- I made a video a couple of weeks ago explaining why a solution
of sugar will always twist polarized light to the right. I recommend you watch the original video if you haven't already,
but if you don't want to then the short answer is because
sugar molecules are chiral. I didn't actually use the word, "Chiral," in the original video, which is a shame 'cause it's a good word. I talked about handedness instead, which basically means the same thing. It just means that the molecule of sugar doesn't have mirror
symmetry in the same way that my left hand doesn't
have mirror symmetry. If you look at my left hand in the mirror you won't see another left
hand, you'll see a right hand. It's the same thing with sugars. If you buy sugar from the shops, then all those sugar molecules in there will be right-handed. They will all turn light to the right. To use the formal language,
all chiral molecules have a left-hand version
and a right-hand version. They're called the
enantiomers of the molecules. So look, here's one enantiomer of glucose and there in the mirror,
that's the other enantiomer. And when you buy glucose from the shops, it's always, always, always
the right-hand enantiomer that you get. (enantiomer clicking) Now, many of you were
not entirely satisfied with my explanation and quite rightly because there's a big question mark. Why are all the sugar molecules that you buy from the shops right-handed? And it's a good question because if you were to make
some sugar for yourself in a chemistry lab and mixing
some chemicals together, you would get a 50/50
mix of left-handed sugar and right-handed sugar. But we don't make sugar in the lab. We get sugar from plants and plants always make right-handed sugar. So now you should be satisfied. "But wait. "Why do plants always
make right-handed sugar? "You haven't given us a
full explanation at all!" Well, plants don't make sugar
by mixing chemicals together, like we would do in a lab. Instead a plant make sugar
one molecule at a time, using tiny molecular machines. When I say tiny molecular machines, I'm talking about enzymes. These large proteins that
bring the ingredients together and stick them together. And these proteins are chiral themselves. There's a left-hand version
and a right-hand version, but you only ever see
one of those enantiomers. So if all the enzymes that are involved in the production of sugar
all have the same handedness, then they will produce sugar molecules that all have the same handedness. And so now you should be satisfied. "But wait, hold on a second. "Why do all the enzymes
have the same handedness? "That doesn't make sense? "You need to explain that as well!" All right. So there's this thing in
nature called, "Homochirality." All the molecules found
in nature, or at least all the chiral ones, are only ever found with one of the two possible handednesses. So for example, all sugar is right-handed. All amino acids are left-handed. DNA is always right-handed. Any kind of molecule that's chiral, you only ever find one of the enantiomers and never the other. And that needs to be explained. Homochirality pops up in
lots of different places, by the way, not just biology. For example, screws are homochiral. The vast, vast majority
of screws that you can buy are right-handed screws. And that's a good thing. Imagine if manufacturers produced sometimes left-handed,
sometimes right-handed, the phrase, "Righty tighty, lefty loosey," would only apply 50% of the time. Fusilli pasta is mostly homochiral. Though I did find one brand
that does it the other way. Scissors are chiral.
(scissors snipping) Yeah, they're designed
for right-handed use. Look, as I'm cutting the page here
(scissors cutting) I can see where the blade
(scissors cutting) is cutting through the paper.
(scissors cutting) But if I try and do that with my left hand the blade is obscuring
the location of the cut. I'm left-handed actually and I
once went to a shop years ago in central London that sells
things for left-handed people, anythinglefthanded.co.uk. And I tried out the left-handed scissors. I couldn't get used to it
because I'd spent my whole life learning to use right-handed
scissors with my right hand. How sad is that? Imagine if growing up I had
access to left-handed scissors? I'd be cutting away like anything. It'd be amazing. I'd be this grand cutter of things. As it is I'm a suboptimal cutter of things with my non-dominant hand. - [Director] Get on with it.\! - Yeah, all right. So we need to explain why
all the molecules in nature are only ever found in one of the possible mirror image options and not the other. It's all to do with
interoperability and efficiency. Like, imagine if there were
two versions of everything. Like, if there were both
versions of the sugar, then the enzymes that we
use to metabolize sugar and get the energy from it, you would need both
versions of those as well. And if you did. If you had both left and
right versions of sugar and left and right versions of the enzymes that we use to metabolize sugar, then... Even if you had that
system like half the time, you'd have the wrong kind of sugar bumping into the wrong kind of enzyme. So it would be hugely inefficient. It's like, if we didn't
agree to shake hands with our right hand, occasionally you'd go to
shake hands with someone and it would fail because you're shaking with your left hand, they're
shaking with their right hand. You might not be wondering
why you don't have some life forms that do it all one way and other life forms that
do it all the other way. But in reality, life is so interlinked. You know, this species is
gonna be linked to this species through mutualism, or predation. You know, then this
species's gonna be linked to this species. The whole of biology is linked in this massive web of
things eating other things. So you can't sustain a
