The important thing from the photosynthesis point of view is that it's this membrane. And on the outside of the membrane, right here on the outside, you have the fluid that fills up the entire chloroplast. So here you have the stroma. And then this space right here, this is the inside of your thylakoid.
So this is the lumen. So if I were to color it pink, right there. This is your lumen. Your thylakoid space. And in this membrane, and this might look a little bit familiar if you think about mitochondria and the electron transport chain. What I'm going to describe in this video actually is an electron transport chain.
Many people might not consider it the electron transport chain, but it's the same idea. Same general idea. So on this membrane you have these proteins and these complexes of proteins and molecules that span this membrane. So let me draw a couple of them. So maybe I'll call this one, photosystem II. And I'm calling it that because that's what it is. Photosystem II.
You have maybe another complex. And these are hugely complicated. I'll do a sneak peek of what photosystem II actually looks like. This is actually what photosystem II looks like. So, as you can see, it truly is a complex. These cylindrical things, these are proteins. These green things are chlorophyll molecules. I mean, there's all sorts of things going here. And they're all jumbled together. I think a complex probably is the best word.
It's a bunch of proteins, a bunch of molecules just jumbled together to perform a very particular function. We're going to describe that in a few seconds. So that's what photosystem II looks like.
Then you also have photosystem I. And then you have other molecules, other complexes. You have the cytochrome B6F complex and I'll draw this in a different color right here. I don't want to get too much into the weeds. Because the most important thing is just to understand. So you have other protein complexes, protein molecular complexes here that also span the membrane. But the general idea-- I'll tell you the general idea and then we'll go into the specifics-- of what happens during the light reaction, or the light dependent reaction, is you have some photons.
Photons from the sun. They've traveled 93 million miles. And actually in photosystem II-- well, I won't go into the details just yet-- but they excite a chlorophyll molecule so those electrons enter into a high energy state.
Maybe I shouldn't draw it like that. They enter into a high energy state. And then as they go from molecule to molecule they keep going down in energy state. But as they go down in energy state, you have hydrogen atoms, or actually I should say hydrogen protons without the electrons. So you have all of these hydrogen protons. Hydrogen protons get pumped into the lumen. They get pumped into the lumen and so you might remember this from the electron transport chain.
In the electron transport chain, as electrons went from a high potential, a high energy state, to a low energy state, that energy was used to pump hydrogens through a membrane. And in that case it was in the mitochondria, here the membrane is the thylakoid membrane. But either case, you're creating this gradient where-- because of the energy from, essentially the photons-- the electrons enter a high energy state, they keep going into a lower energy state.
And then they actually go to photosystem I and they get hit by another photon. Well, that's a simplification, but that's how you can think of it. Enter another high energy state, then they go to a lower, lower and lower energy state.
But the whole time, that energy from the electrons going from a high energy state to a low energy state is used to pump hydrogen protons into the lumen. So you have this huge concentration of hydrogen protons. And just like what we saw in the electron transport chain, that concentration is then-- of hydrogen protons-- is then used to drive ATP synthase.
So the exact same-- let me see if I can draw that ATP synthase here. You might remember ATP synthase looks something like this. Where literally, so here you have a huge concentration of hydrogen protons. So they'll want to go back into the stroma from the lumen. And they do. And they go through the ATP synthase. Let me do it in a new color. So these hydrogen protons are going to make their way back. Go back down the gradient. And as they go down the gradient, they literally-- it's like an engine.
And I go into detail on this when I talk about respiration. And that turns, literally mechanically turns, this top part-- the way I drew it-- of the ATP synthase. And it puts ADP and phosphate groups together. So that's the general, very high overview. And I'm going to go into more detail in a second. But this process that I just described is called photophosphorylation.
Let me do it in a nice color. Why is it called that? Well, because we're using photons. That's the photo part. We're using light. We're using photons to excite electrons in chlorophyll.
As those electrons get passed from one molecule, from one electron acceptor to another, they enter into lower and lower energy states. As they go into lower energy states, that's used to drive, literally, pumps that allow hydrogen protons to go from the stroma to the lumen. Then the hydrogen protons want to go back. They want to-- I guess you could call it-- chemiosmosis.
They want to go back into the stroma and then that drives ATP synthase. Right here, this is ATP synthase. Now, when I originally talked about the light reactions and dark reactions I said, well the light reactions have two byproducts.
It has ATP and it also has-- actually it has three. NADP is reduced. It gains these electrons and these hydrogens. So where does that show up? Well, if we're talking about non-cyclic oxidative photophosphorylation, or non-cyclic light reactions, the final electron acceptor. So after that electron keeps entering lower and lower energy states, the final electron acceptor is NADP plus. Now, I also said that part of this process, water-- and this is actually a very interesting thing-- water gets oxidized to molecular oxygen.
So where does that happen? So when I said, up here in photosystem I, that we have a chlorophyll molecule that has an electron excited, and it goes into a higher energy state. And then that electron essentially gets passed from one guy to the next, that begs the question, what can we use to replace that electron? And it turns out that we use, we literally use, the electrons in water. So over here you literally have H2O.
And H2O donates the hydrogens and the electrons with it. So you can kind of imagine it donates two hydrogen protons and two electrons to replace the electron that got excited by the photons.
