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- Hello, my name's Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this video, I want to show you how cells

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have to do battle with the laws of physics

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in order to get the energy they need.

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This battle goes on in every
cell, every day on our planet

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and it forms the key concept

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in chapter four of my book
"Biology: The Whole Story."

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(birds chirping)

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(frog croaking)

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It's easy to think of
life as truly miraculous,

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but there's no magic in life.

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It has to obey the rules.

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All matter, in fact,

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is bound by something called
the laws of thermodynamics.

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The first law is quite easy to understand.

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It simply says that matter

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can neither be created nor destroyed

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so life has become brilliant at recycling.

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The second law though

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states that entropy must always increase

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and that's a bit harder
to get your head around.

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So entropy is a measure of disorder,

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but if disorder has
always got to increase,

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then how is it that life
can go about creating order?

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Because it clearly does
that, cells, for example

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joined together smaller
molecules to make larger ones.

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And when they have enough
of those large molecules,

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then they can make another cell.

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Well, the way they can do
that is that they've got to

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create a lot more disorder somewhere else

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and that means they have
to invest a lot of energy

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in the process of the
making and the building.

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And by doing that, that tends
to create disorder elsewhere.

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So in order to get the energy they need,

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cells are going to have to
harness chemical reactions.

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Now, in order to understand
chemical reactions,

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we need to look inside atoms.

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And if you remember, an atom
has quite a simple structure.

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In the centre is a nucleus

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that contains some
positively charged protons

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while orbiting around,

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there are negatively charged electrons,

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and those electrons are housed

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in what are sometimes called shells.

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So let's start by having a look

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at a couple of atoms
that are very unreactive.

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The first is helium.

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Helium has just two electrons.

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And because the first outer
shell that atoms can have

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only holds two electrons, the
outer shell of helium is full.

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Now, let's take a look at neon.

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Neon has 10 electrons so it
has two in the first shell

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and then the second shell,

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which in this case is the outer shell,

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is also full because that shell

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will only hold eight electrons.

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And neon's got eight electrons in there.

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Now, helium and neon are
both called noble gases.

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And noble gases are notoriously unreactive

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because electrons like
being in a shell that's full

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and that means that they
are in a low energy state.

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So now let's contrast that
with two different atoms.

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So the first is hydrogen.

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Now, hydrogen only has a single electron

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and that means that that
electron is on its own in a shell

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which does have room for two electrons.

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And that means that
that electron is unhappy

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and in a high state of energy.

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And this atom is oxygen,
superficially a bit like neon,

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but it only has 8
electrons rather than 10.

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So that means in the outer shell,

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there are six electrons
when there could be eight.

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So both hydrogen and
oxygen are electron hungry.

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The good news for both of them

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is that they could get together

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and form a new molecule
and share their electrons.

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And by doing that, they kind
of get a full outer shell.

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So both the hydrogen
atoms in this molecule

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have a share of two electrons

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and the oxygen atom has a
share of eight electrons.

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And that makes this
molecule very stable indeed.

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And it's called water.

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Now, maybe it's hard to imagine

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that combining hydrogen
with oxygen to make water

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is really gonna set the world on fire.

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But another name for liquid
hydrogen is rocket fuel.

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In July, 1969, three men were sitting

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atop a Saturn V rocket taller than Big Ben

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and three times heavier
than the London Eye.

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This was the biggest rocket

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that the world had ever seen,

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and 85% of its weight
was just liquid hydrogen

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and liquid oxygen, the fuel
that was going to push against

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the Earth's gravity and get
that rocket off the ground

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and on its way to the moon.

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When the fuse was lit,

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the oxygen and the hydrogen
atoms started to combine,

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brought together by
those unhappy electrons

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who wanted to form water, that stable molecule.

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And as they were doing that,

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the electrons shed all
of that excess energy.

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And cells have worked out how
to do exactly the same thing.

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So we've just seen that
this reaction of hydrogen

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and oxygen to form water
is an explosive reaction

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releasing a large amount of energy.

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But most of the reactions inside the cell

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only need a tiny bit of energy.

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So the cell actually has
a system of batteries

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which can just deliver very
small amounts of energy,

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and it recharges those batteries

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using this explosive reaction.

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So let's go to the membrane

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where this reaction is taking place.

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And the first thing we see

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are turbines embedded in the membrane.

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And as the turbines turn,

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these little batteries are recharged.

