So from our last lesson video, we already know that biological membranes are semi-permeable or selectively permeable, which really just means that they're really picky about what types of molecules they allow across the membrane freely. In other words, being semi-permeable just means that it is capable of preventing the diffusion of some types of molecules across the membrane depending on the properties of those molecules. But here's a question for you: Which types of molecules are able to freely cross membranes and which types of molecules are not able to freely cross membranes? Well, that's exactly what we're going to focus on here in this video.
Some molecules can freely diffuse across a membrane without any facilitation from a protein whatsoever. The molecules that can freely diffuse across a membrane without any facilitation from a protein whatsoever are going to have very specific properties that include having a very small size, being uncharged with a neutral net charge of 0, and being nonpolar or hydrophobic. Again, these three features—being small in size, uncharged, and being nonpolar or hydrophobic—are all features that correspond with molecules that can freely diffuse across a membrane without any facilitation from a protein.
On the other hand, molecules that cannot freely diffuse across a membrane without any facilitation from a protein are going to have the opposite features. Instead of being really small, these molecules are going to be really large. Instead of being uncharged, these molecules are going to be charged with an overall net charge that is positive or negative. And instead of being nonpolar or hydrophobic, these molecules will be polar or hydrophilic.
If we take a look at our image down below, we'll be able to get a better understanding of which molecules are able to freely diffuse across the membrane and which ones cannot freely diffuse across the membrane without facilitation from a protein. In this image, note that in the middle, we're showing you our biological membrane. On this side of the membrane, this is representing the inside of our cell or the cytosol, the cytoplasm. And on the outside over here, of course, it's representing the outside of our cell.
Notice that we have these different categories of molecules, and we're looking at their ability to freely cross the membrane without facilitation from a protein. The first block of molecules that we have here are all molecules that are relatively small, uncharged with a neutral net charge of 0, and nonpolar or hydrophobic molecules. These molecules include oxygen gas, carbon dioxide gas, and nitrogen gas. Being small, uncharged, and nonpolar, you might recall are all tendencies that correspond with being able to freely diffuse across the membrane without facilitation from a protein. That means that these molecules will easily be able to cross this membrane, from the outside of the cell to the inside of the cell. Notice that we have this really big green thick arrow going across the membrane to represent that these molecules have no problem whatsoever freely crossing the membrane since they have all of the correct tendencies and correct properties that allow them to do so.
If we look at the next category of molecules, notice that we have mixed features that correspond with being able to freely diffuse and that correspond with not being able to freely diffuse across the membrane. Notice that we have molecules that are going to be small and uncharged, which correspond with features of molecules that can freely diffuse, but these molecules are also polar. Being polar or hydrophilic is a feature of not being able to freely diffuse across the membrane. These molecules include water, the steroid that you see right here, and glycerol. Notice over here this other category of molecules are molecules that are either charged or polar molecules. Being charged or polar are features of not being able to freely diffuse across the membrane without facilitation from a protein. These molecules here include molecules that are sugars, molecules that are charged ions, and amino acids like this one which again are going to have these charges.
Last but not least, this category of molecules here are large macromolecules. Recall that the root 'macro' means large, and again being large is going to be a feature that corresponds with not being able to freely diffuse across the membrane. These large macromolecules include substances like polypeptides or proteins, polysaccharides or long carbohydrates, as well as nucleic acids such as DNA or RNA. Notice here that we have this red arrow that is bouncing off the membrane, indicating that these large molecules cannot freely diffuse across the membrane without any facilitation.
This here concludes our brief lesson on which types of molecules can freely cross membranes and which ones cannot. Again, the molecules that can freely diffuse across a membrane without protein facilitation are going to be small, uncharged, and nonpolar hydrophobic, whereas the ones that cannot freely diffuse across the membrane without facilitation are going to be large, charged, and polar hydrophilic. We'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.
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