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  • Hello, and welcome to the kitchen. I wanted to invite you here today because last week

  • we started off in my bathroom and kinda feel bad about that. And also because

  • as I'm making lunch today I wanted to sort of use it as a lab. During this time in my

  • kitchen I'm going to talk to you about three different things:

  • 1) the three most important molecules on the Earth

  • 2) possibly the grossest sandwich I'm ever going to eat

  • 3) an obscure scientist who taught us almost everything we know about urine.

  • So far we've talked about carbon and we've talked about water, and now we're going

  • to talk about the molecules that make up every living thing

  • and every living thing in every living thing. I don't care if you're a bacterium or

  • a blue whale or if you're Lady Gaga, of if you're a mite living on the Queen of England's eyelashes.

  • They're called biological molecules. These aren't just building blocks, these are the

  • molecules necessary for every living thing on Earth to survive. They are essential sources

  • of energy. They are the means of storing that energy. They are also the instructions that

  • all organisms use to be born and grow and to ultimately pass those same instructions

  • on to their future generations.

  • They are the ingredients for life.

  • And we call them: the carbohydrates, the lipids, the proteins, and the nucleic acids.

  • Today we're going to be talking about the first three. It's no coincidence that we

  • classify them in the same way that we classify food, because they're food. [Hank so smart!]

  • And for this classification, we have to thank a little-known English physician who hundreds

  • of years ago dedicated his life to the study of human pee.

  • Oh, my goodness... I'm back in the-

  • That must mean that it's time for the most awkwardly named segment here on crash course:

  • Biolo-graphy.

  • His name was William Prout. In the early 1800s, he became fascinated with human digestion

  • especially our urine. That's because he thought the best way to understand the

  • human body was through chemistry, and the best way to understand body's chemistry was

  • to understand what it does with food. By day he was a practicing physician, but every morning

  • before breakfast he did research in his home laboratory in London. And there he did many

  • great things. Like, being the first to discover that our stomachs contained hydrochloric acid.

  • And writing a breakthrough book about kidney stones called

  • An Inquiry Into the Nature and Treatment of Gravel, Calculus, and Other Diseases Connected

  • with a Deranged Operation of the Urinary Organs.

  • And he was, of course, the first person to discover the chemical composition of pure

  • urea, the main component of urine.

  • For the record, here it is: CO(NH2)2. And in the presence of water, urea gives off ammonia,

  • which is why your pee smells.

  • Through his years of studying urine, Prout came to the conclusion that all "foodstuffs"

  • fell into three categories: the saccharinous (carbohydrates), oleaginous (fats), and albuminous

  • (proteins). Indeed, he went so far as to say that in order to be healthy, you needed to

  • eat all three of these things and not just sheep kidneys and gin, which is what most

  • of London was living on at the time.

  • But like many great minds, Prout was overlooked in his own lifetime, because while he was

  • studying actual science, everyone else was walking around believing that the color of

  • your urine was determined by your personality.

  • This guy looks like a total jerk to me!

  • And if you can tell that much by color I wonder what you could tell by taste.

  • Now he isn't understand that there were biological molecules

  • He didn't understand what these things were, but he did understand that there were three

  • ingredients necessary for life. And it turns out that all organisms either need to synthesize

  • or ingest those ingredients in order to live.

  • We're going to start out with the most basic of these ingredients for life: Carbohydrates.

  • You've no doubt heard of them. You may, in fact, be avoiding them like the plague.

  • But fact is that nothing, and no one, can avoid carbohydrates, because they are the

  • source of all energy that we have available to us.

  • Carbohydrates are made up of sugars, and the simplest of them are monosaccharides. Mono

  • for one, and saccharides for the actual root of the word sugar. The star of the show here

  • is glucose, because it's truly fundamental, by which I mean like Number One on the global

  • food chain. Because it comes from the sun.

  • All biological energy is originally captured from the sun by plants as glucose through

  • photosynthesis. And every cell that needs energy uses glucose to get that energy through

  • a process called respiration.

  • In addition to glucose there are other monosaccharides, like fructose, which has the same molecular

  • formula (C6H12O6), but arranged differently. These subtle chemical difference do matter.

  • Fructose, for example, is significantly sweeter than glucose. It's also processed by our bodies

  • in different ways.

  • And then there are disaccharides which -- like the name says -- are just two monosaccharides

  • put together. The most famous of these is sucrose, which is simply a glucose molecule

  • and a fructose molecule joined by a covalent bond.

  • Mono- and disaccharides are pretty much little niblets of energy that are really easy for

  • our body to process, but when these carbohydrates start to form into longer and longer chains,

  • their function and their roles change as well. Instead of being sources for instant energy,

  • they become storehouses of energy or structural compounds.

