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- [Voiceover] Mitochondria are organelles that are found in cells, and they are responsible for producing ATP, or adenosine triphospate. ATP's a molecule that serves as a principle source of energy for cells, and for this reason, mitochondria are known as the powerhouse of the cell, because they provide the cell with power or energy. So, here's a picture of a mitochondria that was kind of sliced down the middle. And let's take a look at its structure. So it has an outer membrane. And the outer membrane is made up of a lipid bilayer. And you might recognize that term as describing the cell membrane of the cell. And this lipid bilayer is not permeable to most things, but it is permeable to very small molecules. And that's because it has certain proteins embedded in it that allow small molecules to pass. The mitochondria also has an inner membrane, as you can see And the inner membrane is also made up of a lipid bilayer, but it is not permeable to small molecules. And you'll see in a few moments why this is an important fact to keep in mind. And you may have noticed that the inner membrane is not smooth in the way that the outer membrane is, but rather, it has many folds. And these folds are known as, well, individually each fold is a crista, and in plural, we call them cristae. And the reason that the inner membrane is folded into these cristae, is because on the inner membrane, there are lots of different proteins that are necessary for cellular respiration. And by folding the membrane, we just increase the surface area and allow a greater amount of membrane to be in a smaller space. So it basically just gives us more room to work with the enzymes and more room for cellular respiration to happen. And this space between the outer and inner membrane is known as the intermembrane space. I'm gonna write it over here on the side. And then, the center of the mitochondria, we call that the matrix. So, now that we've discussed the structure of mitochondria, let's see how that relates to cellular respiration and what happens here. Let's go through the steps of cellular respiration. Which is the process by which we make ATP, and then let's see how it relates to the mitochondria. So the first step of cellular respiration is glycolysis. And what happens during glycolysis is the molecule glucose, which is a six carbon molecule, gets split into two molecules of pyruvate. Pyruvate is a three carbon molecule. And glycolysis actually does not happen in the mitochondria. It's the only step of cellular respiration that happens in the cytoplasm. The next step of cellular respiration is what's known as the PDC, or the pyruvate dehygrogenase complex. And what happens during the PDC is that pyruvate, and remember we have two of them, gets converted into a molecule known as acetyl-CoA. And you may recognize acetyl-CoA as the molecule that enters the Krebs cycle. The PDC happens in the matrix of the mitochondria, so all the enzymes that are involved in the PDC are found in the matrix, of course, as well. The next step of cellular respiration is the Krebs Cycle. And in the Krebs Cycle, acetyl-CoA is going to undergo a series of reactions, which I'm not going to go into the details of that right now. And this happens also in the matrix of the mitochondria. And what I'm going to focus on right now is that at the end of the Krebs Cycle, we produce two molecules, NADH and FADH2. And these are electron carriers. You'll see in a moment why they're so important, and the last step of cellular respiration, is the electron transport chain. The electron transport chain happens on the inner mitochondrial membrane, so on the membrane itself. And this is the part where we're actually going to make ATP with the help of these electron carriers that we made during the Krebs Cycle. So let's take a closer look at the inner mitochondrial membrane and see what happens during the electron transport chain. So here's a more close-up diagram of the inner mitochondrial membrane. Let's just orient ourselves. Let's say that over here is the outer membrane, which would make this area over here the cytoplasm of the cell. And let's say that over here is the matrix of the mitochondria. And here, just label it as the inner membrane. So you can see the inner membrane is studded with a bunch of these enzymes, and these are the enzymes that are involved in the electron transport chain. And in case you want to know the names of these various enzymes, they're over here. Some of them are pretty long, so we're just going to refer to them by numbers. This one over here is going to be one. That's NADH reductase. Then this white one, that's cytochrome Q. This green one is succinate dehydrogenase. Then we have number three. I'm not going to mention the names. You can read them if you want. Then we have cytochrome C over here. And then there's number four right over here. So we have NADH and FADH2 which were produced during the Krebs Cycle, and these are our electron carriers. And I'm going to describe what happens to NADH, but the same thing happens to FADH2. So NADH is going to be oxidized, or lose electrons, so let's write out that reaction. NADH will turn into NAD+ plus two Hydrogen ions, plus two electrons, so it got reduced. And those two, I'm sorry, it got oxidized. It lost electrons. Those two electrons are going to go on to enzyme number one. So while NADH lost electrons and got oxidized, the first enzyme gained electrons or got reduced. But enzyme one is not going to hold onto the electrons. It's going to pass them on to the next enzyme, which is cytochrome Q. So now enzyme number one gets oxidized because it loses electrons, but enzyme cytochrome Q gets reduced because it gains electrons. And then, the same thing will happen with the next enzyme. Cytochrome