Alright. So now that we've fully differentiated between the 2 different types of passive membrane transport, simple diffusion and facilitated diffusion, in this video we're going to talk more details about facilitated diffusion. And so first, we need to recall that facilitated diffusion is a type of passive transport, which of course means that there is no energy required. So facilitated diffusion is a non-energetic type of diffusion. And as its name implies, facilitated diffusion is facilitated by a membrane protein. And really, it's this membrane protein here that differentiates facilitated diffusion from simple diffusion. Now this integral membrane protein that's involved in facilitated diffusion can either be, number 1, a carrier/transporter, or number 2, a porin/channel.
Now the carrier/transporter is an integral membrane protein that undergoes conformational changes in order to move the solute across the membrane. Whereas the porins or channels, these are integral membrane proteins that do not undergo conformational changes. Instead, the pore and channels will form an obvious membrane-spanning tunnel that will allow it to move solutes across the membrane. And so if we take a look at our map down below, what you'll notice is again we're exploring the leftmost branch and we've already covered passive transport in terms of differentiating simple and facilitated diffusion. But what you can see now in this map is that facilitated diffusion, that membrane protein, can either be a carrier/transporter or a porin/channel.
And so down below over here on the left-hand side, what we have is the carrier/transporter. So in some of your textbooks, they'll refer to them just as transporters. In other textbooks, they'll refer to them just as carriers. And, just in general, those two terms are interchangeable for these types of membrane proteins. And so, as you can see, this is the carrier/transporter, right here embedded in the biological membrane. And notice that there are a bunch of molecules on this side of the membrane in high concentration, and so they want to diffuse down their concentration gradient still non-energetically through the protein transporter. So the molecule would bind to this transporter, and then the transporter would have to undergo a conformational change in order to allow that molecule to be transported across the membrane, and so it will release the molecule like this, and then once it releases the molecule, it can return back to its original position to continue transporting the other molecules. But one thing to note is that when you take a look at the structure of a carrier/transporter, there is really no clear and obvious membrane-spanning tunnel. And so the really only way that this molecule can transport molecules across the membrane is by undergoing conformational changes. So that's really the big takeaway of these carriers/transporters, that they undergo conformational changes.
Now over here on the right-hand side, what we have are the porins and the channels. And so in our previous lesson videos, we had already talked about in some of those videos, and so we know that they can form these structures here that create an obvious membrane-spanning tunnel. And so here we're showing a porin that can allow this molecule to move directly through the membrane, through the tunnel that it's forming. And there is no conformational change that takes place with the porins. Now aquaporins are specific types of porins that allow for water molecules to diffuse at a faster rate through the membrane. And so, aquaporins, will also come up later in our course, as we start to talk more and more about different parts of biochemistry. But aquaporins, the aqua part you can see is specific to water, and the porins suggest that it's creating a pore through the membrane, a channel or a tunnel through the membrane. And then over here on the far right what we have are these ion channels, and we'll be able to talk more about ion channels as we move forward in our course. But they allow specific types of ions to diffuse through the membrane non energetically. And so all 3 of these that you see here are creating some kind of obvious membrane-spanning tunnel, And so that is really what defines the porin and channels. And so as we move forward in our course, we'll be able to talk more and more about these different types of integral membrane proteins that are involved in facilitated diffusion. But for now, that concludes our introduction here to the types of membrane proteins involved in facilitated diffusion, and I'll see you guys in our next video.
