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PROFESSOR: OK.

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So I need to move on a little
bit now, and I want to talk

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about in fact, the earlier way
that nature developed to make

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energy using proton gradients.

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And this part actually preceded
the development of

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respiration as you'll see.

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It's what I somewhat flippantly
refer to as

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photosynthesis release 1.

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In my first lecture,
when I was giving

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you a sketch of evolution.

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Who knows, I mean these are very
rough numbers, but may

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have evolved about $3.4 billion
years ago and early

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life had begun to exhaust
this sea of chemicals

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that had been produced.

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And it's known as cyclic
photophosphorylation.

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And it's a way of taking
the energy in

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sunlight and making ATP.

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So whatever organism figured
this out, this was

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a really big deal.

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Because now instead of having
to use the sort of natural

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reserves like the way the food
around is a depleting resource

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just like our petroleum
reserves, this was able to

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take the abundantly-available
sunlight and

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use it to make energy.

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So that's the principle.

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It uses the energy in sunlight,
and the way it does

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it, is it uses the sunlight to
establish a proton gradient

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just as we've been discussing
earlier, and then uses

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that to make ATP.

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And there's a special molecule
that's involved in absorbing

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the energy from the sunlight.

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We've all heard it I'm
sure, chlorophyll.

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There are a couple of two major

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variants of this molecule.

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Here's one of them, chlorophyll
a, and you don't

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have to memorize
the structure.

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But I want you to notice
a couple of things.

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One is there's a metal in the
middle, a magnesium, and then

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it's coordinated with the
cyclic ring system.

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And notice all the conjugated
double bonds.

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So this chlorophyll was tuned
to absorb energy from the

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visible range of sunlight.

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And when it absorbs a photon
of energy, it kicks an

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electron up to a
higher orbital.

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And if the electron's in a
higher orbital, it's more

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easily lost.

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And this has a consequence.

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So the way this system works
is you have a molecule of

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chlorophyll, that's what
I'm abbreviating here.

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Oh, let me tell you something
else too.

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It's more sophisticated
than this.

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So this is the molecule that
absorbs a particular

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wavelength from sunlight.

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But this is embedded in a
multi-protein structure that

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has a bunch of other molecules
that absorb at different

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wavelengths and then funnel that
energy down to the one

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the chlorophyll comes in.

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So in fact, the whole thing is
like a big antenna that's able

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to absorb quite a bit of
energy from different

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wavelengths in sunlight
and get it down to the

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chlorophyll.

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When the chlorophyll absorbs
energy, it goes up to an

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excited state.

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And as I said, now that the
electron's in a higher

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orbital, it's lost
more easily.

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So this has become a better
reducing agent.

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It's able to give its electrons
to things that it

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couldn't do down in
this energy state.

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So we come down one of these
thermodynamic hills that

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you're hopefully starting to
get used to where it comes

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down in little hops to a carrier
that has this set

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level of energy, down,
free energy, down.

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And similarly, to the principle
that we talked about

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in respiration, a proton is
pumped from what I'm going to

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say, in this case, I'll show you
what I mean, but I'll say

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from a proton that's out
to a proton that's in.

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And by doing that, it
establishes a proton gradient,

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and that gives rise to ATP.

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At the end of this cycle, we
have this chlorophyll minus

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the electrons.

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These come, flow back.

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That's why it's called cyclic
photophosphorylation.

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The electrons go through these
carriers, and then they return

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to chlorophyll.

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So wonderful system.

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I seemed to have accidentally
advanced this.

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OK.

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There are still bacteria around
that run this system.

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So if you remember, we talked
about biosynthesis, the need

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for energy.

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Well, here we are.

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We got ATP.

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But there is something else
hopefully you now appreciate

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in that is that ATP is not
enough to take carbon dioxide

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and make it into sugars
or carbon compounds.

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We need a source of reducing
power as well.

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Because remember, carbon dioxide
is the most oxidized

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form of carbon.

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So these early organism solved
it by making reducing power

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from another source.

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Many of them used hydrogen
sulfide as a source, and they

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used NADP plus.

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Now, this is a minor
variation of NADH.

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It's got one more
phosphate on it.

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You can look it up
in your book.

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This variant of NAD is used
preferentially for

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biosynthetic purposes.

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But everything I've told you
about NAD in terms of banking

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electrons applies here.

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So the electrons from
here are grabbed.