So, you're literally stripping electrons off of water. And when you strip off the electrons and the hydrogens, you're just left with molecular oxygen. Now, the reason why I want to really focus on this is that there's something profound happening here. Or at least on a chemistry level, something profound is happening.
You're oxidizing water. And in the entire biological kingdom, the only place where we know something that is strong enough of an oxidizing agent to oxidize water, to literally take away electrons from water. Which means you're really taking electrons away from oxygen. So you're oxidizing oxygen. The only place that we know that an oxidation agent is strong enough to do this is in photosystem II. So it's a very profound idea, that normally electrons are very happy in water.
They're very happy circulating around oxygens. Oxygen is a very electronegative atom. That's why we even call it oxidizing, because oxygen is very good at oxidizing things. But all of a sudden we've found something that can oxidize oxygen, that can strip electrons off of oxygen and then give those electrons to the chlorophyll. The electron gets excited by photons. Then those photons enter lower and lower and lower energy states. Get excited again in photosystem I by another set of photons and then enter lower and lower and lower energy states.
And the whole time it entered lower and lower energy states, that energy was being used to pump hydrogen across this membrane from the stroma to lumen. And then that gradient is used to actually produce ATP. So in the next video I'm going to give a little bit more context about what this means in terms of energy states of electrons and what's at a higher or lower energy state. But this is essentially all that's happening. Electrons get excited. And as the electron gets excited and goes into lower and lower energy states, it pumps hydrogen across the gradient.
And then that original electron that got excited, it had to be replaced. And that replaced electron is actually stripped off of H2O. So the hydrogen protons and the electrons of H2O are stripped away and you're just left with molecular oxygen. And just to get a nice appreciation of the complexity of all of this-- I showed you this earlier in the video-- but this is literally a-- I mean this isn't a picture of photosystem II.
You actually don't have cylinders like this. But these cylinders represent proteins. Tying in directly to this problem are topics such as deforestation and the Kyoto protocols. When you look around, it's a bit hard to comprehend we are really the newcomers on the planet. We've built so much and seem to be expanding everywhere.
With our building and expansion, we use ever more fossil fuel to run our homes, our cars and the planes we fly in. Burning these fuels creates CO2 as one byproduct of the process. Even the batteries that run our cell phones, Ipods and other gadgets use electricity for charging, which in most instances creates more CO2.
As it turns out, CO2 is also a very important gas in the life processes. Plants use CO2 in the chemical process mentioned above, photosynthesis. Photosynthesis is an extremely complex process. In its simplest form, this important reaction convert CO2 [carbon dioxide] and H2O [water] plus energy [sunlight] into O2 [oxygen] and C6H12O6 [glucose]. The oxygen goes into the air you breathe. They can be represented by C6H10O5 n.
Cellulose C6H10O5 n is a long-chain polymeric polysaccharide carbohydrate, of beta-glucose. What in the world is that you say This forms the primary structural component of green plants. The green plants primary cell wall is made largely of cellulose and the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which, in the form of wood, is the most common biopolymer on Earth.
Now, once again, where does the matter come from that forms the mass of wood, branches and leaves? Trees store carbon, sequestered from the air, as the wood and plant material that makes up their mass. OK, so plants most use carbon in as a part of their life processes Knowing this, how much CO2 is sequestered by an average tree? Well, it very much depends on type of tree. For our example we'll use a mature pine tree Pinus radiata. Let's look at part of your "carbon imprint", your home, and make an estimate for the CO2 it might be producing each year.
I don't want to get too much into the weeds. They've traveled 93 million miles. Or at least on a chemistry level, something profound is happening. This happens through one of the most amazing chemical reactions you can imagine, photosynthesis. More on this a bit further down, but as a lead in to the story we'll ask a question whose answer may astound you
The Calvin cycle Video transcript In the last video we learned a little bit about photosynthesis. I make them green on purpose because the chloroplasts contain chlorophyll. And your average plant cell-- and there are other types of living organisms that perform photosynthesis, but we'll focus on plants.
So they'll want to go back into the stroma from the lumen. Imagine the largest tree you've ever seen even in a picture - where does the matter come from that forms the mass of wood, branches and leaves? They've traveled 93 million miles.
NADP is reduced. And this is an organelle. Respiration is almost the exact opposite reaction to photosynthesis. They can be represented by C6H10O5 n. So on this membrane you have these proteins and these complexes of proteins and molecules that span this membrane.
Because it's reflecting. That's the photo part. Let me do it in a new color. That then teamed up with other cells and said, hey, if I produce your energy maybe you'll give me some food or whatnot.
These green things are chlorophyll molecules. Well, if we're talking about non-cyclic oxidative photophosphorylation, or non-cyclic light reactions, the final electron acceptor. So this right here is a thylakoid. Now, the reason why I want to really focus on this is that there's something profound happening here. Most reactions take place during daylight hours while some occur during the night.
We've built so much and seem to be expanding everywhere. Let's say your house is about 2, square feet, you have a heater, an air conditioner and a water heater.
And then they actually go to photosystem I and they get hit by another photon. How many acres of trees are needed to offset your home? They clean it, and in doing so produce the oxygen we, and all animals need to survive.