Transcription
Okay, so there is a lot going on in this slide. So let's, uh, break it down piece by piece. This is an important slide with information on, um, specifically how molecules cross the plasma membrane. So the plasma membrane, another way of saying this is the cell membrane. So once again, how are we going to know this is a cell membrane? Well, we should see the phospholipid bilayer here, um, with the heads and the tails. So let's say this is outside the cell and we're going to say this is inside the cell. And what we're dealing with here is different types of molecules that have different characteristics and what can and cannot cross the membrane with or without help.
All right, so the first thing we're going to talk about is water. Now, water molecules are very small. And even though they have partial charges, like we know there's a partial negative on this oxygen and we know there's partial positives on these hydrogens, so, so water molecules, um, have these partial charges and water molecules are going to be very unhappy in this area of the membrane. Remember, this area of the membrane is the hydrophobic core of the membrane where the tails of the molecules are clustering to stay away from the water on the outside, the inside of the cell. However, because, um, we talked before about how these hot, these phospholipid molecules are constantly in motion, they're jiggling around because your body has a certain degree of, um, is it 37 degrees Celsius? So they're moving. And when these molecules move, occasionally, um, little chinks can open up between them. The one on the left will dance to the left and one on the right will dance to the right and they'll be like a little gap open up. And these water molecules can then kind of wiggle into the gap. And as soon as the water molecule gets in there, the water molecule is like, this is very uncomfortable. It's very hydrophobic in here. I really don't want to be here. So the water molecule can either turn around and go back out the way it came, or it can dive out the other side. But either way, we're showing that the water molecule can get through the membrane. It doesn't need a transport protein like we talked about earlier, those protein tubes or tunnels. It doesn't need that necessarily. There are some tunnels that help water move in and out, particularly quickly. But in general, water is able to move across a membrane, um, because it's so small. And even though it has charges, which normally would make it not happy in the hydrophobic core, it's, it's because it's so small, it can wiggle through. So we're going to say that water can pass through the membrane without the need of a transport protein. And actually, that process is called osmosis. And what decides whether the water molecule turns around and goes back the way it came, or whether it continues on through, is the solute concentration. And you'll get into that in the lab, so I'm going to leave that to one side. But that is the deal with water.
Now, there are some other molecules over here that can also pass through the membrane, um, without a tunnel. And so that, so they have certain things in common. Number one, if they are very, very small, like water. Like water's only three atoms big, OH, right? It's really small. Um, so another small molecule will be something like oxygen or carbon dioxide or even urea. It's another small molecule, nitrogen. These are all small molecules. Now, the first three, CO2, oxygen, and nitrogen, these aren't charged. They're nonpolar. So because they're nonpolar, they're already hydrophobic. And so this area here is not a barrier. They're quite happy being in here. So they can wiggle through into here. They're not repelled and they can wiggle out the other side. So those molecules can move through. Ethanol is slightly different. Ethanol is has partial charges like water does, but it's also very small like water. So ethanol can also wiggle through the membrane. And then steroids. So steroids, we already know from unit one that steroids are lipids. The cholesterol is a form of lipid. And lipids are all hydrophobic by definition. And so because this is, this is supposed to be cholesterol, right? Or some, maybe it's testosterone, who knows. Um, because they are hydrophobic, uh, again, this area of the membrane, which would normally be off-putting for any molecules with charges or partial charges, they're totally fine with it. And so that's why steroids, even though they're big molecules, they can get through the membrane because the hydrophobic center piece here is through here is not a barrier to them. They like hydrophobic stuff. They mix with it well. And so they can pass through.
Um, and a good example of this, or of how we use this in everyday life, is, um, thinking of something like a birth control patch. All right, um, so a birth control, so you can take birth control orally, like you can just take the pill, or you can have, um, a patch that's infused with the birth control molecules and stuck it on your skin. It's usually stuck on your hip. So birth control is a measured amount of, um, different hormones. And it's mostly progesterone and and estrogen. And so we learned already in the previous unit that those, um, sex hormones, estrogen, testosterone, progesterone, those are all based on cholesterol. And so they are hydrophobic. And so you can take that medicine by just sticking it on your skin because it can cross actually into your cells from just directly into them from the outside because it can cross the membrane.
Now, can you take something, uh, like aspirin or, um, ibuprofen, like sticking it onto you? No, because those molecules are big, just like estrogen, but they also have charges. And those charges make it so that they can't cross through the membrane. So how you deliver drugs into a system has a lot to do with what that molecule, that drug molecule looks like. And if it's a hydrophobic molecule like estrogen, progesterone, or even like nicotine, you can deliver it into the body through just through the skin because it can cross into the membranes. That's good in those situations, or at least you can harness it to get the drug into your system. But it also means you have to be very careful with those kinds of molecules because they can just get into your body through your skin, um, which adds a level of danger to like handling them or that kind of stuff.
And so if you've ever stayed up really late and seen these like late night infomercials on TV on my cable, um, for testosterone stuff, like male enhancement, T1000, T1 billion, or whatever it is, and it's supposed to be like this testosterone substitute or whatever. What they're really selling there is very, very dangerous. They're selling like a, a molecule that is mimicking testosterone. It's not exactly testosterone, but it's very close to the point where it supposedly can fool your body into thinking it's testosterone and reacting in such a way and reacting like your body would react to normal testosterone. And that, that molecule can actually just pass through your membranes directly just by touching it. And if you listen to the like the disclaimers on those infomercials, they'll say things like, pregnant women should not even handle the tablets. And it seems like, whoa, really? Like, really? And the answer is, yes, really. Um, because the whole point about hormones, the definition of hormone is that it can work at very, very, very low concentrations. It doesn't take much to have an effect. And so if someone is pregnant and is growing a baby inside of them, that baby is developing based on the molecules that are washing over it, the hormones and all that kind of stuff, and controls how the baby develops. And so touching a tablet and getting a dose of hormones delivered directly across the membrane into your cells and then into your bloodstream could potentially, you know, get delivered to a developing fetus.
Most, most stuff that you touch, your skin is supposed to be a barrier. Like it's a very good barrier. Most stuff doesn't get in. Most stuff can't cross your skin. So it's very, very good as a barrier. It's just these few kind of molecules that are hydrophobic that have this potential to be able to cross, um, the membranes because they are not repelled by the hydrophobic core right here.
All right, I'm going to continue the next piece on another, another video because this one's long.