Transit Across Membrane: Active A (Cells & Membranes Series)

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Transit across the membrane and this is part A of the active portion, so it just finished up the passive part and in terms of our outline, we've covered these topics and in this lesson, I'm actually only gonna be looking at these terms. Originally I thought the second part was all gonna be one lesson, but there's just a lot to cover.

And so we're looking at primary and secondary active transport. Active transport is, well, it's the movement of molecules that go against the concentration gradient. And this requires cellular energy that is provided in the form of ATP. If this molecule, for instance, is going to move out of the cell, it's going to need a little push because it's going against the concentration gradient.

It need something to overcome, the passive movement that would, well, that would otherwise occur and this is what happens with the famous sodium-potassium pump and I'm gonna cover that in quite a bit of detail in a little bit here. But first I wanna talk about the two major ways in which ATP is relevant to the process and that's the process of powering molecules against the gradient. Primary and Secondary.

Primary Active Transport makes direct use of ATP to push the molecules against the concentration gradient. In Secondary Active Transport the process is fueled by what you can think of as an indirect form of ATP. We still have the molecules being pushed against the concentration gradient. ATP is involved but it's use comes in a roundabout form.

But the main thing to know right now, the short story, is that ATP powers other molecular changes in the cell. And the consequences of those changes create the energy needed to move molecules over the membrane across their concentration gradient but it's a secondary process, it's because other changes were already set into motion by primary active transport.

Let's go through each of these primary secondary in more detail. The classic example of active transport is the sodium potassium pump. This pump uses ATP to move three sodium ions out, and two potassium in. In this process, ATP is converted to ADP. And it's the energy of this cleavage from the conversation, it's used to drive the pump.

So this is what we consider to be primary active transport. There's a ton of sodium outside, a ton of potassium inside. And that's why we need the pump to go against the gradient. And if you want a trick for remembering just the basics of the sodium potassium pump, you can remember the mnemonic three strikes out to propel an inning. Which is three sodium out two potassium in.

When we were talking about secondary active transport we're referring to the movement that is what's called electrochemical potential driven, and what that means is that there's a coupling of power from the primary active transport. That primary active transport is the sodium potassium pump and it generates a serious of events that lead to energy that's used as secondary power, secondary transport.

The example we're going to use here is that of the countertransporter. And there are a few different kinds of countertransporters in the body. The one I'm showing you here is that of sodium hydrogen. So we know there's going to be a lot of sodium around the circumference of the cell. Because of this pump, there's going to be even more sodium, then there are otherwise would be because we're having this system in place that continuously pushes it out.

And there's already more sodium on the outside. So we have this sodium that's trying to get back into the cell. Wherever it can. And that's happening all the time. The pump pushes the sodium out against the gradient in this movement here. It makes the extracellular sodium that much more driven to come in.

So we have a situation where sodium is moving back in even more enthusiastically. It's moving in with more energy than it otherwise would. And even without this pump, it would still be trying to come in, because of the concentration gradient the pump creates a situation where this process is a little stronger. And so let's go back to this countertransporter here.

The sodium is trying to come in. And this is one place it's coming in through, and that movement in is indirectly being powered by the sodium potassium pump. The energy of this movement into the cell then creates a new situation in the cell. And here we have a situation where the sodium ions coming in cause hydrogen to move out.

Now, this isn't the case everywhere in the cell membrane, and this not even the case for all countertransporter mechanisms. But the one I'm showing you here is sodium hydrogen, this protein has a selected affinity for hydrogen. Other countertransporters focus on other ions. I'm gonna give you more example of those.

But in this situation we have hydrogen moving out while it happens so that hydrogen is found in greater quantities outside of the cell than inside of the cell. And so, because it's going in that direction, we can say it's going against the gradient. And therefore, by definition, this is an example of active transport for sure, it's moving against the gradient.

But it's also secondary, because there is no ATP that's directly being used. There is no pump, we have no primary pump, sodium potassium. And yet, of course, it was the pump in the first place,that set the stage for this elaborate orchestra. Therefore we could say that ATP is indirectly powering this hydrogen out of the cell.

In the body we see a variety of countertransporters so I just showed you one. We also see a variety of co-transporters and all of these are secondary active transport. Another word for countertransporter is antiport, a nice intuitive name we know that counter and anti all those go together.

Transporter processor, antiport process is given to describe situations where two molecules are being transported in opposite directions at the same time through some kind of integral protein like we saw in the last picture. Examples include what we just looked at, like the sodium hydrogen countertransporter. That's found, for instance, in the nephron of the kidney.

And in the situation we have the cell that's pushing excess hydrogen out of the cell. And that's really important for regulating pH balance, in that regulation is a primary job of really vital function that the kidney performs. Another kinda transporter or antiport, which is found in even greater numbers of places in the body, is the sodium calcium transporter.

And again, as with all of these, we have sodium ions that are moving in with some degree of power, some degree of force. But we have only one sodium moving in and three moving out. For the sodium hydrogen it was an even 1, 1. The sodium calcium antiport is really important for neuronal activity. The other form of secondary active transport involves transport proteins that are called co-transporters, or symports.

Where two or more molecules are transported in the same direction at the same time. And an example of this would be the movement of sodium with glucose into the cell. You might have heard about or studied SGLT. There are all kinds of SGLTs, so I'm talking about it in a general sense here, but they have different numbers.

Two and three, that stands for sodium-dependent glucose co-transporters, kind of a mouthful. And these symports are, well, they're found in regions of the intestines and the kidneys and it's still all being powered indirectly by the sodium potassium pump because it's that pump that effects the sodium concentration. In all these cases, the movement is sodium moving in, and in this symport, glucose moves in with the sodium.

This is just one of many ways that glucose gets into the cell. In other situations we have glucose actively moving out. Glucose gets in and out of the cell through facilitated diffusion as well as active transport. You remember that from the previous lesson. But of course, when we have this happening in active transport, we're looking at situations where the glucose is moving against the concentration gradient.

And so we have a situation where the inside of the cell actually has more glucose than is on the outside and yet glucose is coming in. So the question might be, why would the body want this to happen? Well, with SGLT, it's trying to make use of glucose usually that's already undergone. Some kinda filtering process, that's trying to bring in this excess glucose, that's already been filtered.

That's a lot of what the kidney does, and without this mechanism, we'd be secreting too much glucose, into the urine. So I'll give you more of a physiological, or medical example here, clear up a little bit of room. SGLT helps the body re-absorb filtered glucose. In diabetes 2, there are some treatments, that are designed to inhibit SGLT, and that's SGLT 2 specifically, as a way of lowering blood glucose.

And actually, it's sometimes used for diabetes 1 as well, but when that is the case, it's typically used in lower doses because, well, people with diabetes 1 have to be careful of their blood sugar in both directions. They have to worry more about ketoacidosis, and so anything that's given to people with diabetes 1 has to be really carefully measured, administered, monitored.

Where isn't there much risk in lowering the blood sugar too much among patients with diabetes 2. Here's a review of what we've covered in part A of Active Transport. And this is what's to come in the next lesson.

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