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Main content
Current time:0:00Total duration:8:06

Introduction to passive and active transport

AP.BIO:
ENE‑2 (EU)
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ENE‑2.E (LO)
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ENE‑2.E.1 (EK)
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ENE‑2.E.3 (EK)

Video transcript

let's say that you have decided to go canoeing and right over here this is a top view of our River right here this is our River and let's say that the current the river is going towards the right so there's two different directions so there's two different directions that you could be canoeing in you could imagine someone who is canoeing in the same direction as the current so they are canoeing that way and then you could imagine another person who's canoeing the other way so someone who's canoeing upstream this person is canoeing downstream this person is canoeing upstream so they are going in that direction so pause this video and think about which person is going to have to expend more energy or which person is going to have to be more active and which person is going to be more passive well yes this wasn't an incredibly hard question if you are going with the flow of current as the person in yellow is here they don't even have to take their paddles out they can just take a nap and the boat will naturally go with the current they would be they could be just moving passively while the person in blue here they're gonna have to work really really really hard they're gonna have to paddle some just to not even move to the right and then even paddle even more to actually go against the current so this person would have to be very active and so this is really just a metaphor for what we're going to talk about now and that's the idea in biology of active versus passive transport so let's start with maybe the the more pleasant one in either situation and that is passive transport so passive transport is when something goes with the gradient so what am i mean by that so you could have a concentration gradient so let's say that on let's say I have a tube of some kind and let's say it's it's filled with water and dissolved in that water at this end of the tube I have a high concentration of some molecule or something right over here well on the right hand side I have a low concentration so what do we think is going to happen well these things are just going to naturally move around and over time they're going to bounce their way so that after a little bit of time has passed a lot of these things are just going to passively move to the right and so at some point you might have an equal concentration or roughly equal throughout this entire container and so this movement along your concentration gradient here you're moving from high concentration to low concentration this would be passive transport this is a concentration gradient that we were moving along let me write that down this is our concentration gradient but you could also have an electrical gradient so let's take a similar type of container maybe it's filled with water and on the left-hand side imagine if you have a bunch of positive particles or molecules and on the right you have a bunch of negative particles or molecules well the positive ones repel each other sort of the negative ones but they attract the positives attract the negative and the negative attract the positive and so you would think that things would naturally move down their electrical gradient the positives want to go away from each other and they are drawn to the negative similarly the negatives want to get away from each other and they are drawn to the positive so whether you're talking about a concentration gradient or an electrical gradient and sometimes you have a combination of both an electrochemical gradient when you're moving along with your gradient you don't have to use any energy that's known as passive transport so no no energy energy need needed it's just going to happen naturally now the opposite is the notion of active transport active transport and this is when you go against the gradient so an active transport would be somehow let's say you're in this situation right over here somehow getting one of these particles let me do in that same color somehow getting one of these particles instead of moving to go in that direction it will actually go against its gradient in that direction or another situation is imagine if you have a positive particle right over here instead of making it instead of it naturally just going to that direction somehow you make it go against its gradient and you make go closer to the other positive particles well this is going to require energy to do and probably the most cited example or the most common example that we're going to see in biology class of active transport is what's known as a sodium potassium pump which we will study in detail in other videos but let's say that this thing that I'm drawing here in white this is a cell membrane and I'm drawing these gaps for a reason and what you have on the outside of the cell membrane you have some potassium ions on the outside but you have a lot more on the inside so these are all potassium ions on the inside of your cell and then so let me just write k plus K a plus k plus k plus k plus and you'll have some sodium ions on the inside of your cell and a plus but you have a lot more on the outside of your cell and in general the outside of your cell is going to have many more positive ions than the inside maybe you already see where this is going and a plus na plus na plus I think you get the idea and a plus and a plus now if on this membrane unless ignore this part right over here if I just had a channel right over here that was open only to potassium so only potassium can go through so only potassium can go through this channel right over here what do you think is going to happen well you would have passive transport these positively charged potassium right over here they would go down their concentration gradient there's more likely to have a potassium ion just bump in the right way just right over here so that goes through the channel because there's just more potassium Zout on on the inside of the cell then there would be on the outside and so this this potassium is going down their concentration gradient from high concentration to low concentration through this channel this would be passive transport passive transport but you could imagine there's also active transport and that active transport is what pumps the sodium ions inside the cell outside of the cell even though it's not only against its concentration gradient it's also against its electrical gradient the outside is more positive so you wouldn't think a positive ion would naturally go outside and the outside has more sodium's than it does inside but the sodium potassium pump still pumps those sodium's outside and as I hinted at it does this using energy so it'll sometimes see a sodium potassium pump drawn like this and once again I'm not gonna go into depth on it we have a whole video on it but the general idea is is that the sodium's bind over here and then some ATP which is the powerhouse of cells which we will study in more depth later on in biology it leverages its energy to change the shape of the proteins that make up this sodium potassium pump to then pump these sodium's outside of the cell so it's going to go from this shape and then it's just going to you could view it as opening it up that way the real enzymes look quite different but that's the general idea you use energy in the form of ATP to pump the sodium's out against both their concentration gradient and their electrical gradient and that's why it's called active transport
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