Paul Andersen explains the process of photosynthesis by which plants and algae can convert carbon dioxide into useable sugar. He begins with a brief …

Hi. It's Mr. Andersen and in
this podcast I'm going to talk about photosynthesis.
I love photosynthesis because it gives me
two things that I need. I need to breathe,
so it gives me oxygen. And I need to eat.
And so it's going to give me food. And so
I love photosynthesis. You might think it's
only found in these things, plants, but it's
also found in bacteria. It's found in algae.
And so it's found in protists. It's found
everywhere. And so photosynthesis has been
around a long time. It's super important that
you understand how it works. And so let's
start with the site in eukaryotic cells of
photosynthesis. And that's the chloroplasts.
So this is a number of cells. And you can
see how many chloroplasts we could have in
a typical cell. So there's a whole bunch of
them. There are a few terms you should be
familiar with. And where they are. First one
is a thylakoid membrane. Thylakoid membrane
is going to be organized like this. And basically
that's where the light reaction is going to
take place. If you've got a stack of thylakoids
like this together we call that a granum.
The other big thing to understand photosynthesis
is that this is filled with a liquid. And
that liquid is called the stroma. And that's
going to be the site of the Calvin cycle.
If we were to grind up a leaf what we would
find is that there's not only one pigment,
chlorophyll A that does photosynthesis. But
theres a number of them that are working together.
And so if you grind up a leaf into some chromatography
paper and then you put it in a solvent, what
you'll get is chromatography. It's going to
separate into all of its different parts.
And so this right here would be chlorophyll
A and chlorophyll B. And this would be like
carotene and xanthaphylls. And they're all
working together. You'll see this other pigments
in the fall when the chlorophyll moves back
into the leaf and is reabsorbed. But if we
look at what light they absorb, here's chlorophyll
A and here's B. This is what's called their
absorption spectrum. And what color of light
they are able to absorb. And you can see that
they absorb a lot of the blue. A lot of the
red. But they don't absorb a lot of this in
the middle, this green. And so a quick question
could be what is their least favorite color,
plants? And the the right answer would be
green. Because the reflect that green light.
Now this is actually puzzled scientists for
a long time. And we really don't have a definitive
answer as to why plants are green. Know this
that if they were black they probably would
get a little bit too hot. They would absorb
too much light. And so let' start with an
equation. Because this is simply a chemical
reaction. It's a chemical reaction with a
number or steps. But what are the reactants?
Water and carbon dioxide. And so how does
a plant grow? It's basically taking water
in from its roots and it's taking carbon dioxide
in through its leaves. Through its stomata.
The other thing it needs is light. And so
it's just taking these simple ingredients.
And then it's weaving those together into
glucose. This monster molecule here. And then
oxygen. And so this is the food that I get.
And this is the oxygen that I breathe. Now
are plants just nice? No. They're making this
sugar for themselves so they can break it
down using cellular respiration. And in fact
if I put this arrow in the other direction,
that becomes cellular respiration. So they're
making food for themselves and they're also
going to make some of the structure. So like
the cellulose in the cell walls of a plant
is made from that as well. Okay, so whenever
I try to think what are the different steps
in photosynthesis? I always image this picture
right here. There's photo and synthesis in
the word. Photo means light. And synthesis
means to make. And so there are two steps
in photosynthesis. The light reaction. And
those are going to take place in the thylakoid
membrane. And then the Calvin cycle. We used
to call this the dark reactions which is a
silly term. Doesn't happen during the dark.
It happen during the light. And so basically
the person who worked this all out is Melvin
Calvin and so we named it after him. Where
does this take place? You guessed it. It takes
place in the stroma or this liquid portion.
And so let's kind of do a cartoon version
of photosynthesis. What are the reactants
again? Water, light and carbon dioxide. What
are going to be the products that come out
of this? It's going to be oxygen and glucose.
