Kind: captions
Language: en
Now, in a previous video, I showed you
that you can only suck up a straw that's
10.3 m long. And that's even if you can
create a perfect vacuum inside your
mouth. If you haven't seen the original
video, check it out. But that raises an
interesting question, which is how can
these trees, which are 100 m high, get
the water all the way from their roots
up to the leaves. The argument, as
you're alluding to, is that if you're
sucking water up through a drinking
straw, for example, you can only have a
straw 33 ft long.
The reason for this height limit is
because the weight of the water in a
column must be supported by the pressure
difference between the top and the
bottom. So, at the bottom, you're going
to have atmospheric pressure, and the
lowest pressure you can produce at the
top would be a vacuum, that is zero
pressure. So atmospheric pressure can
support a column of water that is only
10 m high. What's worse is if you were
able to create a vacuum, the water would
start boiling spontaneously. That's
called cavitation. And uh obviously that
can't be taking place within a tree. So
how are they doing it? Well, we started
to develop some different theories.
My guess though is people talk about
this being continuous water column and
what they I think when when you say that
you think this big like empty pipe,
right? That's what we're picturing. Uh,
but I think what's more likely to be the
truth, this big tube, which you're
saying needs to be filled with water, is
actually made up of cells.
The tree effectively has valves in it.
So you don't have a column of water that
is much higher than 33 ft or less. So
that the water is pumped up by um
osmotic pressure due to differences in
concentration of sugars and so on, but
each individual stage is just quite a
small one. My guess is that it's
probably more like a bucket brigade
where once you know here we're at the
end there's sunlight coming in
and it heats up the water and the water
evaporates and so the water goes off as
water vapor vaporized off. So now this
guy's like my bucket's empty. I want
some more water and and this is a cell
into the cell here but this one can give
the water there because locally like
it's surrounded by water and it's a
little bit of water will go there via
osmotic pressure. Now, another theory is
that osmotic pressure at the base could
actually push the water all the way up
the tree. If the solute concentration is
different enough between the roots and
the water in the surrounding soil, then
water would actually want to push into
the roots in order to equalize the
solute concentrations that could create
a positive pressure which would push the
water up the tree. And this hypothesis
led me to being challenged to blow water
up a tube. Now, the water, as you can
see, is veritassium color. Three, two,
one, go.
Unbelievable. Stop. Stop. Stop.
Yeah. Yeah. Nailed it.
I hate you, D. [laughter]
Now, my ability to blow water up that
tube was impressive. But I don't really
think that a tree would be able to get
so much osmotic pressure at the roots
that it could push the water up 100 m.
Well, some people may be wondering why
we haven't talked about capillary action
yet. That's due to the adhesion between
the water molecules and the walls of a
tube. So, you can suck water up through
perforated materials. Now, I'm not sure
that the tubes inside a tree are small
enough for this effect to have a
significant impact, but it may. Well, I
don't want to give you the complete
answer yet. Uh, I'd like you guys to
tell me what you think and maybe post a
video response. I'll tell you that I had
a significant misconception that was
stopping me from working this out. So if
you can spot what that is, uh do let me
know. And let me give you a summary of
the ideas we came up with. Uh one that
the tree does not contain a continuous
water column. Number two, osmotic
pressure at the roots, maybe pushing the
water up the tree. Number three, osmotic
pressure throughout the tree uh helps
pull the water up. And number four,
capillary action. So, let me know what
roles you think those different factors
play in allowing a tree to draw the
water up 100 m.
And if you don't want to do that work,
then subscribe to the channel and I'll
post the answer in a week.