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  • >> Hi, everyone, and welcome to the Penguin Prof Channel.

  • Today I want to introduce you

  • to the wonderful world of cell membranes.

  • Okay, so if you ask people to draw a cell,

  • generally what you'll get is a circle,

  • and most people don't really think too much

  • about the circle itself, but it's actually really important.

  • The circle really defines, you know, you versus everything else

  • in the universe that's not you, but it's more complicated

  • than that because inside the cell you're going

  • to produce wastes that need to get out, and there's going

  • to be food and good stuff that needs to get in.

  • How are you going to do it?

  • What happens if you puncture this barrier and goodness

  • from the inside leaks out and bad stuff

  • from the outside leaks in?

  • How are you going to manage all this?

  • So the circle itself is actually worthy of our investigation.

  • So what the heck are these membranes like?

  • Are they sort of like a container or, you know, a bag?

  • I mean, that separates, you know, inside from outside.

  • I mean, oh my God.

  • That's so cute.

  • Is it like a barrier?

  • I mean, if you get a hole in your barrier, that's not good.

  • The truth of the matter is that membranes really aren't

  • like any container or barrier that we're familiar

  • with in our everyday life,

  • so that makes them a little bit harder

  • to imagine and talk about.

  • Membranes have to control what goes in and out of the cell.

  • They have to be free to move and signal and communicate

  • and repair and do all kinds of other things.

  • I mean, so much of what the cells do happens

  • at the cell membrane, but you always have this balance

  • of control versus freedom.

  • So how are you going to build a barrier

  • that can do all of those things?

  • One of the more amazing things I think that membranes do is

  • that they form all by themselves,

  • and if you puncture them, they reseal immediately.

  • It's kind of like magic.

  • How the heck are we going to do that?

  • The magic of cell membranes comes from the fact

  • that they're built from molecules called amphiphiles.

  • Now, that may not be a term that you're familiar with,

  • but it comes from the Greek, and there's a word

  • that you probably do know, and that's amphibian.

  • Amphis means both, so amphibians live both on land

  • and in the water, so that will help you to remember it.

  • In chemistry, we're talking about molecules

  • that have both a hydrophilic -- that is, a polar end --

  • and a hydrophobic -- that is, a nonpolar end.

  • So these are really cool molecules

  • because they kind of play both sides.

  • There are actually amphiphiles that are present

  • in our regular life in the kitchen and so forth.

  • We use amphiphiles for detergents as well as for soaps.

  • We use them for surfactants.

  • They help to break up fat.

  • They make fats dissolvable.

  • This is a really cool little mnemonic device

  • that you can remember.

  • Amphibians hold amphiphiles.

  • So this little image can help you to remember that word.

  • The secret then is not in the sauce;

  • it's in the amphipathic lipids that build cell membranes.

  • So we have on every molecule of a phospholipid a hydrophilic --

  • that means water loving -- polar head.

  • This polar head is comprised of a glycerol molecule,

  • a phosphate, and then there's the side chain in our group.

  • We'll see what those are here in a second.

  • And then there is a nonpolar end that's made

  • up of two fatty acids tails, and they are hydrophobic.

  • They are water fearing.

  • So you have this amazing, amphipathic molecule

  • that has one side that is polar and likes water and one side

  • that is nonpolar and fears water.

  • Oh my gosh.

  • We're going to see why that's such a great thing,

  • but before we do that, I said I would show you some

  • of the actual particulars

  • about phospholipids and also glycolipids.

  • If you need to know a little bit more detail about them,

  • probably one of the more common varieties

  • in animal cell membranes is this guy, phosphatidylcholine.

  • So these are the different options, and then the rest

  • of it, the, these are the fatty acids that are all trailing.

  • These are the hydrophobic parts of the beast, okay,

  • if you needed a little bit more detail there.

  • So so what?

  • What you got to remember is that in chemistry, like likes like.

  • So polar molecules stick together

  • and nonpolar molecules stick together, but polar

  • and nonpolar molecules do not mix.

  • You have seen this if you have poured oil in water.

  • Everybody knows that oil and water don't mix.

  • The reason is that water is polar and oil is nonpolar.

  • They hate each other.

  • So you'll get this film of oil on top of water.

  • So what if you have a molecule that plays both sides?

  • And that's the key.

  • We've got these amazing phospholipids

  • that will spontaneously form a bilayer all by themselves.

