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Video transcript

so let's talk about all of the different ways that molecules can be transported across a cellular membrane so perhaps the most basic the most passive of the passive transports would be straight-up diffusion and if you have a small enough molecule let's say this is molecular oxygen it's small it doesn't have any charge it has no polarity that will be able to diffuse down its concentration gradient through the cellular membrane but as we start to talk about things with more charge or things that might be larger then we're going to need some help now the first type of help is just help to allow things to flow down their concentration gradient and that we call facilitated diffusion we have a whole video on facilitated diffusion and one form of facilitated diffusion hey just open up a tunnel and let things flow down their concentration gradient we saw that with the potassium ion channels where potassium builds up on the inside of the cell because of the sodium potassium pump let me just be clear down here is this is the inside this is the outside of the cell and then but then these channels allow the potassium to flow down their concentration gradient going to be put in check because of its charge it's more positive outside but we talked about that in other videos but it's just a simple tunnel now sometimes that tunnel is gated it's only going to be open if a certain trigger is hit and we see that on when we talk about signals going down a neuron voltage-gated channels once the voltage hits a certain amount then the channel opens and then the sodium that has a higher concentration outside can flow down its concentration gradient inside but both of these this is considered passive transport it's facilitated diffusion passive transport we're allowing things to flow down their concentration gradient you can see here the sodium is going in or sorry the potassium is going in the direction of its concentration gradient it's high concentration inside low concentration outside so allowing it to flow down the concentration gradient here the sodium is high concentration outside low concentration inside and this happened because of the sodium potassium pump but we're allowing it to now flow down its concentration gradient now let's talk about active transport so passive transport doesn't or doesn't require any energy to make this stuff happen it's just about things flowing down their gradient in active transport we're either directly using energy to make something go against its gradient or we're using some energy from a previous active transport to to help facilitate something else going against this gradient so first let's talk about primary active transport because this might be a little bit more easy to think about or none of them are that daunting and the best case of this if we're going about animal cells is the sodium potassium pump the sodium potassium pump super important for establishing resting membrane voltage I guess you could say resting membrane potential but the the concentration gradients that establishes are also very important the sodium having a it establishes it pumps sodium ions out of the cell against its concentration gradient so as we say the sodium ions already have a higher concentration outside but it keeps pumping them out and to do that it needs to use ATP it breaks up ATP into ADP and a phosphate group it hydrolyzes it and so that's why it's sometimes called an ATPase it's an enzyme that helps break up ATP but it uses that and it uses that energy to pump sodium out of the cell and potassium into the cell and then as we'll see that sodium that's pumped out of it that kind of forms a potential energy because it starts to build a chemo electrochemical gradient which can later be used to power secondary active transport we'll talk about that in a few seconds now this is an animal cells the analog in plant cells fungi protists prokaryotes is the is the proton ATPase or the proton pump which does the same thing but it does it instead of doing it in two directions it does it for protons it pumps the protons out of the cell against their concentration gradients so even though you have a higher concentration outside than inside it'll continue to pump them out but to do to power it it uses ATP to change its conformation in the right way and so that's why it's off this is often called a proton ATPase this is called sodium potassium ATPase that's our friend the sodium potassium pump this is called proton ATPase and you wouldn't see these in the same in the same cell so maybe I'll little line over here this would be in plants fungi protists things like that this would be in animal cells but both of them are actively using energy they're directly using ATP to transport things against their concentration gradient which is why we call it active transport now because you have these concentration gradients or these electrochemical gradients are established those can then be used a fuel to do other forms of active transport and that's what we call secondary active transport so this right over here this is my little depiction of a symporter and this is the sodium glucose symporter and what it does is it leverages the sodium flowing down its concentration gradient and once again that was established with this sodium potassium pump so it's flowing down its concentration gradient but it's leveraging that energy you can catch them imagine kind of a you know putting a little wheel under a waterfall to make it spin to also make glucose to transport glucose against its concentration gradients so the glucose already has a will have a high concentration gradient here low over here but it's transporting glucose against its concentration gradient we talked about in other videos and then another example of secondary active transport is an anti porter or an it or an exchanger in the symporter a co transporter they're both going in the same direction even though one is going with its concentration gradient that's essentially powering it and the other one is going against its concentration gradient that's why it's active transport with an exchanger they're going in opposite directions so that you have the sodium calcium ion exchanger and here the sodium is going down its concentration gradient and that fuels taking the calcium ions outside of the cell against its concentration gradient so once again anytime something is going against its concentration gradient and once again in this case it's calcium it's going to be active transport but since the sodium ions the sodium ions are going in a different direction than the calcium ions we call this an anti porter while this right over here is a co transporter a symporter now you might say hey well you know is it the sodium potassium pump isn't this an anti porter things are going in different directions and the difference is both of these this is primary transport and the sodium-potassium pump both of these things are going against their concentration gradient in a true anti Porter it's really secondary active transport one of them is going with their concentration gradient going down it which is providing the energy to take the other thing against its concentration gradient so anyway hopefully this gives you a high-level overview of the various forms of transports and gives you more appreciation for how beautiful and intricate and and and and mesmerizing cellular membranes and all the different things that cells have to do actually are
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