6. The Membrane
Active Transport
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So now that we've covered primary active transport, in this video we're going to focus on secondary active transport. And so recall from our previous lesson videos that secondary active transport is not directly driven by ATP hydrolysis. That is primary active transport. Instead, secondary active transport is going to be directly driven by another molecule's concentration gradient. And it's going to be powered by another molecule's concentration gradient instead of being powered by ATP hydrolysis like primary active transport is. However, that being said, secondary active transport, although it may not be directly driven by ATP hydrolysis, it is indirectly driven by primary active transport and ATP hydrolysis. And the reason for this is because the concentration gradient that directly drives secondary active transport is actually built using primary active transport or PAT which we've abbreviated, here for our lesson. And so, in order to better understand secondary active transport, we're actually going to take a look at a classic example in the sodium glucose secondary active transporter. And so down below we have an image of this sodium glucose, transporter, and notice that the image has these numbers 1, 2, 3, and 4, and these numbers that you see down below in the image correspond with the numbers that we have up above here in the text. And so this is really showing you the 4 steps that there are for this sodium glucose secondary active transport example. And so in the very first step here what you'll notice is that sodium ions can be transported against their concentration gradient using primary active transport. And so when you take a look at our image down below, notice that the sodium ion, which we're showing you down below here in step number 1, is being pumped across the membrane in this direction and it is being pumped towards the area of higher sodium concentration, whereas down below there's a lower sodium concentration inside the cell. And so because sodium is being pumped against its concentration gradient from low to high concentration it's going to require energy. And notice that ATP is directly linked to this process of pumping sodium across the membrane. And because ATP is directly linked here it is a form of primary active transport just like what we mentioned up above. Sodium being transported against its concentration gradient using primary active transport. But we've already covered primary active transport so really this is not, anything new to us. And so essentially in number 2 what this is going to generate is a higher concentration of sodium ions, generated on the outside of the cell. So the outside of the cell here has a higher concentration of sodium. You can see there are way more sodium ions, much a much much higher concentration than there are inside the cell, so there's a lower sodium ion concentration inside. And this is because, again, this primary active transport generates this, concentration gradient of sodium. But then what we need to also realize is in this image, in number 3, there's another molecule that's also involved here, glucose, which we have in green. And so notice down below we have glucose as well we have, these green hexagons here and also down below here. And notice that the glucose molecules, they actually have a higher concentration on the inside of the cell, which is opposite to that of the sodium. The sodium has a higher concentration on the outside of the cell. The glucose has a higher concentration on the inside of the cell. And so this is really where secondary active transport comes into play because sodium, is going to be transported down its concentration gradient from an area of high concentration to an area of low concentration, and that does not require any energy. In fact, it actually can provide and release energy. And so as the sodium gets transported down its concentration gradient, it's actually going to provide the energy. It's going to power the transportation of glucose against its concentration gradient from the area of low concentration of glucose towards the area of high concentration of glucose. So once again, to better understand this let's take a look at this example over here. And what you'll notice is that sodium is going to be transported down its concentration gradient from an area of high concentration down towards an area of low concentration. And that does not require any energy for molecules to move down their concentration gradients. Instead, it's going to release energy, and that released energy can be used to power the movement of glucose against its concentration gradient from an area of low glucose concentration on the outside of the cell towards an area of much higher glucose concentration on the inside of the cell. And this is really, a type of active transport since molecule is being transported against its And so because it's not driven directly by ATP, and so because it's not driven directly by ATP, it's going to be driven by the concentration gradient of, sodium being transported down its concentration gradient. And so that makes this a classic example of secondary active transport. And so once again as we mentioned up above with secondary active transport, it's gonna be driven, by the concentration gradient of another molecule instead of ATP hydrolysis. So over here with secondary active transport, notice that there's no ATP at all in this vicinity, and really it's just this concentration gradient of sodium going down its concentration that powers the secondary active transport here of, glucose being transported against its concentration gradient. And so this here concludes our introduction to secondary active transport and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all on our next video.
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