Transport across a cell membrane
Passive Transport by Facilitated Diffusion
- [Voiceover] Alright, so in this video we're going to talk about passive transport. So let's start off taking a look at a cell. So there's the cell right there, and why don't we just zoom in up here. And let's focus in on the membrane. And as you know it's a lipid bilayer which basically means that we have these hydrophilic heads that are sitting over here. So I'll draw just a few of them, so three of them right here and then three of them right here as well. And they've got hydrophobic tails. These are fatty acid tails that come down like this. So they are kinked like that, and that's what makes the hydrophobic core of our lipid bilayer. And I could go on and draw this all the way but I'm just going to have this blue outline as our hydrophilic head, down here as well, hydrophilic head and the hydrophobic tail is inside, this gray area. And so the question arises then, when we have a small particle, let's say this is a chloride ion, Cl negative, that's sitting right there, how do we get it from the outside area, the extracellular space, so I'll say out, this is from the outside, to the inside right here. And in our gut, which is where this example is going to take place, we do that by passive transport, and as we talked about earlier, there are four different types of active transport, and the main type we're going to focus on in this video is what's called Facilitated Diffusion. Facilitated Diffusion, because we're diffusing or we're moving across a certain space. But the question here is, what's facilitating that movement? And therein lies the answer of what we're going to focus on for this video. So along our cell membrane there are also going to be proteins that are embedded along the cell membrane, And so what's going to facilitate the movement of this chloride ion across from the outside to the inside is this chloride channel right here. And so this channel is built in a funny way that has this little pocket that sits right there and it's shaped like that. And the idea behind this pocket is that it's the site where chloride, and specifically chloride, can sit down. So if I were to go through and draw it sitting in here it would look a little bit like this. Not too different from what I just drew, but there's a very important distinction that we're going to have to make here. Now, what happens is that now that our chloride is sitting here, we've made this chloride channel right here uncomfortable. Now we're uncomfortable, it doesn't want to sit like this now because now that we're holding the chloride ion we want to change our posture. And so what happens is that our chloride ion will actually snap and change its shape. And it'll snap in such a way that it'll flip. And when it flips like this, which you'll notice is at the part that was holding on to our chloride, the part that was holding on to our chloride will now suddenly be facing the intracellular face of our cell membrane, which sure enough means that the chloride ion can now leave. Leaving behind then the same protein that we started off with right here. But now it's a little different. Now, as you can see, it's got the same shape, but instead it's clearly missing our chloride ion. And so what's happened along the way is that when we made the switch here from this uncomfortable shape to this relatively more comfortable shape it allowed our chloride to exit. And now that it's exited into the inside of our cell, our protein has become uncomfortable again. And so now that it's uncomfortable it needs to shift. And when it shifts, what shape do you think it's going to take? I think you probably guessed it. It's going to look exactly like what the protein did before it made contact with the chloride in the first place. And so now it's comfortable again, so it's nice and comfy. And in fact it's ready to take on another chloride ion. And so what our protein is doing along the way is that it's changing... it's changing its conformation, and the conformation is just the pose the protein is taking. And it changes its conformation, whether it's binding the chloride ion, as it did here to become more comfortable, or if it's not holding the chloride ion, as it did here when it was uncomfortable, to shift always to a more comfortable position. And through this process, in a very step-wise way our chloride ion is able to enter the cell without using any energy whatsoever. No ATP is used at all, instead this protein is used to facilitate the movement of the chloride into the cell.