Video Transcript: ATP & Respiration

ATP & Respiration Crash Course Biology #7

Oh, hello there. I'm at the gym. I don't know why you're here. But I'm going to do some pushups. So you can join me on the floor if you want. No, I'm not doing this just to show off or anything. I'm actually doing this for science, okay? You see what happened there, my arms moved, my shoulders moved my back and leg muscles moved, my heart pumped blood to all those different places. It is pretty neat.

Well, it turns out that how we make and use energy is a lot like sports or other kinds of exercise: it can be hard work and a little bit complicated. But if you do it right, it comes with a tremendous payoff. But unlike hitting a ball with a stick, it's so marvelously complicated and awesome that we're still unraveling the mysteries of how it all works. And it all starts with a marvelous molecule that is one of your best friends, ATP.

Today, I'm talking about the energy in the process our cells and other animal cells go through to provide themselves with power. Cellular respiration is how we derive energy from the food that we eat, specifically from glucose, since most of what we eat ends up as glucose. Here's the chemical formula for one molecule of glucose. In order to turn this glucose into energy, we're going to need to add some oxygen, six molecules of it to be exact through cellular respiration, we're going to turn that glucose and oxygen into six molecules of CO₂, six molecules of water, and some energy that we can use for doing all of our pushups. So that's all well and good. But here's the thing, we can't just use that energy to run a marathon or something. First, our bodies have to turn that energy into a really specific form of stored energy called ATP, or adenosine triphosphate. You've heard me talk about this before, people often refer to ATP as the currency of biological energy. Think of it as an American dollar. It's what you need to do business in the US, you can't just walk into a Best Buy with a handful of Chinese yen or Indian rupees and expect to be able to buy anything with them even though they're technically money. Same goes with energy. In order to be able to use it ourselves need energy to be transferred into adenosine triphosphate to be able to grow, move, create electrical impulses in our nerves and brains. Everything well back for instance, we talked about how cells use ATP to transport some kinds of materials in and out of its membranes to documentary about that, you can watch that episode right here.

Now before we see how ATP is actually put together, let's look at how cells can cash in on the energy that stashed in there. Well, adenosine triphosphate is made up of a nitrogenous base called adenine with a sugar called ribose, and three phosphate groups attached to it. So one thing you need to know about these three phosphate groups is that they are super uncomfortable sitting together in a row like that, like three kids on a bus who hate each other all sharing the same seat. So because the phosphate groups are such a terrible company for each other ATP is able to do this nifty trick where it shoots one of the phosphate groups off the end of the seat creating ADP or adenosine diphosphate. Because now there are just two kids sitting on the bus seat. And this reaction when the third jerk kid is kicked off the seat energy is released. And since there are a lot of water molecules just floating around nearby, an O H pairing that's called a hydroxide from one of the H₂O's comes over and takes the place of that third phosphate group and everybody is much happier. By the way when you use water to break down a compound like this, it's called hydrolysis hydro from water and lysis from the Greek word for separate.

So now that you know how ATP is spent, let's see how it is minted nice and new by cellular respiration. Like I said, it all starts with oxygen and glucose. In fact, textbooks make a point of saying that through cellular respiration, one molecule of glucose can yield a bit of heat and 38 molecules of ATP. Now it's worth noting that this number is kind of a best-case scenario. Usually it's more like 29, or 30, ATP, but whatever, people are still studying this stuff. So let's stick with that number 38. Now cellular respiration isn't something that just happens all at once glucose is transformed into ATP is over three separate stages glycolysis, the Krebs cycle and the electron transport chain. Traditionally, these stages are described as coming one after the other, but really everything in the cell it's kind of happening all at the same time. But let's start with the first step of glycolysis or the breaking down of the glucose. Glucose of course is a sugar you know this because it's got an "ose" at the end of it and I call this this is just the breaking up of glucose is six carbon ring and to two three carbon molecules called pyruvic acids or pyruvate molecules. Now, in order to explain exactly how glycolysis works, I need about an hour of your time a giant cast of finger puppets each playing a different enzyme and though it would pay me to do it I would have to use words like phosphogluco isomerase.

But a simple way of explaining it is like this if you want to make some money, you got to spend some money. Glycolysis needs the investment of two ATP is in order to work and in the end it generates four ATP is for a net profit if you will have two ATP in addition to those four ATP is glycolysis results in two pyruvate and two super energy rich morsels called NADH, which are sort of the love children of a B vitamin called NAD⁺ pairing with energized electrons and a hydrogen to create storehouses of energy that will later be tapped to make ATP though, but keep track of all the awesome stuff we're making here.

Let's, let's keep score. So far, we've created two molecules of ATP and two molecules of NADH, which will be used to power more ATP production later. Now worried about oxygen. Like I mentioned, oxygen is necessary for the overall process of cellular respiration, but not every stage of it. Glycolysis, for example, can take place without oxygen, which makes it an anaerobic process. In the absence of oxygen, the pyruvate is formed through glycolysis gets rerouted into a process called fermentation. If there's no oxygen in the cell, it needs more of that NAD⁺ to keep the glycolysis going. So fermentation frees up some NAD⁺ , which happens to create some interesting byproducts, for instance, and some organisms like yeasts, the product of fermentation is ethyl alcohol, which is the same thing as all of this lovely stuff. But luckily, for our day-to-day productivity, our muscles don't make alcohol when they don't get enough oxygen, if that were the case, working out would make us drunk, which actually would be pretty awesome. But instead of ethyl alcohol, they make lactic acid, which is what makes you feel sore after that workout that kicked your body. So your muscles used to buy the oxygen they had. And they had to kick into anaerobic respiration in order to get the energy that they needed. So you have all this lactic acid building up in your muscle tissues.

