So from our last lesson video, we know that biological membranes are selectively permeable or semi-permeable, meaning that they act as barriers to prevent the diffusion of some molecules. But which molecules freely cross membranes, and which molecules do not freely cross membranes? Well, that's exactly what we're going to talk about in this video. Some molecules can freely diffuse across a membrane without any facilitation from proteins whatsoever. The molecules that can freely diffuse across the membrane without any facilitation from a protein whatsoever are molecules that are really small in size, molecules that are uncharged or have a neutral charge, and molecules that are nonpolar or hydrophobic. Whereas molecules that cannot freely cross or freely diffuse across a membrane without facilitation from a protein are molecules that are really, really large, molecules that are charged with either a positive or a negative charge, and molecules that are polar or hydrophilic. Once again, molecules with these features right here cannot freely diffuse across the membrane without facilitation from a protein. But molecules that have these features at the top, they can freely diffuse across a membrane without facilitation.
Let's take a look at this image down below to get a better understanding of this idea of the diffusion of molecules across the membrane. Notice over here, in the top left what we are showing you are small, uncharged, non-polar molecules. They have all three of the features: They're small, they're uncharged, and they're nonpolar or hydrophobic molecules. When the molecules have all of the features, these molecules are able to cross the membrane freely without any facilitation whatsoever, and that's why we have a big thick green arrow here to show that these molecules have no problems getting across the membrane. This includes molecules such as gases like oxygen gas and carbon dioxide gas as well as nitrogen gas, all of these molecules are capable of diffusing across the membrane with no problem whatsoever.
Now notice over here we have some molecules that have a bit of mixed features. They are small and uncharged but notice that they are polar, or at least they have some polar components. Polar is a component of features that cannot freely diffuse across. Because these molecules have a bit of polar, notice that they can still get across the membrane, but they don't get across the membrane as easily as these other molecules over here, and that's why we have a thin skinny green arrow to show that they still get across but not as easily as the ones that have all of the features that can freely diffuse. This includes molecules like water and a type of steroid, and a molecule called glycerol. These molecules are small and uncharged but are polar. Hence, they can get through the membrane but not as easily as the other ones over here.
Now notice over here in this box, we're showing you molecules that are charged or polar, and so those are features of molecules that cannot freely diffuse across the membrane. Notice that these molecules cannot diffuse across the membrane. They cannot get through the membrane without any facilitation. And notice that this last category of molecules down below are large macromolecules. 'Macro' is a prefix that means large. These are really large molecules like large polypeptides or large proteins as well as large polysaccharides and nucleic acids. Because they are large, they are features of molecules that cannot freely diffuse across the membrane. Once again, notice that these molecules are not able to diffuse across the membrane, and so they cannot get through.
This concludes our introduction to which molecules can freely cross the membranes and which molecules cannot freely cross the membrane. This little table here summarizes it, what you should know. The molecules that can freely diffuse are going to be very small, uncharged, nonpolar, and hydrophobic, charged either positive or negative, or polar or hydrophilic. We'll be able to get some practice applying these concepts as we move forward in our course, so I'll see you guys in our next video.