PROFESSOR: OK.
So I need to move on a little
bit now, and I want to talk
about in fact, the earlier way
that nature developed to make
energy using proton gradients.
And this part actually preceded
the development of
respiration as you'll see.
It's what I somewhat flippantly
refer to as
photosynthesis release 1.
In my first lecture,
when I was giving
you a sketch of evolution.
Who knows, I mean these are very
rough numbers, but may
have evolved about $3.4 billion
years ago and early
life had begun to exhaust
this sea of chemicals
that had been produced.
And it's known as cyclic
photophosphorylation.
And it's a way of taking
the energy in
sunlight and making ATP.
So whatever organism figured
this out, this was
a really big deal.
Because now instead of having
to use the sort of natural
reserves like the way the food
around is a depleting resource
just like our petroleum
reserves, this was able to
take the abundantly-available
sunlight and
use it to make energy.
So that's the principle.
It uses the energy in sunlight,
and the way it does
it, is it uses the sunlight to
establish a proton gradient
just as we've been discussing
earlier, and then uses
that to make ATP.
And there's a special molecule
that's involved in absorbing
the energy from the sunlight.
We've all heard it I'm
sure, chlorophyll.
There are a couple of two major
variants of this molecule.
Here's one of them, chlorophyll
a, and you don't
have to memorize
the structure.
But I want you to notice
a couple of things.
One is there's a metal in the
middle, a magnesium, and then
it's coordinated with the
cyclic ring system.
And notice all the conjugated
double bonds.
So this chlorophyll was tuned
to absorb energy from the
visible range of sunlight.
And when it absorbs a photon
of energy, it kicks an
electron up to a
higher orbital.
And if the electron's in a
higher orbital, it's more
easily lost.
And this has a consequence.
So the way this system works
is you have a molecule of
chlorophyll, that's what
I'm abbreviating here.
Oh, let me tell you something
else too.
It's more sophisticated
than this.
So this is the molecule that
absorbs a particular
wavelength from sunlight.
But this is embedded in a
multi-protein structure that
has a bunch of other molecules
that absorb at different
wavelengths and then funnel that
energy down to the one
the chlorophyll comes in.
So in fact, the whole thing is
like a big antenna that's able
to absorb quite a bit of
energy from different
wavelengths in sunlight
and get it down to the
chlorophyll.
When the chlorophyll absorbs
energy, it goes up to an
excited state.
And as I said, now that the
electron's in a higher
orbital, it's lost
more easily.
So this has become a better
reducing agent.
It's able to give its electrons
to things that it
couldn't do down in
this energy state.
So we come down one of these
thermodynamic hills that
you're hopefully starting to
get used to where it comes
down in little hops to a carrier
that has this set
level of energy, down,
free energy, down.
And similarly, to the principle
that we talked about
in respiration, a proton is
pumped from what I'm going to
say, in this case, I'll show you
what I mean, but I'll say
from a proton that's out
to a proton that's in.
And by doing that, it
establishes a proton gradient,
and that gives rise to ATP.
At the end of this cycle, we
have this chlorophyll minus
the electrons.
These come, flow back.
That's why it's called cyclic
photophosphorylation.
The electrons go through these
carriers, and then they return
to chlorophyll.
So wonderful system.
I seemed to have accidentally
advanced this.
OK.
There are still bacteria around
that run this system.
So if you remember, we talked
about biosynthesis, the need
for energy.
Well, here we are.
We got ATP.
But there is something else
hopefully you now appreciate
in that is that ATP is not
enough to take carbon dioxide
and make it into sugars
or carbon compounds.
We need a source of reducing
power as well.
Because remember, carbon dioxide
is the most oxidized
form of carbon.
So these early organism solved
it by making reducing power
from another source.
Many of them used hydrogen
sulfide as a source, and they
used NADP plus.
Now, this is a minor
variation of NADH.
It's got one more
phosphate on it.
You can look it up
in your book.
This variant of NAD is used
preferentially for
biosynthetic purposes.
But everything I've told you
about NAD in terms of banking
electrons applies here.
So the electrons from
here are grabbed.
The cell makes NADPH, which you
can use as reducing power
for biosynthesis.
You get elemental sulfur
and hydrogen ions.
So this process an organism
that used this kind of
photophosphorylation to make
ATP would get its reducing
power through a process
something like this.
And then it could make sugars,
and then from that point on,
they can be used to
make all the other
molecules that you need.
The key thing is to get from the
carbon dioxide down into a
more reduced form of carbon.
So that works pretty well.
However, a better system came
up involved in evolution.
This was the one I again
somewhat flippantly called
photosynthesis release 2,
when I was talking.
This is known as, probably came
up who knows again but
maybe 3 billion years ago, and
it's known as noncyclic
photophosphorylation.
And what's important about this
system and why it's an
improvement over the other, is
it uses the energy in sunlight
to make ATP just as we've
learned, but it also uses the
energy in sunlight
to make NADPH.