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Now, these are not real batteries,

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actually a little molecule called ATP.

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Now in our world,

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turbines are turned by
the flow of particles.

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So we might either have a wind turbine

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or we might have a water driven turbine.

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The cell's turbines are driven

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by a flow of charged
particles called protons.

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So protons flow through the turbine.

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And if we look, we can see
on one side of the membrane

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that we've got a whole pile
of protons piled up there.

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And we have to ask ourselves,
well, how did they get there?

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Because piling up protons
together like that

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is creating a kind of order.

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And we know that in order
for cells to create order,

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they've got to invest a lot of energy.

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So if we move a bit
further along the membrane,

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we see some other
structures embedded in it.

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And these look like pistons

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and they are driving
protons across the membrane

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and they need energy in order to do that.

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So if we look back, we see that the cell

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is shuttling hydrogen to the membrane.

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Those hydrogen
atoms consist of a proton

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and this excited electron.

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And when that electron
looks across the pistons

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what does it see but
a waiting oxygen atom.

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And so that electron leaps across,

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and the cell makes sure
that as the energy is shed

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from that electron, it's
used to drive the pistons

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and drive the protons across the membrane.

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And then as the protons
return through the turbine,

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they can combine with
those spent electrons

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and the oxygen to form water.

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Now, you might reasonably be asking,

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but where does a cell get hydrogen from?

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There's no hydrogen in our atmosphere.

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Well, it has to get that hydrogen

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from inside other molecules like glucose.

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Glucose consists of carbon
and oxygen and hydrogen.

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So the cell can dismember
glucose and release the hydrogen

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and shuttle that off to the membrane.

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And the carbon atoms just
end up combining with oxygen

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and being released as carbon dioxide.

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And so these two processes,
the dismemberment of glucose

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and then this process at the membrane

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with the turbines together
form aerobic respiration.

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And we write that equation
as glucose plus oxygen

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gives carbon dioxide and water
and a huge amount of energy,

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and it's absolutely dependent

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on having an oxygen rich atmosphere.

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Without oxygen, you wouldn't get anything

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like the amount of energy
out of that reaction

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as you can when oxygen is present.

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So the oxygen-rich biosphere
has allowed active animals

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to evolve, things that can
run and jump and swim and fly.

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You know, animals are selfish creatures.

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They're guzzling glucose
and sucking down oxygen

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without a thought for
where it's all coming from.

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But animals are lucky

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that there are other things on this planet

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that have chemical tricks
that they do not possess.

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If you wanna make glucose and oxygen,

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then one of the options is
to reverse that reaction

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that we've just seen.

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You can reverse any chemical reaction,

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but the problem is if you get
a lot of energy out one way,

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then when you reverse it,

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you're gonna have to
put a lot of energy in.

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So the reverse reaction is carbon dioxide

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plus water makes glucose and oxygen.

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And you might recognise that.

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It's something called photosynthesis.

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And photosynthesis, the
photo part in that word

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means the energy that you're gonna get

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to make that reaction happen
is gonna come from the sun.

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And that is another
fiendishly complex operation

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that was invented by organisms
called cyanobacteria.

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So they worked out how to
capture the sun's energy

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and the energy is actually used

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to smash up those water molecules.

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Remember, where hydrogen and
oxygen are both very happy.

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You're gonna smash them to
bits, get the hydrogen out,

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attach it to carbon dioxide,
and bingo, you've got glucose.

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And actually that's what
the cyanobacteria want.

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The oxygen's just a waste product.

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Photosynthesis evolved first on our planet

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around two to three billion years ago

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and that oxygen started to
trickle into the atmosphere.

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Now, of course, you might think,

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but it's not cyanobacteria
that do photosynthesis today.

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Isn't it plants?

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Well, it's both.

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Plants have their own story
and we'll get to it later,

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but we certainly rely on the combination

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of plants and cyanobacteria to
generate that oxygen we need.

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So next time you are passing
a gorgeous green thing,

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maybe show it a little more
love than you might normally do.

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Thanks for watching this episode.

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I hope you've enjoyed it.

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And if you have, please share.

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There's a lot more detail about how cells

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dismember glucose and the
details of photosynthesis

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in chapter four of the book

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and the link's below if you're
interested in buying it.

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If you wanna hang on for the next episode,

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that will be coming soon.

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And that'll be based on chapter five

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which is all about bacteria.

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(birds chirping)