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Course: AP®︎/College Biology > Unit 2
Lesson 6: Facilitated diffusionElectrochemical gradients and secondary active transport
How a cell can use a molecule's electrochemical gradient to power secondary active transport.
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What is electrochemical gradient?(10 votes)
An "electrochemical gradient" is a combination of two factors: an "electrical potential difference" between the inside and outside of the membrane and a "concentration gradient."
In Khan's example, the "electrochemical graident" is established due to the sodium/potassium pump and the carrier protein's ability to allow potassium to travel down its concentration gradient. These forces cause two things to happen to the system. First, they create a difference in electrical potential - the outside of the cell is more positive than the inside of the cell. Second, there is a difference in sodium concentration: there is more sodium outside of the cell (high concentration) than inside the cell (low concentration). Therefore, there are two forces (electrical potential difference, concentration gradient) that are acting on the system. We call the combination of these two forces the "electrochemical gradient."(21 votes)
How does the cell store electrochemical energy from the electrochemical gradient?(8 votes)
Basically, for the sodium-glucose pump, because the sodium wants to get in, it physically pushes against the protein and, like a waterwheel, turns it so the glucose can get through. Hope this answered your question!(16 votes)
- What is the overall balance of energy on this process? Building up the electromechanical Gradient of Sodium uses energy in form of ATP. Getting glucose in on the secondary chanel takes advantge of this energy stored in the potential. Than it might give back some energy into the cell by using the glucose to synthesize some ATP. But this can not be zero sum, right? There must allways be some friction in the system.(5 votes)
No, it is not zero-sum since this does not create equilibrium.
In the cell, many processes happen simultaneously and there is not literal stopping after one, and jumping to another one, then after the second process resuming the first one.
While you buildup electrochemical gradient, active transport happens. Once the cell uses an electrochemical gradient as the driving force for the sodium pump, not all gradient is 'used'. And what is most important, both processes happen simultaneously.
There must be some friction in the system. And to answer to you, probably, electrochemical gradient always favors building up inside of the cell, therefore, generating energy for active transport. Otherwise, active transport would not happen one day and the cell would suffer.(2 votes)
Where does osmosis fit in with all these different concentration gradients? Is osmosis irrespective of the nature of the type of solutes, and instead only respective of the ratio of solvent to that-which-is-not-solvent?
It makes perfect sense when we're only dealing with one solvent and one solute, but throwing in all these different types of molecules in aqueous solution makes it harder to guess how the water itself might behave.(3 votes)- The type of solute doesn't matter. What matters is osmolarity, which is a measure of the total amount of particles dissolved per liter. Water will want to go from an area of low osmolrity to a level of high, regardless of what types of particles are involved.(4 votes)
Can th glucose just be carried into the cell by the energy of ATPs? Why does the cell need to establish both chemical and electrical gradient first? Thank you!(4 votes)- Yes, it can, but it is faster to use already prepared potential which comes in handy. An electrochemical gradient is locally tied to the symport of glucose.
It is way faster to utilize that energy from finding ATP from somewhere else.(2 votes)
Why doesn't the electric potential difference allow K+ ions to move into the cell ?]
THanks very much!(4 votes)- Because that would be against the concentration gradient. K tends to leave cell (and that's why it has to be re-uptaken by Na/K pump). However, there are channels were K leaves the cell.
I think electrical potential could possibly allow K influx, but, chemical concentration gradient does not allow it. :)(2 votes)
If the cell keeps bumping Na+ ions otside, that means there is also another pump that would transports them inside so the cycle can continue? by active transport i guess(3 votes)- No there is no another pump, actually depends on what cell we are speaking of, but there are voltage-gated ion channels which open and let Na ions inside and that's how Na circulates. :)(2 votes)
- what is an electrogenic pump?(3 votes)
- The pump which changes the electrical potential of the cell. It pumps (either in or out or both) charged molecules.(2 votes)
Why does glucose move with the Na+ into the cell with the secondary transport? Why not some other molecule?(3 votes)
if an acid and a base were mixed together how would the strength of the Ph gradient compare to electro chemical and concentration gradients?(3 votes)
Video transcript
- [Voiceover] In the video on the sodium-potassum pump, we talk about how it 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 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, 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, they're not gonna want to go up here so much because of their charge. It's more positive here than it is over here. They'd 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 sodiums 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 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. Electrochemical gradient. And I already said it once, but I'll say it again. 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 than on this side, so you're going to have a 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 on 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 let's say 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 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 a turbine or 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, or I guess you'd say its 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 concentration gradient, you're going to have to use active transport. So this concentration gradient, so let me be clear on glucose's 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 in this direction. And over here, the source of energy to go against the concentration gradient, is the stored potential energy from the electrochemical gradient of the sodium. And so this type of active transport, 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 Transport. It's using the stored energy 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.