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  • 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.

  • Now, I'm not doing this to show off or anything. I'm actually doing this for science.

  • [pained grunt]

  • You see what happened there?

  • My arms moved, my shoulders moved, my back and stomach muscles moved, my heart pumped

  • blood to all those different places. Pretty neat, huh?

  • 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 can come with some

  • tremendous payoffs.

  • 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 you best friends: ATP.

  • Today I'm talking about energy and 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 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 [C6H12O6]. 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 6 molecules

  • of CO2, 6 molecules of water and some energy that we can use for doing all our push ups.

  • 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 U.S. You

  • can't just walk into Best Buy with a handful of Chinese yen or Indian rupees and expect

  • to be able to buy anything with them, even though they are technically money. Same goes

  • with energy: In order to be able to use it, our cells need energy to be transferred into

  • adenosine triphosphate to be able to grow, move, create electrical impulses in our nerves

  • and brains. Everything. A while back, for instance, we talked about how cells use ATP

  • to transport some kinds of materials in and out of its membranes; to jog your memory about

  • that you can watch it right here.

  • Now before we see how ATP is put together, let's look at how cells cash in on the energy

  • that's stashed in there.

  • Well, adenosine triphosphate is made up of an nitrogenous base called adenine with a

  • sugar called ribose and three phosphate groups attached to it:

  • Now one thing you need to know about these 3 phosphate groups is that they are super

  • uncomfortable sitting together in a row like that -- like 3 kids on the bus who hate each

  • other all sharing the same seat.

  • So, because the phosphate groups are such terrible company for each other, ATP is able

  • to do this this nifty trick where it shoots one of the phosphates groups off the end of

  • the seat, creating ADP, or adenosine diphosphate (because now there are just two kids sitting

  • on the bus seat). In 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 OH pairing

  • -- that's called a hydroxide -- from some H2O 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 for water and lysis, from the Greek word for "separate."

  • So now that you know how ATP is spent, let's see how it's 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 29-30 ATPs, but whatever -- people are still studying

  • this stuff, so let's stick with that 38 number.

  • Now cellular respiration isn't something that just happens all at once -- glucose is

  • transformed into ATPs over 3 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 a cell is kinda happening all at the same time.

  • But let's start with the first step: 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 glycolysis is just the breaking up of glucose's 6 carbon ring into two 3-carbon

  • molecules called pyruvic acids or pyruvate molecules.

  • Now in order to explain how exactly glycolysis works, I'd need about an hour of your time,

  • and a giant cast of finger puppets each playing a different enzyme, and though it would pain

  • me to do it, I'd have to use words like phosphoglucoisomerase.

  • But one simple way of explaining it is this: If you wanna make money, you gotta spend money.

  • Glycolysis needs the investment of 2 ATPs in order to work, and in the end it generates

  • 4 ATPs, for a net profit, if you will, of 2 ATPs.

  • In addition to those 4 ATPs, glycolysis also results in 2 pyruvates and 2 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.

  • To help us keep track of all of the awesome stuff we're making here, let's keep score?

  • So far we've created 2 molecules of ATP and 2 molecules of NADH, which will be used

  • to power more ATP production later.

  • Now, a word 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 pyruvates formed through glycolysis get rerouted into a process

  • called fermentation. If there's no oxygen in the cell, it needs more of that NAD+ to

  • keep the glycolysis process going. So fermentation frees up some NAD+, which happens to create

  • some interesting by products.

  • For instance, in 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 butt.

  • So, your muscles used up all the oxygen they had, and they had to kick into anaerobic respiration

  • in order to get the energy that they needed, and so you have all this lactic acid building

  • up in your muscle tissue.

  • Back to the score. Now we've made 2 molecules of ATP through glycolysis, but your cells

  • really need the oxygen in order to make the other 30-some molecules they need.

  • That's because the next two stages of cellular respiration -- the Krebs Cycle and the electron transport

  • chain, are both aerobic processes, which means they require oxygen.

  • And so we find ourselves at the next step in cellular respiration after glycolosis:

  • the Krebs Cycle.

  • So, while glycolysis occurs in the cytoplasm, or 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 pyruvates -- and reworks them to create another 2 ATPs

  • per glucose molecule, plus some energy in a couple of other forms, which I'll talk

  • about in a minute. Here's how:

  • First, one of the pyruvates is oxidized, which basically means it's combined with oxygen.

  • One of 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

  • coA. Then, another NAD+ comes along, picks up a hydrogen and becomes NADH. So our two

  • pyruvates create another 2 molecules of NADH to be used later.

  • As in glycolysis, and really all life, enzymes are essential here; they're proteins that

  • bring together the stuff that needs to react with each other, and they bring it together

  • in just the right way. These enzymes bring together a phosphate with ADP, to create another

  • ATP molecule for each pyruvate. Enzymes also help join the acetyl coA and a 4-carbon molecule

  • called oxaloacetic acid.

  • I think that's how you pronounce it.

  • Together they form a 6-carbon molecule called citric acid, and I'm certain that's how you

  • pronounce that one because that'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. But it's 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

  • freaking genius, he was awarded the Nobel Prize in 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, I'm using a lot of ATPs making it.

  • Now, each time a carbon comes off 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 to NAD+ and its sort of colleague FAD.

  • NAD+ and FAD are both chummy 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 in the electron transport chain. In fact, they're so good at it that they show

  • up in a lot of those high energy-vitamin powders the kids are taking these days.

  • NAD+s and FADs are 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 FADH2, respectively.

  • Each pyruvate yeilds 3 NADHs and 1 FADH2 per cycle, and since each glucose has been broken

  • down into two pyruvates, that means each glucose molecule can produce 6 NADHs and 2 FADH2s.

  • The main purpose of the Krebs Cycle is to make these powerhouses for the next and final

  • step, the Electron Transport Chain.

  • And now's the time when you're saying, "Sweet pyruvate sandwiches, Hank, aren't we supposed

  • to be making ATP? Let's make it happen, Capt'n! What's the holdup?"

  • Well friends, your patience has paid off, because when it comes to ATPs, the electron

  • transport chain is the real moneymaker. In a very efficient cell, it can net a whopping

  • 34 ATPs.

  • So, remember all those NADHs and FADH2s 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 the

  • 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. So all of 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 NADHs that came

  • out of the Krebs Cycle have just enough energy to produce roughly 3 ATPs each.

  • And we can't forget our friends the FADH2s. We have two of them and they make 2 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 2 ATPs for each pyruvate during glycolysis.

  • We made 2 in the Krebs Cycle.

  • And then during the electron transport chain we made about 34 in the electron transport chain.

  • And that's 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 and watch any parts of this episode

  • that you didn't quite get and I really want to do this quickly because I'm getting very tired.

  • [Suck it up, Hank]

  • If you want to ask us questions you can see us in the YouTube comments below and of course,

  • you can connect with us on Facebook or Twitter.

  • [manly grunt]

Oh, hello there. I'm at the gym. I don't know why you're here, but I'm going to do some

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B2 中高級

ATP和呼吸。生物學速成班#7 (ATP & Respiration: Crash Course Biology #7)

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    Chi-feng Liu 發佈於 2021 年 01 月 14 日
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