 So why does this turn
into this in the fall?
I live in Boston, which
means that about this time
each year, the
leaves on the trees
start to turn their colors.
Leaves get their color
from three main compounds.
The first is chlorophyll,
which might sound familiar.
Chlorophyll is involved
in photosynthesis,
which is the process
by which plants
turn CO2 and water
into sugar and oxygen
by harnessing the
energy from the sun.
Chlorophyll gives most plants
their distinctive green color.
The second group of compounds
are the carotenoids,
which sounds a little bit
like carrot, which makes sense
because carotenoids give leaves
there yellow and orange hues.
Carotenoids are present with
chlorophyll in organelles
called chloroplasts year round.
And these chloroplasts
are just the organelles
where photosynthesis occurs.
Finally, there's a
group of compounds
known as anthocyanins.
And anthocyanins give
plants reddish hues.
And they're often
found in things
like cranberries,
and blueberries,
and lots of dark red leaves.
These anthocyanins
are actually produced
in the fall when chlorophyll
starts to break down,
but I'm getting ahead of myself.
Now, for most of the
year, chlorophyll
is so bright green
that it blocks out
some of the yellow
and orange hues coming
from the carotenoids.
For most of the growing
season, chlorophyll
is constantly being broken
down and reproduced.
But as it starts
to get colder out,
the cool night air closes
off the veins of the leaves.
And it gets harder for
water and nutrients
to move into the chloroplasts
and rebuild chlorophyll.
So what this means is
that the chlorophyll
is being broken down, but
it's not being reproduced.
So the green color of the
leaves starts to disappear.
This means that the yellows and
the oranges of the carotenoids
can start to show through.
But that's not all.
Anthocyanins, by contrast,
are not present year round
in the leaves.
Instead, they are actively
produced in the fall right
before the leaves
drop off the trees.
So why would a
plant suddenly want
to start producing a red
pigment in a leaf that's
about to drop off anyways?
Scientists haven't reached
a definitive conclusion yet.
But they do have some
pretty good ideas.
One is that the red pigments act
as a sort of direct sunscreen
as the canopy starts
to drop leaves.
The leaves that remain
are exposed to more light.
And those leaves are losing
the sunlight-absorbing powers
of their chlorophyll.
Another cool idea is that the
red pigment might actually
trick migrating insects into
thinking that the tree is
unhealthy and discouraging
them from laying
their eggs on those trees.
So all of this leads us to
some pretty cool science
that you can do at home to
visualize the pigments found
in leaves where you live.
So all you need
for this experiment
is isopropyl alcohol, some
strips of filter paper--
I used coffee filters--
a couple glasses,
scissors, and some leaves.
And if you want
to be super cool,
pick a few different
kinds of leaves,
so that you can compare the
compounds you find within them.
Step one, chop each of your
kinds of leaves up into lots
of little tiny pieces.
We really want to release
all of these compounds out
of the leaves.
So the smaller and the
finer you chop them,
the better this
is going to work.
Next, cover the leaves
in isopropyl alcohol.
Now, this is just
rubbing alcohol,
so it's not very dangerous.
But you do want to make sure
you don't get this in your eyes
or drink it.
The next step is to grind up
these leaves in the alcohol.
Now, ideally, you would be doing
this with a mortar and pestle.
but I don't have that.
And so I am using
a wooden spoon.
Now, at this point,
you should start
to see the alcohol change colors
as those pigments are released
from the leaves.
If you want to speed
this reaction up a bit,
you can set all of your
glasses into a hot water bath
and leave them for
about 30 minutes.
After this, you can really
see the color change
in the alcohol.
Now, I strained the
leaves out of my alcohol,
so that it was a
little easier to see.
But you could probably
leave them in there as well.
Next, cut your filter
paper into strips
and place a strip into each
glass so that one side of it
is touching the alcohol and the
other side is out in the air.
Now, I repeated this
experiment later
with longer strips
of filter paper
and hung one side
out of the edge.
And this actually worked
a little bit better.
Now, you want to let your
glasses sit for about an hour.
So what we're doing here
is called chromatography,
which just means that we
are separating a mixture.
Here, we are using
paper chromatography
to separate the different
compounds within the leaves.
So the alcohol,
our mobile phase,
is being drawn up the paper
strip, our stationary phase,
and it's pulling leaf
pigments along with it.
Depending on their
chemical structure,
molecules will have
different polarities
and will want to interact
with that paper differently.
Some will want to stick
tightly onto the paper,
while others will move more
easily with the liquid.
This means that some compounds
will travel along the paper
faster than others, separating
the compounds along the strip,
and giving you distinct
bands of compounds.
Cool.
So we actually got some
really good separation
between the yellow
carotenoids on top
and the green chlorophyll
on the bottom.
And the longer you leave these,
the better that separation
will get.
Now, this is an experiment
you can easily do at home,
and you should.
I want to see what the
compounds from leaves from all
around the world look like.
So make a video response of
you doing this experiment,
and leave it below.
And I'll check them all out.
And yeah, go on.
Go forth.
Do science.
