In this video, we're going to begin our introduction to membrane transport. It's important to know that biological membranes are actually semi-permeable, which really just means that biological membranes can act as barriers to essentially prevent diffusion of some molecules across the membrane. It's also important to note that the term semi-permeable is synonymous with the term selectively permeable. Selectively permeable and semi-permeable really just mean the same thing. Again, all it's saying is that membranes are really, really picky about what types of molecules they allow to cross the membrane. The term permeable means how penetrable something is, and the term selectively, of course, just means picky. It is going to select the specific types of molecules that are able to cross the membrane freely. The root semi means partial, and so semi-permeable just means partially penetrable. Really, all of this is just saying that biological membranes are selectively permeable or semi-permeable meaning that they're super picky about what types of molecules can cross and what types cannot cross.
If we take a look at our image down below, we'll be able to see an example of a selectively permeable biological membrane. Notice in this cartoon image here in the middle we're showing you our biological membrane, which we know is a semi-permeable membrane or a selectively permeable membrane. Again, all this means is that it's really picky about what it allows to cross the membrane. So, notice on the left-hand side we're showing you two molecules, a sodium ion and an oxygen molecule or an O2 molecule. Notice that only the O2 molecule in this cartoon is able to cross the membrane freely, and the sodium ion is not able to cross the membrane. This is because of the properties of these molecules, and we'll be able to talk more about what exact properties these molecules need to have in order to be able to cross the membrane freely in our next lesson video. But here in this cartoon, notice that the membrane here is saying, "Excuse me, do you have an appointment?" And when it's talking, it's talking about the sodium ion. The sodium ion does not have an appointment in this cartoon. So, he's saying this isn't fair, stupid barrier, I'm not able to get through. Whereas the oxygen molecule here is able to get through, and he's saying we have an appointment, let's go, and it's able to cross. This is just a silly cartoon, but hopefully, it drives home the point of how biological membranes are semi-permeable.
This concludes this video, and I'll see you all in our next one.