  • These are the polysaccharides. Instead of being just two or three monosaccharides put

  • together, polysaccharides can contain thousands of simple sugar units.

  • And because they're so big and burly, they're great for building with. In plants, cellulose

  • is the most common structural compound. It's just a bunch of glucose molecules bound together

  • and it is the most common organic compound on the planet.

  • Unfortunately, it's very difficult to digest. Cows can do it, but humans certainly cannot,

  • which is why you don't enjoy eating grass.

  • Polysaccharides are also really good for storing energy and not just structurally but just

  • as an energy store. And that's where we get bread. Now, really interesting thing here:

  • Bread is made up of starch, the most simple of which is called amylose. Amylose and cellulose

  • looks almost exactly identical, but one is grass and the other is bread.

  • Like, chemistry! [Well said.]

  • Plants store glucose in the form of starch, and it comes in lots and lots of different

  • forms, from roots and tubers to the sweet flesh of fruits, to the starchy seeds of the

  • wheat plant that end up being milled into flour.

  • Ground-up grain is the main ingredient in the bread, of course, most of the calories,

  • or energy content, comes from carbohydrates. When I eat this -- and I am going to eat the

  • hell out of it -- I'm going to be eating all of the chemical energy that this wheat

  • plant got from the sun in order to feed it's next generation of seeds that we then stole

  • for our own use.

  • Now we, as human beings, can't grow fruits or tubers, so we have to store our energy

  • in a couple of different ways. The way that we tend to store carbohydrate energy is in

  • glycogen, which is very similar to amylose or starch, but has more branches and is more

  • complicated. It's basically made up of the glucose we have left over after we eat and

  • it sits in our muscles where it's ready to be used and it's also stored in our livers.

  • It's generally a pretty short term store. If we don't eat for a day pretty much all

  • of our glycogen gets depleted.

  • But over the longer term, the way we store energy is through

  • Fat! All of our mom's worst enemies: the fat. Which turn out to be really important and

  • are the most familiar sort of a very important biological molecule: lipids.

  • Lipids are smaller and simpler than complex carbohydrates, and they're grouped together

  • because they share an inability to dissolve in water. This is because their chemical bonds

  • are mostly nonpolar, and since water, as we learned in the previous episode despises non

  • polar molecules, the two do not mix. It's like oil and water.

  • In fact, it's EXACTLY like oil and water!

  • And if you've ever read a nutrition label or seen this thing called the television,

  • you're probably pretty conversant in the way we classify fats. But then 99% of us have

  • no idea what those classifications actually mean.

  • Fats are made up mainly of two chemical ingredients: glycerol, which is a kind of alcohol, and

  • fatty acids, which are long carbon-hydrogen chains that end in a carboxyl group.

  • When you get three fatty acid molecules together and connect them to a glycerol, that's a

  • triglyceride -- they feature prominently in things like butter, peanut butter, oils, and

  • the white parts of meat.

  • These triglycerides can either be saturated or unsaturated. And I know that when we put

  • the word "fat" and "saturated"  into the same sentence it sounds like an evening

  • at KFC, but here we're talking about being saturated with hydrogen.

  • As you hopefully remember from our first lesson: Carbon is very nimble in how it uses its four

  • electrons. It can form single, double or triple bonds.

  • This means that if the carbon atoms in a fatty acid are connected to each other with single

  • bonds, all of the carbon atoms end up connected to at least 2 hydrogen atoms. And of them

  • even picks up a third. So the fatty acid is saturated with hydrogen.

  • But when some of the carbons atoms are connected to each other with double bonds all of those

  • carbons' electrons are spoken for, so they're not able to pick up those hydrogen atoms.

  • This means that they're not saturated with hydrogen and they are unsaturated fatty acids.

  • To demonstrate, may I direct your attention to this jar of peanut butter?

  • Here you can kind of see both kinds of fats. The liquid stuff you see at the top here:

  • that is the unsaturated fat, which we generally think of as oils. The pasty stuff down here

  • also contains lots of unsaturated fat but also contains saturated fat, which doesn't

  • have any double bonds and so it can pack more tightly and form solids at room temperature.

  • And there are also other fat classifications that you've heard of.

  • Trans fats, which everyone tells you NEVER to eat. They're right, don't eat them! They

  • don't exist in nature and are basically unsaturated fatty acids that instead of kinking go straight

  • across and so they're super bad for you. Don't eat them.

  • Omega-3 fatty acids are unsaturated at the 3 position, which is like, right there [where?].

