(lively instrumental music)
- In 1845, the German chemist,
Christian Friedrich Schonbein, was experimenting
with mixtures of nitric and sulfuric acids
when he spilled a mixture of these two acids on the table.
He grabbed his wife's cotton apron to mop it up,
but when he hung the apron on a nearby stove to dry,
it exploded.
This was the first step towards the discovery of guncotton,
which we know today as nitrocellulose.
And nitrocellulose is used in everything from explosives
to x-rays to early silent films and nail polish,
but nitrocellulose begins its life
as the humble plant molecule, cellulose,
and cellulose makes up so much more in our lives
than just paper.
It's what makes our houses strong, it's in our food,
clothing, plastics, it is everywhere.
In fact, it's one of the most abundant organic molecules
on the planet.
So, how does this tiny molecule
hold up so much of our modern world?
(ethereal instrumental music)
Some of the tallest trees in the world
are in California's Redwood National Park.
Coastal redwoods can reach over 300 feet tall.
The world's tallest tree, Hyperion,
towers over even the other giants at 379 feet.
But like any tree, they start as tiny saplings.
So how do these tiny plants grow into massive organisms?
And how does the cellulose molecule
keep them standing tall for hundreds
or even thousands of years?
Cellulose is the main chemical building block
of plant cell walls, and it's what makes them strong.
It's also one of the most abundant
naturally occurring compounds in the world.
Wood is about 50% cellulose when dry,
while cotton is closer to 90%.
But cellulose starts off, like a lot of things,
as small building blocks, in this case,
carbon dioxide and water.
A tiny sapling tree is going to get much of its mass
from these two molecules.
Plants take in both of these molecules
and turn them into glucose using photosynthesis.
A glucose molecule looks like a ring
and is one of the simplest sugar molecules.
Plants then take these glucose molecules
and use them as building blocks to make long strands,
creating polymers.
Two types of polymers are formed.
One is starch, which stores energy
and the other is cellulose, which helps create structure
in the plant, much like a skeleton
in humans and other animals.
Each strand of cellulose is made up of hundreds to thousands
of glucose molecules, all strung together in a long chain.
This makes it a polymer, a molecule made up
of lots of repeating units, or monomers.
Here, the monomers are the glucose molecules,
and if you want a tall tree,
you're gonna need a lot of cellulose.
While it's made up of thousands of glucose molecules,
it's still small when we think about things we can see.
A long strand of cellulose
could be about 0.04 millimeters long.
If you stood it up next to a fine grain of sand,
it would only reach a quarter of the way up,
and when you consider that the tallest trees in the world
can be about 300 feet tall, which is about 100 meters,
or 100,000 millimeters, well, you're gonna need a lot
of CO2 and H2O to create all of that cellulose.
And honestly, that's great for our planet.
Forests and other plant life around the world
soak up about 25% of human carbon emissions.
Cellulose is really tough.
It's what lets plants stand up straight and not flap over,
reaching those crazy heights.
We humans have learned to use this toughness and strength
to our advantage in lots of different ways,
from building to wearing.
Those cellulose molecules all start off
pretty much the same.
They could end up in lots of different places,
from paper to clothing to the food you eat
or even explosives.
So let's take a look at some of the many paths
cellulose molecules could take.
Probably the most straightforward path
for a cellulose molecule from a tree, a plant,
or a blade of grass is to be eaten as food.
While our human stomachs can't break it down,
many animals like cows, sheep, and horses
have evolved methods of breaking it down
that involve long, slow digestive processes,
lots and lots of chewing, and a whole host
of symbiotic gut bacteria that we don't have.
But that doesn't mean we humans don't snack on a bit of it
every now and then.
Of course, you expect cellulose to be in things
like vegetables, especially tough ones like leafy greens,
but we also add it to other stuff
like shredded cheese and barbecue sauce.
And the cellulose we're adding is usually from wood,
like tree wood, so should you be worried about sawdust
in your pizza cheese?
Well, no.
Cellulose is just one of many molecules present
and it's great as an anti-caking ingredient,
which prevents shredded cheese from clumping together.
And while we could get it from other things like onions
and asparagus and other foods that we eat,
that would kind of be a waste of those perfectly good foods.
Making it from wood is also pretty cheap,
and even though we can't break it apart and digest it
like a cow, it still acts as fiber
that helps keep our digestive system running smoothly.
Have you ever looked up at a tall tree and said to yourself,
hey, that'd make a lot of paper.
Well, you'd mostly be right.
You might remember making paper in art class,
grinding up a bunch of used paper in a blender
and then smushing it together, and I used to think
that that's how all paper was made.
You just grind up some wood and squish it together
and bam, paper.
Sorry, tall tree.
But paper is actually a bit more complicated than that.
In wood, the cellulose molecules are all bound together
with another polymer molecule called lignin,
which acts like a glue holding the cellulose together.
So you have to get rid of the lignin
to get to the cellulose.
To do this, the wood chips are first soaked
in an alkaline solution, which breaks down the lignin,
helping to release the cellulose fibers.
And the longer the cellulose fibers, the stronger the paper.
So this process usually involves processing the wood chips
by trying to beat them rather than chop them up.
Once the cellulose fibers have been released from each other
and washed, the mixture is then bleached
to remove any remaining lignin and turn the fibers white
before they're turned into paper.
And the final part of this process
does involve smushing them together.