So let's watch what happens. In the light
dependent reaction water and light go into
the thylakoid membrane and they produce two
things. They produce oxygen. Oxygen is simply
a waste product. And then they're going to
produce these chemicals. NADPH and ATP. So
they have energy now. Let's watch what happens
to them. Well the energy is going to transfer
to the Calvin cycle where carbon dioxide comes
in and then glucose goes out. And so this
is the big picture of photosynthesis. But
now let's kind of dig in a little bit deep
and talk about the light reaction. Okay, so
where are we? We're in the thylakoid membrane.
So we're in this membrane right here. So if
we were to zoom in to that membrane right
here, that's what this diagram is. Okay. So
what are the two things coming in? Well the
first one is going to be light. So light's
coming in here. Light's coming in here. What's
the next things that we're going to have coming
in? And that's going to be water. Okay, so
let's look at some of the other big features
in this thylalkoid membrane. So this is the
outside, or the stroma. And this is going
to be the lumen or the inside. And so there's
a couple of big things right here. What's
in here? Well these are basically going to
be proteins with chlorophyll on the inside
of it. And so we call that whole thing together
a photo system. So this first one is actually
called photo system II. And then we go to
photo system I. And the reason we go backwards
is that that photo system I was discovered
first. So basically what comes in? Light.
What's that light used to do? Well that light
is used to power the movement of an electron
through an electron transport chain. So that
electron is going through proteins, carrier
proteins. And eventually that electron is
going to go to here. It's going to go to NADPH.
Because remember that's one of the products
of the light dependent reaction. Okay. What
happens to the water then? So the water is
going to be split right away. If you split
water what do you get? Well you get oxygen.
So that's the O2 that's going to diffuse out
of a cell. And that's the oxygen that you're
actually breathing right now. And then we're
going to have these protons which are simply
hydrogen ions. So they're hydrogen atoms that
have lost their electron. Okay, so this is
getting kind of messy. So let's look what
happens next. As that electron moves through
the electron transport chain, and again it's
powered by the introduction of light here
and light here. That electron is going to
be moving all the way down here and every
time it goes through one of these proteins,
it's pumping protons to the insides. So it's
pumping protons to the inside. Now protons
have a positive charge. So basically what's
happening is that you're building up a positive
charge on the inside. So there's a positive
charge in here. If you know how cellular respiration
works, you'll realize that this is the opposite
of that. So now we have all of these positive
charges on the inside. Where do they go? Well
there's only one hole that they can go through.
And that's is to go through this protein here.
As those protons move out, they're moving
through a protein called ATP synthase. And
it works almost like a little rotor. And every
time a proton goes through we make another
ATP. So what have we made in the light dependent
reaction? We've made NADPH and we've made
ATP. And what's nice about that is they're
now just sitting right here in the stroma
and so they're able to go on to the Calvin
cycle which is going to be the next step in
this process. And so who's providing the energy?
Light. Whose providing the electrons? Water.
And then a waste product of that is simply
going to be oxygen. Okay. Let's go to the
Calvin cycle then. So what's happening in
the Calvin cycle? You can see here's those
reactants. So we've got our ATP here, ATP
here and NADPH. What are they providing? Simply
energy. We also have this molecule here. It's
called RUBP. Basically it's a five carbon
molecule. And then we have carbon dioxide
coming in. So it moves through the stomata
of the leaf. And it's going to diffuse its
way in. Carbon dioxide is a one carbon molecule.
So basically there's an enzyme here called
rubisco and it's going to attach this one
carbon molecule to a five carbon molecule.
It immediately breaks into three carbon molecules.
And then it gets energy from ATP and NADPH.
And when we're done it's creating this chemical
down here, called G3P. What does G3P become?
Well it can be assembled quickly into glucose
or sucrose or maltose or whatever they need
to do, that's going to be produced right in
here by the G3P. So that's where we're synthesizing.
