- 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 to hit, "Subscribe,"
and I'll see you next time.
(funky instrumental music)