pocket of mirror image because it will fail. It won't be able to cooperate with the rest of the biological system. One fanciful consequence is that, if through some weird space-time anomaly you found yourself in a mirror
version of this universe, you would be absolutely screwed. So there's the answer. Well, not quite because you might now ask, "Well, why is it that way
and not the other way? "There's two options. "Why did life choose this option? "Why is it right-handed sugar,
left-handed amino acids, "right-handed DNA? "Why isn't it left-handed
sugars, right-handed amino acids, "left-handed DNA?" You can reframe the
question by talking about the primordial soup. The conditions on Earth
just before life emerged. Scientists think that it was
a mixture of simple molecules, like sugars, amino acids, nuclear bases. Those are all things
that can occur naturally. You don't need life to get
those building blocks for life. It's reasonable to assume
that the primordial soup was a 50/50 mix of left-handed
and right-handed molecules. So why was one set chosen
over the other for life? One hypothesis is that you've
got these simple molecules bumping into each other, randomly
forming larger molecules. And just by chance, at some point, they bumped into each other in such a way as to create a larger molecule
that is self-replicating. It's a rare event, so it
only happens that one time. And just by chance it happens
with right-handed nucleotides instead of left-handed nucleotides. And so you've got this
replicating molecule. You're getting more and more of it. It's evolving as well, so it's getting even
better at replicating, creating more and more of itself. So the population of
right-handed nucleotides and the corresponding right-handed sugars and left-handed amino acids are increasing in number in the world. Eventually the world becomes
full of homochiral molecules and all those original molecules
of the other handedness decay away without being replaced. Another possibility is that the emergence of self-replicating
molecules is not that rare. And it happened more than once. So you have this 50/50 mix
of replicating systems. One has one handedness, the
other has the other handedness. But that 50/50 mix is
an unstable equilibrium. If one of the populations grows slightly compared to the other just by chance, then it has an advantage
because it's surrounded by more of those molecules
that it finds useful. So the unstable equilibrium of a 50/50 mix quickly moves away to a
homochiral population. The problem with both of those hypotheses is that they don't fit
well with the chemistry that most scientists assume
was driving life at the time. It's called the, "RNA world hypothesis." And it assumes that the initial replicating molecules
of life were RNA-based. And it makes sense because
RNA, which remember, is the single-strand version of DNA. It can be created spontaneously through polymerization of a nuclear bases. That can happen without
the presence of life. It can act as a store of
information just like DNA can. But unlike DNA it can also act as the replicating machinery of life. I didn't know this, but the ribosome, you know, that big
molecule inside your cells that reads genetic code
and spits out proteins built from amino acids. That molecule itself
is not a protein built from amino acids. It's actually a tangled up RNA molecule. In other words, you can have a system of molecular replication
that is just RNA-based. The problem is this. We know experimentally that
if you try to grow RNA chains by polymerizing nuclear bases, you'll do fine, so long
as all the nuclear bases have the same handedness. If you introduce the
opposite handedness base, it will shut down the process. You can't grow RNA chains in
a 50/50 mix of nuclear bases. So either there's a system
of molecular replication that predates the RNA world model and that system was able to create the homochiral environment needed for the RNA world to thrive, Or there's somehow not a 50/50 mix of left and right-handed
molecules in the primordial soup. And that needs to have arisen
through non-biological means. There are hypotheses that
support both those options. For example, there are chemical reactions that are self-catalyzing. So if you have a 50/50
mix of chiral molecules, but one is slightly larger than the other, so long as those chiral
molecules are self-catalyzing, as in, they create more of
themselves as catalysts, then you'll very quickly
get to a homochiral mix. We've never seen experiments
of that happening with the basic building blocks of life. So there's still work to be done there. Another option is that a
bias for one handed molecule over the other in the
primordial soup came from space. Some scientists have
looked at the amino acids found inside meteorites. And if they found a bias for
one handedness over the other, it's always been for
left-handed amino acids, like we see here on Earth. It could be that those
meteorites are contaminated, so more work needs to
be done there as well. It also pushes the question back. Like, why would you find a
non-50/50 mix of amino acids on a meteorite? In other words, we're getting
to the point now where no one knows for sure. And while that doesn't
work particularly well for a YouTube video,
it's good for science. That's where the
interesting stuff happens. I'm gonna go on a slight tangent now. Making homochiral molecules
in a lab, or in a factory is really difficult, but it's also sometimes really important. The reason it's difficult
is because, normally when you mix chemicals
together, you're gonna end up with a 50/50 mix of the chiral product. So you then have to filter
out the ones you don't want. It's difficult to filter out the chemicals because they're identical
in almost every way. So maybe you could use an enzyme. So you use an enzyme to
