>> Hello, and welcome to the "Penguin Prof Channel".
In today's episode, I'm going to try to explain photosynthesis.
It's tough.
This is a tough subject.
So I need you to take a nice, deep breath and relax.
And now think about where did that oxygen come from.
If you said plants, you're on the right track.
If you said rainforests,
that's where most people think their oxygen comes from.
About twenty percent of the oxygen
that you're breathing right now was made by plants
from a rainforest, but most
of the oxygen was not made by plants at all.
So who's doing all the photosynthesis?
Well, it's happening in the oceans
by little tiny microscopic critters called phytoplankton.
All these little green blobs.
If you blow them up, they're actually really pretty.
Photosynthesis allows life as we know it to exist.
Photosynthesis produces oxygen that we need to breathe
and produces carbohydrates that we use to burn for fuel.
There's two parts of photosynthesis, and luck for us,
there's two parts to the word photosynthesis.
The word photo, the photo part is about the capturing
of light energy into chemical bond energy.
The synthesis part of photosynthesis is
about the production of carbohydrates.
So the photo part is going to involve light energy captured
by photosynthetic pigments of the plant, and these photons
of energy will be converted to chemical bond energy
in the form of NADPH and ATP.
If these molecules and the concepts of oxidation
and reduction are really foreign to you, please check
out my video on redox reactions.
I will put the link in the description box below.
These high-energy molecules are going to be used
as energy sources to fix carbon,
atmospheric carbon into carbohydrates.
Not into glucose directly but into molecules
that can be used to make glucose.
So basically what we do is we're going to divide the processes
of photosynthesis into the light-capturing part,
which conveniently are called the light reactions,
and then the carbon fixation part, which we refer
to as the Calvin cycle, or the light-independent reactions.
So let's talk about light first.
Here's the electromagnetic spectrum.
There's a teeny tiny little slice in there
that is actually visible to us, and that's the same sliver
that plants can use for photosynthesis.
Now how plants capture this light is with pigment molecules
like chlorophyll A. Here's chlorophyll B. There are other
pigments, though.
This is beta carotene.
Beta carotene is what gives things like carrots their color.
Now, what you can do is look at the absorbance spectra
of different pigments, and you'll see that they all peak
in different places, and this actually allows photosynthetic
organisms to be very efficient in their capturing
of different wavelengths,
but most photosynthetic pigments have a low in absorbance right
around here, and that, as you can see in the color bar below,
corresponds to the color green.
So plants do not use green light very efficiently at all,
and that is why green is transmitted and reflected,
and that's why plants appear green.
You can actually extract chlorophyll yourself
in your own kitchen.
I like to use spinach.
You're going to need a blender and some cheesecloth,
and you're going to need to blend it up not in water,
that won't work, but in an organic solvent
like pet ether, acetone, methanol.
I know you got that in your kitchen.
Don't drink this smoothie.
If you filter it through the cheesecloth, you're going
to get a nice solution of chlorophyll, and what you can do
with it is shine light on it, and you're going
to see something really cool.
It's going to glow red, and this is called fluorescence,
and in order to understand the light reactions,
it's actually important that you understand why chlorophyll
fluoresces when you shine light on it like this.
So here's what's happening.
If you isolate a chlorophyll molecule, and you shine light
on it, what's going to happen is electrons at the center
of the chlorophyll are going to increase in their energy state.
They're going to get excited.
Ooh, I like that.
It's very exciting.
Now, as you know, anything that goes up must come down,
and in isolated chlorophyll molecule, there's nowhere
for those electrons to go.
So as soon as they get excited, they immediately fall back
down to what we call the ground state, and when they do that,
they give off some energy in the form of heat and light.
And that light, we call fluorescence,
and that's why it glows.
Now if you go out into the world,
hopefully you have never seen a glowing plant unless you take
some very interesting medication.
So plants don't naturally fluoresce,
and the reason is plants capture the energy
of the electron before it falls back down.
So you have the striking of the chlorophyll,
and just like before, the electrons
in the chlorophyll molecule, they increase
in their energy state, but instead of being allowed
to fall back down and having all that energy wasted,
the chloroplast has proteins which capture the energy
of the electron as it falls, kind of like water going down a,
a water wheel, and that energy is then used to do work.
And that's basically what the light reactions are about.
These chlorophyll molecules are actually organized
in a complex system that we call a photosystem, and the photons
of light actually bounce
around until they hit a reaction center,
and the whole thing is very beautiful and complicated.
This is what is actually looks
like with all those green blobs representing chlorophyll
in there, and it's actually quite beautiful.
In order to understand how this works, we're going to have
to understand the structure of a chloroplast.
A chloroplast is an organelle with not one but two membranes,
an inner and an outer membrane, and then inside, there's going
to be fluid that we call stroma, and that's where all
of the carbon fixation is going to happen.
And then most of the chloroplast is actually made of
yet more membranes that we call thylakoids,
and they're organized in stacks that we call grana.
So this would be one granum, and in the thylakoid membranes,
this is where all of the light reactions are going to occur.
So we're going to talk about the light reactions first,
and then later we're going to talk about this carbon fixation.
OK, so the first thing that we're going
to need is a source of electrons.
The electrons come from water.
Believe it or not,
photosynthetic organisms use sunlight to split water.
So water molecules are going to be split, and, of course,
you're going to get out of that oxygen, hydrogen, and electrons.
This is called photo oxidation.
What's going to happen is the electrons are going
to fuel the entire system of the light reactions.
So we're going to watch the electrons here in just a second.