Back to the score. Now we have made two molecules of ATP through glycolysis let yourselves really need the oxygen in order to make the other 30 some molecules that they need. And that is because the next two stages of cellular respiration, the Krebs cycle, and the electron transport chain are both aerobic processes, which means that they require oxygen and so we find ourselves at the next step in cellular respiration. After glycolysis comes the Krebs cycle, so well glycolysis occurs in the cytoplasm, where the fluid medium within the cell that all the organelles hang out in the Krebs cycle happens across the inner membrane of the mitochondria, which are generally considered the power centers of the cell. The Krebs cycle takes the products of glycolysis, those carbon rich pyruvate and reworks them to create another two ATPs per glucose molecule, plus some energy and a couple of other forms, which we'll talk about in a minute. Here's how first one of the pyruvate is oxidized, which basically means that it's combined with oxygen to the carbons off the three carbon chain bonds with an oxygen molecule and leaves the cell as co2. What's left is a two-carbon compound called acetyl coenzyme A, or Acetyl Co A, then another NAD⁺ comes along, picks up a hydrogen and becomes NADH. So our two pyruvate is created another two molecules of NADH to be used late as in glycolysis. And really all life enzymes are essential. Here. They are the proteins that bring together the stuff that needs to react with each other. And they bring them together in just the right way. These enzymes, for example, bring together a phosphate with an ADP to create another ATP molecule for each pyruvate enzymes also helped join the Acetyl Co A and A four carbon molecule called oxaloacetic acid. I think that's how you pronounce it together and they form a six-carbon molecule called citric acid and I'm certain that that's how you pronounce that one because yeah, it's the stuff that's in orange juice.

Fun fact, the Krebs cycle is also known as the citric acid cycle, because of this very byproduct. However, it is usually referred to by the name of the man who figured it all out, Hans Krebs an ear, nose and throat surgeon who fled Nazi Germany to teach biochemistry at Cambridge, where he discovered this incredibly complex cycle in 1937. For being such a total freakin' genius he was awarded the Nobel Prize for Medicine in 1953. Anyway, the citric acid is then oxidized over a bunch of intricate steps, cutting carbons off left and right to eventually get back to oxaloacetic acid, which is what makes the Krebs cycle a cycle and as the carbons get cleaved off the citric acid there are leftovers in the form of CO₂ or carbon dioxide, which are exhaled by the cell. And eventually, by you, you and I as we continue our existence as people are exhaling the products of the Krebs cycle right now.

Good work. This video by the way, using a lot of ATP making, know which type of carbon comes off of the citric acid, some energy is made, but it's not ATP. It's stored in a whole different kind of molecular package. This is where we go back back to NAD⁺, and it's sort of colleague FAD, NAD⁺ and FAD⁺ are little enzymes that are related to B vitamins derivatives of niacin and riboflavin, which you might have seen in the vitamin aisle. These B vitamins are good at holding on to high energy electrons and keeping that energy until it can get released later and the electron transport chain in fact, they're so good at it that they show up in a lot in those high energy vitamin powders that the kids are taking these days and it is an FADs aren't like batteries, big awkward batteries that pick up hydrogen and energized electrons from each pyruvate which in effect charges them up the addition of hydrogen turns them into NADH and FADH  two respectively. It pyruvate yields three NADH is and one FADH two per cycle. And since each glucose has been broken down into two pyruvate that means that each glucose molecule can produce 60 NADH is and to FADH2 is the main purpose of the Krebs cycle is to make these powerhouses for the next and final step, the electron transport chain now go to the time when you're saying sweet pyruvate sandwiches Hank, aren't we supposed to be making ATP here? Let's make it happen captain. What's the holdup? Well, friends, your patience is finally paying off because when it comes to ATPs, the electron transport chain is the real moneymaker and a very efficient cell it can net a whopping 34 ATP. So remember, all those NADH is in FADH tubes that we made in the Krebs cycle? Well, their electrons are going to provide the energy that will work as a pump along a chain of channel proteins across the inner membrane of the mitochondria where the Krebs cycle occurred, these proteins will swap these electrons to send hydrogen protons from inside the very center of mitochondria across its inner membrane to the outer compartment of the mitochondria. But once they're out, the protons want to get back to the other side of the inner membrane because there's a lot of other protons out there. And as we've learned, nature always tends to seek a nice peaceful balance on either side of a membrane while these anxious protons are allowed back in through a special protein called ATP synthase. And the energy of this proton flow drives this crazy spinning mechanism that squeezes some ADP and some phosphates together to form ATP. So the electrons from the 10, NADH that come out of the Krebs cycle, have just enough energy to produce roughly three ATPs each, and we can't forget our friends, the FADH twos, we have two of them, and they make two ATPs each, and voila, that is how animal cells the world over make ATP through cellular respiration. Now just to check, let's reset our ATP counter and do the math for a single glucose molecule. Once again, we made two ATP is for each pyruvate during glycolysis. We made two during the Krebs cycle and then during the electron transport chain, we made about 34 and that is just for one molecule of glucose. Imagine how much your body makes and uses every single day.

Don't spend it all in one place. Now you can go back, watch any parts of this episode you didn't quite get. I really want to do this quickly because I'm getting very tired. If you want to ask us questions, you can see us in the YouTube comments below. Of course, you can connect with us on Facebook or Twitter.