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.