We're also going to get oxygen, and this is the source
for atmospheric oxygen, and you're going to get protons,
and the protons are going to contribute
to the proton gradient in the thylakoid lumen that we're going
to see in a little bit.
That's going to be used to make ATP.
OK. So let's see what happens to the electrons.
When light strikes photosystem two, it's going to get bounced
around until it hits the reaction center, and, there,
the electrons in the center are going to be increased
in energy [inaudible], just like we saw before,
but instead of the electron just falling back down
and fluorescing, that electron's energy is going to be captured
by proteins in the thylakoid membrane called the electron
transport chain.
Kind of like water wheels.
As the electron passes through these proteins,
the energy in the electron is going
to be harnessed to do cell work.
So the electron is going to fall through these proteins,
and the energy is going to be used to pump protons
against their gradient into the thylakoid lumen.
So we're creating a gradient of protons here.
Now another photon of light strikes the center
of photo system one, and that's going
to re-energize the same electron to a high-energy state again.
Where the electron ends up is
in what's called photo reduction of NADP.
So NADP gets reduced to NADPH.
So you should see overall the flow of electrons goes
from water through photo system two,
through the electron transport chain, through photo system one,
and it ends up in NADPH.
Wow. Isn't that incredible?
What about the production of ATP?
Well, that's where the proton gradient comes in.
So the proton gradient comes from photo oxidation of water
as well as the electron transport chain,
constantly pumping protons into the thylakoid lumen.
Now the protons are not happy here.
They desperately want to get out,
but the only place they fit is through a little slot
in a protein called the ATP synthase,
and what will happen is the energy of the proton going
down its gradient, going in the direction it wants
to go fuels the phosphorylation of ADP to make ATP.
Those are the light reactions.
Now what we're going to do is focus in on the Calvin cycle
and see what all of that ATP and NADPH is going to be used for.
The Calvin cycle goes by a lot of names, as you can see.
Melvin Calvin, yes, that was his real name, is the person
who is responsible for figuring this out.
He used carbon 14 to trace carbon through photosynthesis.
He wrote an autobiography about his journey,
which is a nice little read, and he won the Nobel Prize
in chemistry for this discovery.
The star of the show turns out to be an enzyme called RuBisCO,
and RuBisCO is the most abundant protein on Earth.
At any given time, there's probably forty billion tons
of it on the planet, and it is responsible
for incorporating atmospheric carbon from carbon dioxide
into an organic molecule.
I know you want to know how this works.
RuBisCO is the enzyme that is the key to carbon fixation.
That doesn't mean that carbon is broken.
It just means that the carbon from CO2 is going
to be incorporated into organic compounds
which will be made into glucose.
This really is the key to life on Earth as we know it.
So I can't overemphasize enough the importance of this enzyme.
So here's what it actually does.
RUBP is a five-carbon molecule that actually cycles
through the Calvin cycle, as you'll see in a minute,
and the addition of CO2, that additional carbon,
this carboxylation of RUBP happens because of RuBisCO,
and what you form is an unstable intermediate that's six carbons
in length.
It immediately gets split into two molecules
of three phosphoglycerate, and this happens
at the beginning of the Calvin cycle.
Now the whole of the Calvin cycle,
the sum of all the reactions looks like this.
Things to notice.
There's the source of the carbon that's going to be incorporated
into the new carbohydrate.
This NADPH, that was made in the light reactions.
We need some water, and we need some ATP.
What's going to come out of it is not glucose.
Now, a lot of textbooks will say that glucose is the result
of the Calvin cycle, and that's actually not true.
What you're making is G3P, glyceraldehyde-3-phosphate.
That is actually the carbohydrate that gets made,
and that will be used to build glucose,
or it could be incorporated into storage molecules like starch,
structural molecules like cellulose,
or anything else that the plant needs.
There are three phases to the Calvin cycle.
Phase one is the incorporation of carbon dioxide to RUBP.
This guy right here, and that's done
because of the amazing enzyme RuBisCO.
Phase two is reduction phase, and we are using up some
of the ATP and some of the NADPH that we made
in the light reactions.
Phase three is the regeneration of RUBP,
and this is why it's a cycle.
OK, so let's just talk about numbers for a second.
If you have three RUBP molecules entering the cycle,
you can make six glyceraldehyde-3-phosphates,
six G3P's, but it cost you five of those
to make the three RUBP's.
So that's actually the, the cyclical part of it.
The only thing that gets to leave is one G3P, right,
because you make six, but it cost you five.
This is kind of confusing.
So if you want to see how expensive this really is
to get one G3P released from the cycle,
it's going to cost nine ATP's and six NADPH's.
So this is a really expensive process.
So here is a really good summary of all of this.
We're going to input solar energy in water, and we're going
to run the light reactions, and the products
of the light reactions include oxygen gas, right.
That comes from the splitting of water
and the high energy molecules ATP and NADPH, which is going
to provide reducing power.
Those, along with carbon dioxide,
will feed into the Calvin cycle,
the outputs of which include the sugars, it's actually G3P,
which will be converted into glucose.
That's the chemical energy, and ADP and NADP plus,
which will feed back into the light reactions.
Here's another view of the same thing that kind
of adds the location of everything
in the chloroplast, and that's it.
Wow. So the next time you look at plants, man,
they deserve a lot of respect.
Photosynthesis is an unbelievably complex process,
but it makes life on Earth possible.
As always, I hope that was helpful.
Thank you so much for visiting the "Penguin Prof Channel".
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Good luck.