pluck out the mirror image that you don't want. Leaving you with the one that you do want. And of course the enzyme needs
to be homochiral as well, otherwise it will pluck
out the one you don't want. Where'd you get this enzyme from? Well, you steal it from nature. So you find an organism that
makes the enzyme that you want, you figure out the bit of DNA
that codes for that enzyme. You stick that DNA in a bacteria, you grow the bacteria, the
bacteria makes the enzyme and then you filter out the enzyme and you use it to pluck
out the mirror image that you don't want. Leaving you with the mirror
image that you do want. (sighing) That is one way to do it. One area where it's important to be able to make homochiral molecules is in the pharmaceutical industry because the mirror images of a drug can have completely different effects. Drugs tend to interact
with receptors in the body. And this simplified schematic
of a molecule and a receptor shows why one enantiomer
of a molecule could line up with the receptor while its mirror image wouldn't be able to. One harmless example is ibuprofen. When you buy ibuprofen it's a 50/50 mix of the two mirror images, but only one of them does anything. The other's completely useless. The infamous example is thalidomide. One mirror image helps
with morning sickness, the other mirror image
causes birth defects. And sadly, if you give
someone a dose of just the morning sickness mirror image it actually becomes a
50/50 mix in the body. Some other examples that I found. One an enantiomer of ethambutol is used to treat tuberculosis, whereas the other mirror
image causes blindness. Levomethorphan is an opioid,
whereas its mirror image is an hallucinogenic cough medicine. And the mirror image of meth is sold as a nasal decongestant spray. So my Google search history
now includes the phrase, "Meth nasal spray," Even sugar has an interesting history of commercial synthesis. So the left-hand version of sugar. It still stimulates the
sweetness receptors in the mouth, which is obviously not
sensitive to chirality for whatever reason. But it can't be metabolized in the body. So it would be an amazing
artificial sweetener except that it's incredibly
expensive to make. By the way, the synthesis
of left-handed sugars is intimately linked to our search for extraterrestrial life on Mars. We sent a lander there with some sugar. The idea being that if
there was life on Mars and it was like life on Earth, then it would metabolize that sugar and we'd be able to see the
products of that metabolism. But because we couldn't
be sure that life on Mars had the same handedness as life on Earth, we had to send both
mirror images of sugar. We had to make the
left-handed version of sugar. One last thing that's
absolutely mind-bending. When you think about chiral objects, they have physical extent, right? Like, this is a three-dimensional object. A two-dimensional object
wouldn't be chiral in 3D space. And it would be hard to imagine subatomic particles being chiral. And yet many of them are. For example quarks come in left-hand and right-hand versions. These particles that we
consider to be point-like. In other words, zero-dimensional objects moving around in three-dimensional space. How can they possibly be chiral? And even stranger, the universe
treats them differently. Only left-handed quarks
feel the weak nuclear force. In other words, the universe
has a left-handed bias. I feel validated. That's better. I'm sure you know by now
that I'm a big advocate of online video learning. I've even shared some of the courses that I've been working through
on Skillshare in the past. Skillshare is sponsoring this video. This time I thought I'd
share some of the courses that I think might click
with you, Dear Viewers. This is just a handful of courses from the thousands that they have. And you can try them all
for free if you use my link. If you go to skl.sh/stevemould10, the first 500 people to
use that URL will get two months absolutely
free, no strings attached. So you can try all these out for free. Okay, here's the list. "How to Study for Exams - An
Evidenced-based Masterclass." That's a big one. People waste a lot of time
by studying inefficiently and revising inefficiently. It's amazing how much time you can save using science-based tricks. "Presentation Design for Smart People." I don't know if you watch presentations, or give presentations as part of your job. They generally suck. "Build Beautiful Websites
Faster and Easier "Using Bootstrap 4." I don't know if you're
familiar with Bootstrap, but if you've got an idea for
a website that does a thing and you don't really
care about how it looks, you just don't want it to look
bad, Bootstrap is amazing. It gives you everything you need. "Pricing Your Work - How to Value Your Work as a Freelancer." That's a big one for me. "How To Notion." Notion is this thing. It's like an alternative to Evernote. I'm a big fan of Notion. But it's this really powerful thing. It can do lots and lots
of different things. So, worth doing a course to figure out what the best way for you to use it. "Storytelling 101: Character,
Conflict, Context & Craft." You know, it's so interesting
that if you studied "The Hero's Journey," or, you know "The Story Circle," it's
amazing how you just see it in all the stories that you read or watch. "Email Productivity: Work
Smarter With Your Inbox." That's another big one for me. I might do that one next, actually. I recommend you to just
have a look through. See if anything wets your appetite. Maybe you're already dabbling something and you wanna give it a boost. Anyway, the link on the screen, it's also in the description there. So check out Skillshare today.
(upbeat music) I hope you enjoyed this video. If you did, don't forget
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