  • Oh my God.

  • That's so cool.

  • They actually will form other shapes too, but for the purposes

  • of cell membranes, this is the shape

  • that we are most concerned with.

  • The most favorable shape for phospholipids to make is a shape

  • where all the polar ends face the water because they

  • like water and all the nonpolar ends face each other.

  • The fatty acid tails point inward.

  • Isn't that cool?

  • If you really want to see this, I guess the easiest way

  • to see it is if you play with your soup

  • and you have some oil drizzled on the top of your soup

  • and get yourself a fork, okay.

  • Don't let anybody tell you not to play with your soup.

  • Get yourself a fork and experiment

  • with different amounts of pressure and speed

  • and drag the fork through the oil and see

  • if you can get the big globs of oil to break

  • up into smaller globs, and then see

  • if you can get the little globs to join together

  • and congeal into big globs.

  • If you watch the behavior of the oil, that's really kind

  • of the most similar thing that you can see with the naked eye

  • that is similar to what phospholipids do in solution.

  • So in 1972 after a lot of study --

  • membranes are actually very delicate,

  • so it turns out they're very hard to study --

  • they were able to freeze and then split a membrane

  • and expose the center of the phospholipid tails,

  • and that's what you're seeing right here.

  • And then they, it's called freeze fracture

  • electron microscopy.

  • They were able to actually verify

  • that this is what cell membranes look like

  • and these are the different components of cell membranes.

  • So what you've got, about 75% -- this is for animal cells,

  • by the way -- about 75% of the membrane is made

  • of these phospholipids, the molecules

  • that we just looked at, which the polar end shown here

  • and the fatty acid, the nonpolar end, shown there.

  • And they insert themselves all by themselves

  • into a bilayer like this.

  • In addition to that, we have glycolipids.

  • They make up about five percent of the membrane.

  • And cholesterol.

  • Check out the cholesterol.

  • Cholesterol, about 20% of most animal cell membranes can

  • be cholesterol.

  • That can be as high as 50%, by the way.

  • Depends on the species of animal.

  • The cholesterol will actually stabilize the structure

  • of the membrane, and by stabilizing the phospholipids,

  • animal cells actually get away with not having

  • to have a cell wall, which is pretty cool.

  • The other thing that you notice throughout are proteins,

  • and some proteins span the cell membrane like these.

  • We call them integral proteins.

  • And some of them are only on one side

  • of the cell membrane or the other.

  • Could be the inside or the outside.

  • We call those peripheral membrane proteins.

  • You might think for a minute

  • about how a cell could anchor proteins in a cell membrane,

  • and how you do it is of course you recall

  • that proteins are made of chains of amino acids,

  • so how you anchor something into a structure

  • that is not a solid is you have

  • to make the membrane-spanning portions nonpolar,

  • so they like to hang out with the fatty acid tails.

  • And then on either side of these integral proteins, so here

  • and here, these amino acids would be polar.

  • So they don't like to hang out with the fatty acid tails.

  • They prefer the polar heads

  • and the water that's surrounding this bilayer on both sides.

  • So that's pretty amazing.

  • All of this allows for the amazing variety

  • of membrane functions that we're going to see as we go

  • through biology and physiology.

  • Membranes really do account for a lot

  • of what cells are able to do.

  • One thing that's really important is membranes allow

  • for compartmentalization, and that's going

  • to allow us to create gradients.

  • Something you're going to hear me say all the time is,

  • you know, this is a gradient-driven process.

  • Well, how do you have a gradient?

  • You have to have a separation.

  • You have to have one side which is different

  • from the other side -- different in concentration, in pH,

  • or whatever the variable is --

  • but membranes allow for that to be possible.

  • Membranes are going to allow for cells to recognize each other.

  • They're essential for cell-cell recognition, communication,

  • for one neuron to talk to another, for hormones

  • to be received by receptors, and so much more.

  • So you're going to see as you go through your studies of biology,

  • membranes are essential.

  • I hope that that introduction

  • to cell membrane structure was helpful.

  • As always, I ask for your comments and your subscriptions.

  • Please visit on Facebook and follow on Twitter.

  • Good luck.

>> Hi, everyone, and welcome to the Penguin Prof Channel.

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B1 中級

会う...細胞膜だ! (Meet... The Cell Membrane!)

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    Amy.Lin に公開 2021 年 01 月 14 日
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