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.
