So now that we know from our last lesson video that facilitated diffusion requires transport proteins, in this video, we're going to talk about some of the transport proteins of facilitated diffusion. There are two main types of transport proteins involved in facilitated diffusion, which we have numbered down below: number 1 and number 2. The first type of transport protein involved in facilitated diffusion are the porins, otherwise known as channels. As their name implies, porins and channels, they're going to form an obvious membrane-spanning tunnel that will allow molecules to move across the membrane by traveling through the tunnel that these porins or channels create. Porins imply the word "por," which is like a hole or a tunnel. And channels, once again, are basically like tunnels as well. Aquaporins are specifically used to transport water molecules across or through the cell membrane, essentially facilitating osmosis. Thus, osmosis, the diffusion of water across a semipermeable membrane, can occur at a faster rate, thanks to the aquaporins.

The second type of transport protein involved in facilitated diffusion are the transporters, otherwise known as carriers. Unlike the porins and channels, transporters or carriers do not form an obvious membrane-spanning tunnel. Instead, transporters or carriers are only open on one side of the membrane at a time, and they must undergo conformational changes in order to move molecules across the membrane. Let's take a look at our image down below to get a better understanding of these ideas. On the left-hand side, we are showing porins and channels. Notice that porins and channels create a membrane-spanning tunnel that allows molecules to move through the membrane, through the tunnel, and cross to the other side. Aquaporins, specifically, allow water molecules to diffuse across the membrane at faster rates, essentially allowing osmosis to occur faster. You can see that channels, like ion channels, can allow ions with charges, like a positive charge, for example, to move through the tunnel here and get to the other side of the membrane. Over here, we see obvious membrane-spanning tunnels, and there is an opening on both sides of the membrane.

Now, looking at the right-hand side of the image, we see transporters or carrier proteins. These do not form an obvious membrane-spanning tunnel, like those on the left. Instead, transporters are only open to one side of the membrane at a time. In order to transport these molecules from one side of the membrane to the other side, this transporter or carrier protein must undergo a conformational change. Notice that over here, the transporter or carrier has undergone a conformational change that allows the molecule on the inside to be released to the other side of the membrane. Here, the transporter or carrier protein is only open to one side of the membrane, and it is closed on the other side of the membrane. Transporters or carriers are only open to one side of the membrane at a time, and it is only after a conformational change occurs that they can change their shape and be open on the other side of the membrane. Both of these are types of transport proteins involved in facilitated diffusion, allowing molecules to be transported down their concentration gradients passively without any energy input. This concludes our lesson on this, and we will be able to get some practice applying these concepts as we move forward in our course. I'll see you all in our next video.