So in other words, this second
version gives the cell simply
from the energy in sunlight
everything it needs to take
carbon dioxide and make it
into organic compounds.
And it's a pretty cool system
evolutionarily.
It's built on the older one,
the first arising one.
You'll still see the elements of
the present but with a new
variation added in, very much
the way we do design when
you're doing engineering.
You get something that's
working, and you can use that
as a basis to move to a new
and improved version.
And naturally, if you get a new
and improved version, and
you get a little advantage over
your neighbors, natural
selection makes sure that
that better system gets
established.
So here's here how
this noncyclic
photophosphorylation starts.
We take a chlorophyll and it
absorbs the quantum of energy,
and it kicks itself up
to an excited state.
The chlorophyll, as before,
electrons come down,
energetically downhill and
remember that theme.
I keep saying that's at least
thermodynamic properties.
If we think about free energy,
it doesn't matter
what path you take.
Whether you come shooting right
down or you come down
through it, you get the
same energy back.
What's amazing about the system,
if it didn't have all
this extra apparatus, you'd kick
up the electron, and then
it would just come right back
and you'd get a little
radiated, a little
energy given off.
We wouldn't have accomplished
anything.
And what's terrific about this
photophosphorylation system,
it's able to capture the energy
that's in that excited
chlorophyll.
So at this point, and as it's
coming down as I said, we have
H plus going from H
plus, H plus in.
I'll give you a picture of
what I mean by that.
Then we're getting ATP made.
So this time the difference is
instead of the electrons going
back to that chlorophyll, it was
missing its electrons, the
electrons, instead, go to a
chlorophyll, which is at a
somewhat higher energy level
than the first one.
And it has just absorbed quantum
of energy, and it's
kicked itself up to an even
more excited state.
And these electrons from this
system come on and fill up
this chlorophyll.
So this one over here is
called photosystem II.
And the term used in the field
to describe what I'm about to
tell you here is now called
photosystem I.
So what we have now from this
system is an excited molecule,
chlorophyll, that's even more
excited than we were before.
And so it's even more able to
give off its electrons.
It has more reducing power.
In fact, it has enough reducing
power that it can
reduce NADP.
So NADP plus, electrons coming
downhill, you get NADPH.
So here we are, reducing power
made by using the energy in
sunlight, ATP made using
the energy in sunlight.
So by just using this noncyclic
photophosphorylation
system, the cell's got what it
needs to take carbon dioxide
and put it through a sequence of
reactions that will let it
make sugars and other things.
In this course, I don't have
enough time to go through the
biosynthetic pathway.
It's in your textbooks.
You might find it interesting
to look at.
We're not making a big
issue of it in there.
But it exists, and you can see
that it obviously exists.
So there's one more wrinkle here
which might be sort of
eerily reminiscent of one of the
issue I posed for you when
I asked about whether we could
just keep on doing glycolysis.
I can't just let
the system run.
I forgot about something
so far.
Over here, this guy
lost an electron.
It can't get it back because the
electrons went over there.
They have to come
from somewhere.
Well, the energetics of the
system now are such that it
can get electrons from water.
And what's left over when you
take the electrons from water?
We have half of an
oxygen molecule.
So here's the class.
It's a waste product, if you
will, from this very efficient
noncyclic photophosphorylation
system, but
it's molecular oxygen.
And it was when this system
developed that we started to
have oxygen appear
in this world.
The organelle that carries out
photosynthesis, actually, the
first organisms that learned
how to do this is called
cyanobacteria, which they
sometimes sort of rather
incorrectly call blue-green
algae
because they're bacteria.
But you see cyanobacteria
all the time.
And similarly to what happened
with the mitochondria, there's
no abundant evidence that the
way photosynthesis happens in
plants is a cyanobacterium got
trapped somehow inside a early
plant cell, and is now
a permanent part
of the plant cell.
And it's called a chloroplast.
So it's derived from
a bacterium.
If you see plants are green.
If you can look in and see the
chloroplast inside, this shows
the chloroplast coming in.
And here's their basic
structure.
They too have a double
membrane.
They have an outer membrane.
They have an inner membrane.
They have a part that's called
the stroma, and that's
essentially, like the cytoplasm
of a normal cell.
And they have something in
here called a lumen.
It's a space, and the membrane
that bounds it is a special
membrane called a thylakoid
membrane.
And that gradient is established
by pumping an
electron from the stroma, which
I called out, into the
lumen, which I called in.
Again, the point is this cell
managed to establish a proton
gradient, and it's able to make
the chloroplast, able to
establish a protein gradient,
and make ATP.
And there's a transmission
micrograph of a chloroplast.
You can see the thylakoid
membranes inside.
It's not too hard to imagine
that that was, in fact, a
cyanobacterium that
got in there.
And there's quite a bit of
additional evidence that
supports that.
