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


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]

100 Comments

Add a Comment

Your email address will not be published. Required fields are marked *