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PROFESSOR: One more thing you need to know.
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Whoops.
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The one more thing you need to know is something that's not really a bond or
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a force at all, but it is what we call hydrophobic forces.
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What do I mean by hydrophobic forces?
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What I mean is if I give you a long molecule like this, and all it is is a
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hydrocarbon, lots of carbons with hydrogen around it, and you know that
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all of those bonds are non polar, right?
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Will that make any interactions, any hydrogen bonds with water?
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No, because it doesn't have any polar bonds there.
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So suppose I were to drop this poor, unsuspecting molecule into my highly
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structured cage of water.
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Bounce it around there.
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When it gets in there, like a bull in a china shop, it is disrupting
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hydrogen bonds with the water molecules would like to
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make with each other.
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It's breaking favorable bonds.
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It's energetically unfavorable for this non polar molecule to be coming
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into this polar liquid.
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And when it occupies any space, it is breaking apart hydrogen bonds.
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That's a big energetic cost.
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Suppose I were to take a whole bunch of these non polar molecules and throw
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them into water, and every one of them is there, disrupting the neighborhood,
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breaking up the hydrogen bonds in its neighborhood amongst the water.
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That's energetically very unfavorable.
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The waters would all like to be communicating with each.
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Other hydrogen bonding with each other, and they would like to push out
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these interlopers.
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These non polar molecules.
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And what's going to happen?
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Yep.
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They're going to separate .
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Oil and water.
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That is oil and water.
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If I try to mix together oil and water, they're going to separate.
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I got oil and water.
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Not mixing.
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And that's because the non polar molecules can't hydrogen bond.
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So again, macroscopic.
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Big, observable difference comes from these hydrogen bonds.
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All right.
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So you now know everything.
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Most everything.
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You now know enough.
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You know enough that we can now start applying what we've learned to try to
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explain some of the amazing properties of life.
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Section five.
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Understanding the properties of molecules.
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Let's try something.
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Let's try to understand the cell membrane.
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Example.
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Making a membrane.
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It's arguable that the distinctive critical thing about life is having a
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cell membrane.
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I've got to separate the inside from the outside.
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If I've got a cell, an early cell, I've got to have some bag, some sack
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of material separate from the whole primordial sea.
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And so having a membrane is utterly critical to having life.
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Why does this magical membrane get formed?
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Well, it turns out simple chemistry tells us how the magical membrane gets
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formed, and it's not so magical.
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Let's start with our highly nonpolar molecule over there.
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Let's take that guy.
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I'm going to put six Cs here, but, you know, it could be different numbers.
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And, oh, by the way, I'm beginning to drop my hydrogens.
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And thre will be a whole deep dive in the course about how to write
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molecules, and I'm just going to drop the hydrogens right now.
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Tell me about the property of this molecule.
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Is this going to be a molecule that likes to be in water or not?
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No.
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Not like to be in water we call hydrophobic.
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It's hydrophobic.
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It doesn't like to be in water.
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It fears water, versus hydrophilic.
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Liking water.
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This is a hydrophobic molecule.
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Now, it turns out I can modify this hydrophobic molecule in
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the following way.
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OH.
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A group that you'll come to learn on the homeworks is
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called a hydroxyl group.
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OH is a polar bond.
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It turns out that nearly putting an OH there converts this into from a
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hydrocarbon into what we call an alcohol, and it can dissolve in water.
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The ability to make that polar bond with its OH is enough to make it
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dissolvable in water.
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That's soluble.
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This becomes hydrophilic.
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Now, it's not absolutely hydrophobic, absolutely hydrophilic.
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There are degrees of hydrophobicness this and hydrophobicity and
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hydrophilicity.
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But it becomes more hydrophilic.
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Let me modify this further for you.
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I'm going to put on another group here.
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Gonna put on a carboxyl group.
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Also hydrophilic.
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All good.
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All right, we're practicing our hydrophobics, hydrophilics.
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Now let me carry out a chemical reaction here.
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I'm going to take, boy, I'm getting bored drawing this.
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So how about if I just do this?
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And you'll know what I mean, OK?
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Let's take that, and we're just going to represent our--
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take another one of these guys.
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And another one of these guys.
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Three of these guys.
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Three of these.
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We call these fatty acids here.
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These are fatty acids.
