Kind: captions
Language: en
[Applause]
sometimes the simplest questions have
the most amazing answers like how can
trees be so tall it's a question that
doesn't even seem like it needs an
answer trees just are tall some of them
are over a 100 m why should there be a
height limit I'll tell you why trees
need to transport water from their Roots
up into their topmost branches in order
to survive and that is no trivial task
there is a limit to the height that
water can be sucked up a tube it's 10 m
if you suck on a long vertical straw the
water will go no higher than 10 m at
this point there will be a perfect
vacuum at the top of the straw and the
water will start to boil
spontaneously for a tree to raise water
100 m it would have to create a pressure
difference of 10 atmospheres how would
trees do that when I posed this
conundrum a lot of people said the
answer is transpiration and that's when
water evaporates from the leaf pulling
up the water molecules behind it now
that's clearly a mechanism a tree can
use to create create suction but it
doesn't help us overcome this 10 m limit
the lowest the pressure can go is a pure
vacuum which I imagine is not happening
inside of tree
leaves right right Hank so you might
suspect that a tree does not contain
continuous straw-like tubes the tree
effectively has valves in it so you
don't have a column of water this big
tube which you're saying needs to be
filled with water is actually made up of
cells although these are good
speculations they don't turn out to be
correct scientists who stud trees find
that the xylm tubes that transport water
do contain a continuous water column so
how else could the tree transport water
from the roots to the leaves they don't
suck they don't use a vacuum okay so how
do they do it squeezing like a cow like
you're squeezing a cow all the way up
there's little tree muscles in there
yeah besides being a giant waste of
energy all of the cells that make up the
xylm tubes are dead what about osmotic
pressure If there is more solute in The
Roots than in the surrounding soil water
would be pushed up the tree but some
trees live in mangroves where the water
is so salty that osmotic pressure
actually acts in the other direction so
the tree needs additional pressure to
suck water into the tree then it must be
capillary action the thinner the tube
the higher the water can climb but the
tubes in a tree are too wide at 20 to
200 micrometers in diameter water should
rise less than a meter so how do trees
do it well one of the assumptions we
made is wrong the lowest the pressure
can go is a pure vacuum P vacuum your
vacu in a gas this is true when you
eliminate all of the gas molecules the
pressure is zero and you have a perfect
vacuum but in a liquid you can go lower
than zero pressure and actually get
Negative pressures in a solid we would
think of this as tension this means that
the molecules are pulling on each other
and their surroundings as the water
evaporates from the pores of the cell
wall they create immense negative
pressures of minus5 atmospheres in an
average tree think about the the air
water interface at the pore there is one
atmosphere of pressure pushing in and -
15 atmospheres of suction on the other
side so why doesn't the meniscus break
because the pores are tiny only 2 to 5
nanm in diameter at this scale water's
high surface tension ensures the
airwater boundary can withstand huge
pressures without caving As you move
down the tree the pressure increases up
to atmospheric at the roots so you can
have a large pressure difference between
the top and the bottom of the tree
because the pressure at the top is so
negative but hang on if the pressure at
the top is5 atmospheres shouldn't the
water be
boiling yes yes it should but changing
phase from liquid to gas requires
activation energy and that can come in
the form of a nucleation site like a
tiny air bubble that's why it's so
important that the xylm tubes contain no
air bubbles and they can do this because
unlike a straw they've been water filled
from the start this way water remains in
the metastable liquid state when it
really should be boiling it's just like
super cooled water remains liquid even
though it should be ice so you could say
that the water in a tree is super sucked
because it remains liquid at such
negative pressures and why are trees
moving all this water up the tree I want
you to make a guess say it out loud for
photosynthesis actually no less than 1%
of the water is used in photosynthetic
reactions any other ideas okay what
about growth well 5% of the water is
used to make new cells so what happens
to the other 95% of the water it just
evaporates for each molecule of carbon
dioxide the tree takes in it loses
hundreds of molecules of water wa can
you believe how amazing this is trees
create huge negative pressures of tens
of atmospheres by evaporating water
through nanoscale pores sucking water up
100 Metter in a state where it should be
boiling but can't because the perfect
xylm tubes contain no air bubbles just
so that most of it can evaporate in the
process of absorbing a couple molecules
of carbon
dioxide I will never look at trees the
same way
again eat anything how does that make
any sense don't eat no they don't eat
anything I'd like to say a huge thank
you to Hank Henry and Professor poov for
making on camera hypotheses this is an
essential part of the scientific process
even if your hypothesis turns out to be
wrong as Einstein said a person who's
never made a mistake has never tried
anything new i' always wondered what it
would be like to be on this
side video now I'd be surprised if you
weren't already subscribed to these guys
but if you're not go click on these
annotations and check out their channels
and you may just learn something I'd
also like to thank professor John Sperry
from the University of Utah he walked me
through all of this in an hourong Skype
conversation so I'm going to put a link
to his website in the description we're
looking at pressures here below at
atmospheric is that right that's right
mm below atmospheric this is liquid
pressure not gas pressure so it's a
common misconception that oh you can't
have you know negative pressures because
there's no molecules left you know the
definition of pure vacuum is zero
molecules that's for gas okay so just to
just to be clear I think this was one of
my big big problems in understanding
this this video would have been
impossible without cgp gray when I told
him about this idea in London and I felt
like like mind just blown with this
whole thing he said it was going to be
really hard to explain and you know when
he says it's hard to explain you know
things are going to be tough so thank
you for all your input to this script
and thank you for watching making this
video has been a real Odyssey for me so
thank you for joining me on that journey
I really appreciate all of your comments
and if you haven't subscribed to the
channel already uh you can click The
annotation or click the link above and
join me on my next scientific
Adventure I made a video promising to
make a video about the answer to this I
I proposed the problem like right a
couple months ago and I was like
subscribe to the channel I'll give you
the answer next
week drive it at the right
frequency
oh yes success is frightening