and the chemical gradient the chemical gradient you have higher concentration here lower here you would want to diffuse down more things are going to bump on this side then on this side so you're going to have a net net flow down if you didn't have this membrane here and then when you think about the electric potential more positive on this side than this side so positive ions would want to go down and so you could view this gradient as a source of potential energy and cells in fact use this gradient in fact the sodium electrochemical gradient as a source of energy and so this lets say that this this protein right over here this is what we're going to call a symporter this is a symporter and what it does is it uses the electrochemical gradient of one ion in this case but in this case sodium so it uses the fact that sodium really wants to go through the membrane and it uses that energy imagine like water falling down a waterfall and it can turn it can turn a turbine or it can turn a water mill type of thing and so it uses that energy of the sodium flowing down its electrochemical gradient it wants to go in this direction for two reasons concentration and electric potential so or I guess you'd say it's electrostatic charge and then it uses that energy to transport other things and the most famous symporter with sodium is glucose it's going to use that the sodium and the glucose are going to go together and the glucose is being transported against its concentration gradient and so if you're going to transport something against its act against its concentration gradient you're going to have to use active transport so this concentration gradient so let me be clear on glucose is concentration gradient it looks like this you have high concentration over here and you have low right over here and the cell might not want to waste all this glucose it wants to get as much glucose into the cell or across the membrane as possible and so it's going to have to do some active transport to go against its concentration gradient to go to go in this direction and over here the source of energy to go against the concentration gradient is the stored is the stored potential energy from the electrochemical gradient of the sodium and so this type of active transport where you're using where you're using the energy that was stored up through another form of active transport the sodium potassium pump we call this secondary active transport so what's going over here this sodium glucose symporter this is secondary active transport secondary active active transport it's using its using the stored energy from one from the electrochemical gradient of one molecule it's using that stored energy to drive the active transport of another molecule glucose going against its concentration gradient
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