Q will pass those two electrons on to the next enzyme. And in case you're wondering where this enzyme two comes in, so FADH2, when it gets oxidized, its electrons go directly to enzyme two, from there to cytochrome Q, from there to three, etc. but anyways, back to what's happening to our NADH, so the two electrons are in enzyme three. Then they go to cytochrome C. Then they go to enzyme four, and then, finally, those two electrons are used to reduce oxygen and make water. So I'm going to write one half O2, which is the same as one oxygen atom, plus two H+'s, plus those two electrons, give us water. Two H+ plus two electrons, that's the same thing as saying 2H, so we produce water. Let's go back to the electrons jumping from one enzyme to the next. When these electrons go from one enzyme to the next, they are going from a state of higher energy to a state of lower energy. And when electrons go from a state of higher energy to a state of lower energy, they release energy. So I'm just going to write energy kind of coming out of that, out of those arrows. And that energy is used for something. The enzymes in the inner mitochondrial membrane use that energy to do something. They use that energy to pump hydrogen ions out from the matrix and into the intermembrane space. If you recall, the intermembrane space is the space between the inner and outer mitochondrial membrane. So we're going to have, at the end of this process, a whole bunch of hydrogen ions in this intermembrane space. And that makes the intermembrane space more acidic, sorry, not, not out there, I, the outer membrane. Let's just, to make things clear, so. We're talking about the intermembrane space. So the intermembrane space now becomes acidic while the matrix becomes basic. And we know that, in general, molecules like to go from areas of high concentration to areas of low concentration, and this intermembrane space has so many hydrogen ions. And they just want to get back into the matrix. But, if you recall, we said that the inner membrane is not permeable even to the tiniest molecule, so the H+ ions cannot go through the inner membrane. There's only one way that they can get back into the matrix, and that is they can go through this special enzyme known as ATP synthase. ATP synthase has special channels in it that will allow H+ ions to pass through. So the H+ ions will pass through these special channels in ATP synthase, and when they do, they are going to cause this axle to turn. Let's focus now on the bottom part of ATP synthase. It has, this part of the protein has ADPs. adenosine diphosphates and Ps, or phospates. There are a lot of them. I'm just going to draw one of each. And when the axle spins, as the H+ ions go through, it's going to cause the ADPs and Ps to kind of knock into each other and attach. So ADP will attach to P, and we're finally going to produce ATP, the molecule that we're trying to get to this entire time. Let's just mention two terms that are relevant here. The first is chemiosmosis. Chemiosmosis refers to the hydrogen ions passing through the special channels in ATP synthase and then spinning the axle and making ATP. And another term you should know is oxidative phosphorylation. Oxidative phosphorylation, well, phosphorylation tells you something's being phosphorylated, so we're referring to ADP being phosphorylated or adding a P to it, and we're making ATP. And the term oxidative tells us that the phosphorylation is happening because of oxidation, because of the oxidation of NADH, and subsequently, the oxidation of all these enzymes. Let's just recap everything that happened here. We had NADH and FADH2 which were produced during the Krebs Cycle. They got oxidized. They lost electrons. Those electrons went on to the first enzyme, and from there on, they went from enzyme to enzyme to enzyme. And when those electrons went from enzyme to enzyme, they went from a state of higher energy to a state of lower energy. When electrons do that, they release energy. That energy was used to pump hydrogen ions from the matrix into the intermembrane space, so we have a bunch of these hydrogen ions in the intermembrane space. These hydrogen ions want to get back because there's a high concentration in the intermembrane space and low concentration in the matrix. But, as we explained, the inner membrane is not permeable to H+ ions, so the only way for them to get back is to go through ATP synthase, through the special channels in ATP synthase. When they go through, they spin the axle. That causes this part of the protein to knock ADP and P together. And that, finally, produces ATP. And that ATP then provides the cell with the energy that it needs. There's one more topic about mitochondria that I'd like to discuss. And that is that mitochondria have their own genome. So they have one piece of circular DNA. It's a lot smaller than the amount of DNA that's found in the nucleus, but it allows them to do a lot of things on their own. Mitochondria are also self-replicating, so they can replicate independently of the cell in which they are. And because they have their own genome, they're able to make their own ribosomal RNA, tRNA, that's transfer RNA. They actually make some of the proteins involved in the electron transport chain. I'm just going to abbreviate that ETC, so ETC stands for electron transport chain. And they also produce parts of the protein ATP synthase. That's a rather complex protein, and they do produce some parts of it. However, most of the proteins in the mitochondria are actually encoded for by the nuclear genome. The mitochondria even uses a different system of transcription and translation. And when I say different, I mean different than the nuclear genes, and mitochondria even has its own unique genetic code. So mitochondria are relatively independent organelles.