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The cell makes NADPH, which you
can use as reducing power

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for biosynthesis.

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You get elemental sulfur
and hydrogen ions.

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So this process an organism
that used this kind of

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photophosphorylation to make
ATP would get its reducing

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power through a process
something like this.

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And then it could make sugars,
and then from that point on,

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they can be used to
make all the other

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molecules that you need.

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The key thing is to get from the
carbon dioxide down into a

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more reduced form of carbon.

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So that works pretty well.

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However, a better system came
up involved in evolution.

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This was the one I again
somewhat flippantly called

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photosynthesis release 2,
when I was talking.

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This is known as, probably came
up who knows again but

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maybe 3 billion years ago, and
it's known as noncyclic

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photophosphorylation.

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And what's important about this
system and why it's an

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improvement over the other, is
it uses the energy in sunlight

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to make ATP just as we've
learned, but it also uses the

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energy in sunlight
to make NADPH.

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So in other words, this second
version gives the cell simply

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from the energy in sunlight
everything it needs to take

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carbon dioxide and make it
into organic compounds.

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And it's a pretty cool system
evolutionarily.

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It's built on the older one,
the first arising one.

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You'll still see the elements of
the present but with a new

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variation added in, very much
the way we do design when

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you're doing engineering.

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You get something that's
working, and you can use that

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as a basis to move to a new
and improved version.

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And naturally, if you get a new
and improved version, and

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you get a little advantage over
your neighbors, natural

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selection makes sure that
that better system gets

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established.

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So here's here how
this noncyclic

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photophosphorylation starts.

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We take a chlorophyll and it
absorbs the quantum of energy,

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and it kicks itself up
to an excited state.

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The chlorophyll, as before,
electrons come down,

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energetically downhill and
remember that theme.

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I keep saying that's at least
thermodynamic properties.

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If we think about free energy,
it doesn't matter

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what path you take.

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Whether you come shooting right
down or you come down

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through it, you get the
same energy back.

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What's amazing about the system,
if it didn't have all

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this extra apparatus, you'd kick
up the electron, and then

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it would just come right back
and you'd get a little

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radiated, a little
energy given off.

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We wouldn't have accomplished
anything.

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And what's terrific about this
photophosphorylation system,

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it's able to capture the energy
that's in that excited

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chlorophyll.

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So at this point, and as it's
coming down as I said, we have

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H plus going from H
plus, H plus in.

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I'll give you a picture of
what I mean by that.

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Then we're getting ATP made.

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So this time the difference is
instead of the electrons going

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back to that chlorophyll, it was
missing its electrons, the

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electrons, instead, go to a
chlorophyll, which is at a

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somewhat higher energy level
than the first one.

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And it has just absorbed quantum
of energy, and it's

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kicked itself up to an even
more excited state.

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And these electrons from this
system come on and fill up

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this chlorophyll.

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So this one over here is
called photosystem II.

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And the term used in the field
to describe what I'm about to

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tell you here is now called
photosystem I.

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So what we have now from this
system is an excited molecule,

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chlorophyll, that's even more
excited than we were before.

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And so it's even more able to
give off its electrons.

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It has more reducing power.

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In fact, it has enough reducing
power that it can

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reduce NADP.

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So NADP plus, electrons coming
downhill, you get NADPH.

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So here we are, reducing power
made by using the energy in

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sunlight, ATP made using
the energy in sunlight.

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So by just using this noncyclic
photophosphorylation

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system, the cell's got what it
needs to take carbon dioxide

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and put it through a sequence of
reactions that will let it

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make sugars and other things.

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In this course, I don't have
enough time to go through the

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biosynthetic pathway.

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It's in your textbooks.

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You might find it interesting
to look at.

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00:13:02,455 --> 00:13:04,490
We're not making a big
issue of it in there.

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00:13:04,490 --> 00:13:07,500
But it exists, and you can see
that it obviously exists.

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00:13:07,500 --> 00:13:11,550
So there's one more wrinkle here
which might be sort of

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00:13:11,550 --> 00:13:15,615
eerily reminiscent of one of the
issue I posed for you when

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00:13:15,615 --> 00:13:19,670
I asked about whether we could
just keep on doing glycolysis.

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00:13:19,670 --> 00:13:21,240
I can't just let
the system run.

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00:13:24,840 --> 00:13:27,800
I forgot about something
so far.