In other words we're taking carbon and we're
fixing it. We're making it usable. Now some
of that G3P is released. But a lot of it is
recycled again to make more of this RUBP.
And so that's why it's a cycle over and over
again. What's the big picture? If we don't
have ATP, if we don't have NADPH, then this
process is going to shut down. What's the
other thing that could shut it down? If we
don't have carbon dioxide. Okay, so that's
basically photosynthesis. And again it's been
working for billions of years. But there's
a slight problem. And that problem is called
photorespiration. What is photorespiration?
Well photorespiration occurs only when we
don't have enough carbon dioxide. So if we
don't have enough carbon dioxide, let me cross
that out, well we certainly can't make our
G3P. But something worse happens. Oxygen can
actually jump into the Calvin cycle. And using
rubisco can form another chemical. Now that
chemical doesn't do anything. In other words
it has no purpose. And the cell actually has
to break it down. And so as a result of that
plants, and we call almost all plants C3 plants.
And the reason we call them C3 plants is this
G3P is going to be a 3 carbon molecule. So
for these C3 plants, photorespiration is bad.
In other words, they don't get anything out
of it. And so they're going to lose based
on that oxygen kind of jumping into the Calvin
cycle. And so you might think evolutionarily,
why would this have even evolved? Well remember,
photosynthesis shows up first. And then oxygen
in the atmosphere shows up much later. And
so it wasn't a problem initially, but it became
a problem. Another question you might think
is, when are we not going to have enough carbon
dioxide? When wouldn't we have carbon dioxide?
Well how do they get carbon dioxide? And plant
is going to have a stomata. And it's surrounded
by guard cells. And so basically when a plant
opens up its stomata, carbon dioxide can diffuse
in. And so the only time the plant wouldn't
have carbon dioxide, because we have tons
of carbon dioxide in the atmosphere, is when
it's actually closed. And when would it be
closed in a plant? The only time it's closed
is when it's really, really hot. And a plant
doesn't want to lose water. Because through
transpiration you're constantly losing water.
And so if you're a plant, if it's a hot day
you have this really tough choice. If you
open up your stomata, you're going to lose
water. You could shrivel up. If you close
it, you can't get carbon dioxide in and then
you're going to start doing photorespiration.
And so of course nature has come up with solutions
to this over time. And it's only going to
be found in plants that live in really hot
environment. So here's the first solution.
And this totally makes sense. This is in CAM
plants. CAM plant would be a jade plant or
like a pineapple. Basically what they do is
they only open their stomata at night. And
so at night they open up their stomata. And
then the carbon dioxide will come in and they'll
create malic acid out of it. So they're going
to store it in vacuoles inside the cell. Okay.
So now when it's day time what they can do
is they can close the stomata because they
don't want to lose water. And now they can
actually take that carbon dioxide out of the
malic acid and they can use it in the Calvin
cycle to make sugars. So the great thing about
CAM plant is again they're only taking in
carbon dioxide at night when it's cool. And
then during the day they can close their stomata
and they don't lose water. Another example
of this would be in C4 plants. What they do
is instead of doing it day and night, what
they'll do is they'll take that carbon dioxide
in and they'll actually use enzymes to make
a 4 carbon molecule out of it. That 4 carbon
molecule will move to some cells on the inside
of the leaf called the bundle sheath cells.
And then they can simply introduce carbon
dioxide into the Calvin cycle here. And so
again, both of these solutions are basically
taking in carbon dioxide when you can get
it. Creating a chemical out of it. And then
they can introduce that chemical into the
Calvin cycle and they don't have to wait for
carbon dioxide to diffuse in. Now of course
there's going to be extra steps in here so
it's going to require more energy. And so
we only see this in areas where it's really,
really warm. But an example of a C4 plant
that we all eat and use a lot of, in fact
most of us are just made out of this stuff
is corn. And so that's photosynthesis. A simple
problem is photorespiration, but I hope that's
helpful.

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