Large machines place a suspension of wet cellulose fibers
onto a large screen, where they're pressed
and then dried out to create paper.
But the cellulose in a tree
could also become something a bit more transparent.
We figured out methods to turn cellulose into things
like plastic, too, specifically, cellophane.
Cellophane is that thin plastic film that wraps everything
from crackers to birthday presents.
That cello is from cellulose.
In the early 1900s, Jacques E. Brandenberger
tried to come up with a waterproof fabric coating.
Supposedly, he'd seen another customer at a restaurant
spill a glass of wine on the tablecloth,
and he wondered if he could create a surface
that could be easily wiped clean.
His first attempts involved spraying a waterproof coating
on a tablecloth, but the fabric was way too stiff
to use afterwards.
However, he noticed that a reaction had occurred
between the fabric and the coating
to create a thin, clear film that could be peeled off.
This was cellophane.
Cellophane starts off as a pulp of ground up wood, cotton,
or even hemp, and goes through a chemical process
where, at first, it becomes something called viscose.
And if that name sounds vaguely familiar,
just check out your clothing labels.
Viscose is actually a popular fabric
that's probably hanging in your closet.
Brandenberger's tablecloth started off as viscose,
and even today, we take viscose and treat it
with sulfuric acid and sodium sulfate to make cellophane.
Similar compounds, like cellulose nitrate
and cellulose acetate, were also used for things
like movie film.
However that first one, cellulose nitrate,
had a wee bit of a problem.
It had a tendency to decompose explosively.
You might know it by another name, nitrocellulose.
It easily explodes at low temperatures,
even as low as 38 degrees Celsius.
It's been the cause of a number of fires,
including a nitrocellulose film storage facility
for 20th Century Fox and a fire at the Cleveland Clinic
started in a storage room filled
with nitrocellulose x-ray images.
Nitrocellulose film has to be stored in cool, dry conditions
because its reactivity also makes it prone to degradation.
So careful storage and digital backups are important
to make sure we don't lose film history.
So that big, tall tree, it might just explode one day.
Cellulose is a key component of a number of explosives,
but its discovery as a useful flammable substance
was also partially accidental.
Back to our friend, Schonbein, and his exploding apron.
The cotton apron was full of cellulose molecules.
When it was exposed to the nitric and sulfuric acids,
many hydrogens on the cellulose molecules
were substituted with nitro groups, NO2.
The resulting fluffy white guncotton is unstable
and extremely flammable, which is why it's used
in explosives and rocket propellants.
The key to guncotton's explosivity
is all of these nitro groups.
These groups are relatively unstable, while the gases
that result from their decomposition,
like nitrogen, are very stable.
When we talk about chemical stability,
what we're really talking about
is how many electrons they have.
Remember, from our episode on red tides,
that electrons exist at set distances
from the nucleus of an atom.
Each of these distances is called a shell
and each shell has a specific number of electrons
that it's able to contain.
If it has fewer electrons, it wants to fill them
by reacting with another atom, creating a bond
and sharing their electrons
so that both of their shells are filled.
Nitrogen is an important product of guncotton decomposition,
and its stability comes from the three bonds
that hold the nitrogen atoms together.
When these bonds are formed,
all of the nitrogen atom's outermost shells
are filled with electrons.
This makes it very stable.
Now, these three bonds require lots of energy to break,
but also release lots of energy when they're formed.
This decomposition of guncotton is called exothermic
because the energy is released as heat.
The released gases have a much greater volume
than the starting solids do.
So not only are they hot, but they rapidly expand,
and that combo of heat and expanding gases,
that's an explosion.
And the properties that make cellulose great
as an intentional explosive also make it really good
at being unintentionally explosive, too.
Let's say we cut down our cellulose-filled tree.
Pile of sawdust seems pretty harmless, just kind of dusty,
but its tiny wood fragments full of cellulose
are highly combustible.
If it gets blown up into the air, you now have a mixture
of lots of combustible materials with all of the oxygen
in the air, making a perfect fuel mix for an explosion.
The small sawdust particles have a lot of surface area,
giving the cellulose lots of opportunity
to interact with oxygen.
A tiny flame or spark can cause a huge disaster.
But let's say that we don't turn that cellulose
into paper or food or explosives.
What if that tree grows and grows, and then finally,
falls, crashing down to the forest floor?
What becomes of it then?
Well, it'll die and start to decompose.
Organisms like fungi and bacteria
will start to break down cellulose and lignin,
using their energy to grow.
However, because cellulose is so stable,
the decomposition is very slow.
Think about your favorite hiking trail
and the dead tree lying on the side.
Year after year, it's still there,
only very slowly degraded by fungi and bacteria.
And millions of years ago, the process was even slower.
Though there is some scientific disagreement,
some research suggests that fungi didn't develop the ability
to break down lignin until about 300 million years ago.
This meant that plant matter built up over time
and became oil and coal after being exposed to huge amounts
of pressure and heat.
Cellulose- and lignin-eating fungi today
are one potential reason why the Earth
is no longer producing these fossil fuels
at the same rate it did millions of years ago.
The biofuels industry continues to look for a way
to chemically degrade this most abundant organic molecule
on Earth to make fuels not derived from oil.
Though it starts off as just a long strand of glucose,
cellulose can end up in lots of different places
in the world around us.
Personally, though, if I were a tree,
think I'd rather end up being an explosive piece
of cellulose than one that just ends up
as a cellophane gum wrapper.
(bright orchestral music)