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Over here, this guy
lost an electron.

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00:13:31,630 --> 00:13:34,990
It can't get it back because the
electrons went over there.

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00:13:34,990 --> 00:13:36,720
They have to come
from somewhere.

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00:13:36,720 --> 00:13:40,170
Well, the energetics of the
system now are such that it

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00:13:40,170 --> 00:13:45,515
can get electrons from water.

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00:13:45,515 --> 00:13:48,840
And what's left over when you
take the electrons from water?

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00:13:48,840 --> 00:13:54,040
We have half of an
oxygen molecule.

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00:13:54,040 --> 00:13:59,120
So here's the class.

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00:13:59,120 --> 00:14:05,430
It's a waste product, if you
will, from this very efficient

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00:14:05,430 --> 00:14:08,120
noncyclic photophosphorylation
system, but

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00:14:08,120 --> 00:14:09,880
it's molecular oxygen.

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00:14:09,880 --> 00:14:13,020
And it was when this system
developed that we started to

217
00:14:13,020 --> 00:14:17,810
have oxygen appear
in this world.

218
00:14:17,810 --> 00:14:25,260
The organelle that carries out
photosynthesis, actually, the

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00:14:25,260 --> 00:14:30,940
first organisms that learned
how to do this is called

220
00:14:30,940 --> 00:14:35,060
cyanobacteria, which they
sometimes sort of rather

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00:14:35,060 --> 00:14:36,600
incorrectly call blue-green
algae

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00:14:36,600 --> 00:14:39,650
because they're bacteria.

223
00:14:39,650 --> 00:14:42,710
But you see cyanobacteria
all the time.

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00:14:42,710 --> 00:14:45,620
And similarly to what happened
with the mitochondria, there's

225
00:14:45,620 --> 00:14:50,150
no abundant evidence that the
way photosynthesis happens in

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00:14:50,150 --> 00:14:56,700
plants is a cyanobacterium got
trapped somehow inside a early

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00:14:56,700 --> 00:14:59,280
plant cell, and is now
a permanent part

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of the plant cell.

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00:15:01,550 --> 00:15:02,800
And it's called a chloroplast.

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00:15:06,560 --> 00:15:09,160
So it's derived from
a bacterium.

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00:15:09,160 --> 00:15:13,400
If you see plants are green.

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If you can look in and see the
chloroplast inside, this shows

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00:15:20,430 --> 00:15:21,920
the chloroplast coming in.

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00:15:21,920 --> 00:15:23,870
And here's their basic
structure.

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00:15:23,870 --> 00:15:27,360
They too have a double
membrane.

236
00:15:27,360 --> 00:15:29,100
They have an outer membrane.

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00:15:29,100 --> 00:15:30,360
They have an inner membrane.

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00:15:33,840 --> 00:15:38,910
They have a part that's called
the stroma, and that's

239
00:15:38,910 --> 00:15:44,380
essentially, like the cytoplasm
of a normal cell.

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00:15:44,380 --> 00:15:49,130
And they have something in
here called a lumen.

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00:15:49,130 --> 00:15:53,360
It's a space, and the membrane
that bounds it is a special

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00:15:53,360 --> 00:15:59,040
membrane called a thylakoid
membrane.

243
00:16:06,580 --> 00:16:11,070
And that gradient is established
by pumping an

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00:16:11,070 --> 00:16:17,060
electron from the stroma, which
I called out, into the

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00:16:17,060 --> 00:16:20,190
lumen, which I called in.

246
00:16:20,190 --> 00:16:24,540
Again, the point is this cell
managed to establish a proton

247
00:16:24,540 --> 00:16:29,090
gradient, and it's able to make
the chloroplast, able to

248
00:16:29,090 --> 00:16:33,230
establish a protein gradient,
and make ATP.

249
00:16:33,230 --> 00:16:38,510
And there's a transmission
micrograph of a chloroplast.

250
00:16:38,510 --> 00:16:42,900
You can see the thylakoid
membranes inside.

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00:16:42,900 --> 00:16:45,830
It's not too hard to imagine
that that was, in fact, a

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00:16:45,830 --> 00:16:47,660
cyanobacterium that
got in there.

253
00:16:47,660 --> 00:16:49,980
And there's quite a bit of
additional evidence that

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00:16:49,980 --> 00:16:51,230
supports that.