  • And that's the only difference, but the reason that these are important is because we can't

  • synthesize them ourselves. They're essential fatty acids meaning that we need to eat them

  • in order to get them.

  • All this is starting to make me pretty hungry, but before we get to more food stuff there

  • are some unappetizing sort of lipids that we also need to talk about.

  • So remember that triglycerides are three fatty acid chains connected to a glycerol? Swap

  • one of those fatty acids for a phosphate group and you have a phospholipid. These make up

  • cell membrane walls. Since that phosphate group gives that end polarity, it's attracted

  • to water. And the other end is nonpolar and it avoids water. So if you were to scatter

  • a bunch of phospholipids into some water, they would automatically arrange themselves

  • like this with hydrophobic ends facing each other, and hydrophilic ends sticking out to

  • face the water. Every cell in your body uses this natural structure to form its cell membrane

  • in order to keep the bad stuff out and the good stuff in.

  • Another class of lipids is the steroids. Steroids have a backbone of four interconnected carbon

  • rings, which can be used to form hundreds of variations. The most fundamental of them

  • is cholesterol, which binds with phospholipids to help form cell walls. But they can also

  • be activated to turn into different lipid hormones.

  • And so now we approach the most complicated, powerful, polymorphously awesome chemicals

  • in our body: the protein.

  • And by complicated I mean that they are probably the most complicated chemical compounds on

  • the planet. In fact, they are so amazing that we're going to so a separate episode on them,

  • and how they're created by DNA. But right now, in you, there are tens of thousands of

  • proteins doing everything they can to keep you alive.

  • There are enzymes regulating chemical processes, helping you digest food. There are antibodies

  • connecting themselves to invaders like bacterium and viruses so that your immune system can

  • get them. There are protein endorphins that mess around with your brain and make you feel

  • emotions [Gross!]

  • But they're everywhere, they do EVERYTHING!

  • And proteins do all of this stuff using only 20 different ingredients. And these are the

  • amino acids.

  • Just like fatty acids, amino acids have a carboxyl group on one end. On the other end

  • they have an amino group. Amino acid!

  • Now hey, I don't know if you've noticed this, but this is the first time nitrogen

  • has shown up in our food. This is super important, because despite the fact that nitrogen is

  • everywhere -- it's like 80% of the air -- we can't just pull it out of the air and put

  • it into our bodies. We have to get nitrogen from food. And so we have to eat foods that

  • are high in protein like this egg, which by it's very virtue, because all the white part

  • is protein, it contains a goodly amount of nitrogen.

  • Now in the middle of the amino and the acid group is a carbon. It shares one of its electrons

  • with good ol' hydrogen, and the other electron is free to be shared with "R"

  • Which is just a kind of fill in the blank. We call it "The R group"

  • It can also be called a side chain, and there are 20 different kinds of side chains. Whatever

  • fits in that blank will determine the shape, and the function, of that amino acid.

  • So if you put this in there, you get valine, an amino acid that does a lot of stuff, like

  • protecting and building muscle tissue. If you put this in there, you get tryptophan,

  • which may be best known for its role in helping you regulate mood and energy levels.

  • Amino acids form long chains called polypeptides. Proteins are formed when these polypeptides

  • not only connect but elaborate and frankly really elegant structures. They fold. They

  • coil. They twist. If they were sculptures, I would go the museum every day just to look at them.

  • And I'd walk straight past the nudes without even looking.

  • But protein synthesis is only possible if you have all of the amino acids necessary,

  • and there are nine of them, that we can't make ourselves.

  • Histidine, isoleucine, leucine

  • Lysine, methionine, phenylalanine

  • Threonine, tryptophan, and valine.

  • By eating foods that are high in protein, we can digest them down into their base particles

  • and then use these essential amino acids in building up our own protein.

  • Some foods, especially ones that contain animal protein, have all of the essential amino acids

  • including this egg

  • And that concludes this triple-decker sandwich of biological awesomeness, which is all we

  • need to be happy, healthy people.

  • And I'm sure, because of that, it's going to be delicious.

  • Nope.

  • Thank you for watching this episode of Crash Course. I will be discussing something else

  • very interesting next week. I don't even know what it is.

  • Don't forget. Go back and reinforce what you've learned today by going back and watching bits

  • that you feel like you may not have got completely.

  • We'll also, of course, be available on Facebook and twitter if you would like to ask us questions

  • or give us suggestions there.

  • [BURP]

Hello, and welcome to the kitchen. I wanted to invite you here today because last week

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Biological Molecules - You Are What You Eat: Crash Course Biology #3

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    Chi-feng Liu   に公開 2013 年 04 月 23 日
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