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  • 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.

Hi. It's Mr. Andersen and in this podcast I'm going to talk about photosynthesis.

Subtitles and vocabulary

B1 INT US carbon carbon dioxide dioxide calvin photosynthesis calvin cycle

Photosynthesis

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    wshuang999   posted on 2014/11/22
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