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And I'm gonna do a chemical reaction with the following molecule.
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The chemical reaction I'm going to do is I have those two hydroxyls pointing
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at each other, and I'm going to do a reaction you're going to see again and
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again and again and again and again, which is what's
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called dehydration synthesis.
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Two Hs and an O are kicked out to make a water molecule.
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Water comes off, water comes off, water comes off.
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And I now just instead have a bond here.
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All right.
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So I have my three long hydrocarbon tails.
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I've got rid of my hydroxyls completely.
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I just have that bond there.
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Oxygen connected here, oxygen connected here.
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This molecule, turns out, is called, let's see. it's got three of these
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fatty acids.
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It's sometimes called a triacylglyceride, or just amongst
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friends, a triglyceride.
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This molecule here is called glycerol.
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But this thing here, when we combine them, is called a triacylglyceride, or
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just plain triglyceride.
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I bet, I don't know whether you've had it done, but your triglyceride levels
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are measured by the doctor.
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That's actually part of your cholesterol and fat and other-- and
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they do an analyses, they'll measure your LDL cholesterol and your HDL
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cholesterol, and you triglyceride levels.
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Things like that.
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That's what triglyceride is.
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It's a hydrophobic molecule.
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It's mostly very hydrophobic.
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If I mix that with water, what would happen?
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Separate.
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OK.
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So now, now, one more thing.
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One more thing.
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We'll get rid of this.
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If I take my triglyceride, C, double O, O, CH2 here, CH3 here.
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Two here, sorry.
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C. Wiggle, wiggle, wiggle, wiggle, wiggle.
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And I take off one of those three tails.
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I just have two of these tails.
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And instead, I put on here CH2.
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I put on, whoops, a phosphate group, which is negatively charged.
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What do you think a phosphate group is in terms of hydrophilic or
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hydrophobic?
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It's got these charges, philic, right?
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It's got charges.
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If it's got charges there, it interacts happily with water.
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The other part of the molecule is hydrophobic.
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This part of the molecule is hydrophilic.
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Is this molecule hydrophobic, or is it hydrophilic?
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Both.
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It is confused.
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This is a confused molecule.
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Confused, technically is called amphipathic.
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That's a word for confused, all right?
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This molecule here has these two, I'm now going to even
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simplify my picture further.
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I'm just going to say wiggle, wiggle, wiggle, wiggle, wiggle, wiggle, and
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I'm going to say this head here that's kind of negative, very polar, and this
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is nonpolar.
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And if I have that molecule, it's terribly confused.
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And If I throw it into water, the head is saying, let's go hang out with the
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water, and the tail is saying, you know, the water is pushing away the
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rest of the tail, because the tail is interfering with all those nice
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hydrogen bonds the water wants to make.
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So if I mix this up, what's going to happen?
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It's going to adopt a configuration that best resolves the problem.
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And the way the low energy configuration that it's going to adopt
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could be like this.
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Point your charged hydrophilic heads out into the water solution and stick
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those obnoxious hydrophobic tails in here, and can those hydrophobic tails
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interact with each other?
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Yeah, by van der Waals interactions.
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Not very impressive, but they'll have a good little van der Waals party in
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the middle.
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And they've got these charges outside, and that's a structure that resolves
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the problem.
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That's one way to do it.
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There's another structure that resolves the problem.
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If I had sort of a glass, they could decide to do this.
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Hydrophilic head groups out there, and then another layer here.
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And what I would have is what's called a lipid bilayer.
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This structure goes by the slightly funky name of micelle,
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spelled like that.
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This is a lipid bilayer, because very cleverly it's arranged itself to point
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the hydrophilic groups out, and the hydrophobic groups in.
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There's one other way to get this lipid bilayer to work, other than
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having it going to the edges of my glass here, which is the following.
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Water on the inside, water on the outside, lipid bilayer
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all around in a sphere.
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I don't need anything fancy.
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If I shake it up, spontaneously, just by resolving this bonding property, I
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get a membrane.
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That's where membranes come from.
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I don't need magic.
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I just need those laws of bonding to get it to work.
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So I get this very pretty structure here.
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Somewhat better drawn there, of this lipid bilayer, and this is
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the basis of cells.
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Take a few minutes to think about what you've just heard by
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answering this question.
16201
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