Kind: captions
Language: en
this video is part of what is
potentially the largest collaboration
ever on YouTube along with my friends
mr. beast and mark Rober Destin from
smarter every day and many many others
we're trying to get 20 million trees
planted before the end of this year and
each tree costs $1 so essentially we're
trying to raise 20 million dollars if
you want to help us get to that target
well then go to team trees org and
donate now most of the youtubers are
making original videos for this
collaboration but since I'm obviously
currently travelling in Sydney I don't
have time for that so I am reposting one
of my favorite videos of all time which
is about trees and how they bend the
laws of physics and do things that
engineers and scientists can't yet
replicate so if you haven't seen that
please check it out 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 hundred
meters Why should there be a height
limit I'll tell you why trees need to
transport water from their roots up
until 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 ten
meters if you suck on a long vertical
straw the water will go no higher than
10 meters 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
a hundred meters it would have to create
a pressure difference of ten 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 and
that's clearly a mechanism a tree can
use to create suction but it doesn't
help us overcome this 10 meter limit
below as the pressure can go as if your
vacuum which I imagine is not happening
instead of tree leaves right right Hank
so you might suspect that a tree does
not contain continuous straw like tubes
the tree effectively has vowels in it so
you don't have a column of water this
big - what you're saying needs to be
filled with water there's actually made
up of cells although these are good
speculations they don't
to be correct scientists who study trees
find that the xylem 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 wheezing like a cow
like you're squeezing the cow water all
the way up there's little three mussels
in there yeah besides being a giant
waste of energy all of the cells that
make up the xylem 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 micro meters
in diameter water should rise less than
a meter so how do trees do it well one
of the assumptions we made is wrong
below if the pressure can go is the pure
vacuum your vacuum you're back 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 minus 15 atmospheres in an
average tree think about the air water
interface at the pore there is one
atmosphere of pressure pushing in and
negative 15 atmospheres of suction on
the other side so why doesn't the
meniscus break because the pores are
tiny only two to five nanometers in
diameter at this scale waters high
surface tension ensures the air water
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 is
negative 15 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
xylem 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 supercooled 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 five percent
of the water is used to make new cells
so what happens to the other 95 percent
of the water it just evaporates for each
molecule of carbon dioxide that tree
takes in it loses hundreds of molecules
of water
whoa 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
a hundred meters in a state where it
should be boiling but can't because the
perfect xylem 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
[Applause]
trees are some of the biggest organisms
on the planet but where do they get that
matter to grow which nutrients a it the
grain status or any yeah goodness said
of the soil I suppose comes out of this
soil yeah
goodness goodness why isn't there a big
hole around the tree where it's taken
out all the soil does it's a gradually
let the soil has time to recover now I
think it's intuitive to believe that the
tree gets most of its mass from the soil
because you can see those roots digging
into the soil and they must be taking
something out of there and I mean a tree
looks like dirt and it feels solid like
dirt but it's not in the early 1600s a
scientist named Johann Baptiste event
Helmholtz tried to figure out where the
mass of a tree was coming from so he got
a pot of soil and very carefully
measured the amount of soil in there
then he planted the tree and took care
of it for five years making sure that no
soil left or was added to his pot and at
the end of this experiment he laid the
tree to find that it was 72 kilograms
but the mass of soil had only decreased
by about 60 grams this was pretty strong
evidence that the mass of the tree does
not come from the soil I've never
thought about that actually because they
don't really eat anything
don't eat me no no they don't eat
anything water it's always absorb that's
all they eat yeah they don't eat
anything else no that's all they eat
well presumably from the water and the
nutrients from the soil is there
anything else that you need besides the
soil and the water this will be late
isn't it to make other than the original
seed for that particular trail the seed
and the soil and the water and that
makes this big tree of course
Johann Baptista van Helmholtz did
conclude that the tree was made entirely
of water now while that's not correct at
least he was on the right track
realizing that the matter of a tree
doesn't come out of the soil the Sun
energy yeah
the Sun energy are they converting
energy into maths or do you know what I
mean yeah like that there wasn't there
wasn't stuff and then there was like
where did that stuff come from my
question is where do they get that mass
to grow big it from the rain and the Sun
presumably like a sunlight and the
sunshine the sunshine does it does the
sunshine add mass to the trip well it
yes it wouldn't they wouldn't grow
without it I don't know they'd adds mass
but they wouldn't grow without it of
course the sun's energy is needed for
the tree to build the matter into its
branches and leaves but the Sun itself
the energy is not matter
oxi didn't the trees need the arcs are
indeed the earth and I guess oxygen the
oxygen of course the oxygen are there
any ingredients that we're missing come
dog so it's carbon dioxide carbon
dioxide so would it surprise you to find
out that 95 percent of a tree is
actually coming from carbon dioxide
trees and largely made up of air so as
it turns out trees are mostly made out
of air out of the carbon dioxide that
they take in and what's interesting is
that we breathe out carbon dioxide and
water that's how we lose mass but it's
the exact same substances that trees
breathe in to gain mass so if you can
imagine a closed system where it was
just you and a tree you would breathe
out that carbon dioxide and water the
tree would take it in so you would get
smaller while the tree is getting bigger
and in a sense you're becoming the tree