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 gonna talk about in this video. Now some molecules can freely diffuse across a membrane without any facilitation from proteins whatsoever. And so the molecules that can freely diffuse across the membrane without any facilitation from a protein whatsoever are molecules that are really, really small in size molecules that air uncharged or have a neutral charge, and molecules that are non polar or hydro phobic, 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 air polar or hydro filic. And so, 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. So 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. So notice over here in the top left what we're showing. You are small, uncharged, non polar molecules and so notice that they have all three of the features. They're small, they're uncharged, and they're non polar or hydrophobic molecules. And so 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. And so this includes molecules such as gas is like oxygen, gas and carbon dioxide gas, as well as nitrogen as well 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 little bit of some mixed features here. They're small and uncharged, but notice that they are polar or at least they have some polar components. And polar is a component of features that cannot freely diffuse across. And so because these molecules have a little bit of polar notice that they can still get across the membrane. But they don't get across the membrane as easily as thes 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. And so this includes molecules like water, and, uh, this molecule here, which is a type of steroid, and this molecule over here, which is a molecule called glycerol. And so these molecules are small and uncharged, but yet they are polar, and so they could get through the membrane, but not as easily as these 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 alright here, charged or polar of molecules that cannot freely diffuse across the membrane. So notice that these molecules they cannot diffuse across the membrane they cannot get through the membrane without any facilitation, and notice that this last category molecules down below are large macro molecules. Macro is a prefix. That means large. So these air really, really large molecules like large poly peptides or large proteins. And a swell is large polly sack rides and nucleic acids. These molecules, because they're large, they are features of molecules that cannot freely diffuse across the membrane. So once again notice that these molecules are not able to diffuse across the membrane and so they cannot get through. And so really, this year concludes our introduction to which molecules can freely across the membranes and which molecules cannot freely across the membrane. And once again, this little table here summarizes it. What you should know. The molecules that can freely diffuse are going to be very, very small, uncharged and non polar and hydrophobic, which, whereas the molecules that cannot freely diffuse across the membrane, will be large charged, either positive or negatively charged or polar or hydro Philip. So 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