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  • It may seem counterintuitive, but every day that you've barely been able to get out

  • of bed and every time that you struggled to keep your eyes open while cramming for a test

  • at 1am, you've actually had a lot of energy.

  • I'm not talking about motivation or enthusiasm, I mean real, physical, energy.

  • We might be used to energy as a big picture concept, that having energy allows us to move

  • our bodies and do work.

  • That's totally true, but cells also need energy to move their bodies, manufacture new

  • proteins, and make chemical reactions happen.

  • And that's the focus of this episode, energy at the very tiny level.

  • Today, we're going to learn how we turn the molecules from food into usable energy.

  • What do we mean when we say that energy is important?

  • Well, some of our biological processes require energy to turn reactants into productschemistry

  • terms for the chemicals you start with and the chemicals you end with.

  • Our bodies have a few ways of doing this, namely extracting energy-rich molecules from

  • the food you eat and turning it into energy.

  • That's where a molecule called adenosine triphosphate, or ATP, comes in.

  • As the name implies, this molecule has three phosphates.

  • But it's the bonds between them that we're more interested in.

  • The chemical bonds that hold those phosphates together hold a lot of energy.

  • When one of those phosphates is broken off, that ATP becomes ADP, or adenosine diphosphate

  • plus one loner phosphate.

  • That transformation of ATP to ADP results in usable energy that our cells can use to

  • power our biological processes. So that begs the questionwhere does ATP come from?

  • Well you see, when an adenosine and a triphosphate fall in love, they

  • I'm kidding!

  • Our bodies' main way of making ATP involves using carbohydrates, especially a simple but

  • important molecule called glucose.

  • Try not to think of carbohydrates as mini tortillas floating around in your cells, but

  • as what they really are: molecules of carbon, hydrogen, and oxygen, hence carbo-hydrate.

  • The first thing we do is put that glucose molecule through the process of glycolysis.

  • Glyco for sugar, and lysis because we're breaking it apart.

  • That glucose molecule gets broken down into another molecule called pyruvate, plus another

  • molecule that becomes useful a little later on.

  • Glycolysis actually takes a little bit of ATP to happen, but it ends up netting us two

  • ATP molecules, which is awesome, we will definitely use those two ATP.

  • This entire reaction happens without oxygen, it's anaerobic.

  • But our bodies can extract way more ATP if they use an additional aerobic pathway, meaning

  • they do use oxygen.

  • Some simple organisms, like the bacteria that causes botulism can fuel their entire existence

  • off of anaerobic pathways like glycolysis.

  • But in our bodies, glycolysis is just the first step to cranking out a lot of ATP.

  • By the end of glycolysis, we have two pyruvates, two ATP molecules and two molecules of NADH,

  • a molecule that we can't extract energy from but we can repurpose as ingredients in

  • a different reaction.

  • Now, glycolysis happens in the cytosol, the liquid within your cells.

  • But like we learned in the last episode, we've got a secret weapon, a BEHEMOTH of energy

  • production.

  • We have mitochondria.

  • I know!

  • We spent so much of the last episode talking about mitochondrial DNA and endosymbiosis

  • and today we finally get to talk about its powerhouse-ness.

  • I'm excited too, okay, let's go.

  • So after glycolysis, we shuttle that pyruvate and those NADH towards the mitochondria, where

  • we're going to make even more ATP.

  • After all is said and done we're going to end up with more than thirty ATP.

  • Our cells like to work smarter, not harder, so they use molecules called enzymes.

  • Enzymes are chemicals that lower the amount of energy required for that reaction to happen.

  • I like to think of enzymes as coupons for chemistry.

  • You get the same product in the end, but instead of spending a lot of energy, you apply an

  • enzyme, and you get the same product for a lot less energy.

  • And this next process, the Krebs Cycle, has multiple enzymes helping it along.

  • Wait!

  • Don't click away yet!

  • Look, I get it.

  • I've had to memorize this thing three separate times throughout my schooling because I kept

  • on forgetting it.

  • That's the real Kreb's Cycle.

  • You learn the Kreb's cycle, then forget the Kreb's cycle, so you learn the Kreb's

  • Cycle and then you're spiraling forever in the nerdiest episode of Black Mirror ever

  • written.

  • But once you see the big picture of this cycle, you'll come to appreciate it as I have.

  • Remember, the purpose of all this is to make ATP, and we can make a lot of ATP if we can

  • use oxygen.

  • But we need to prep our materials in such a way that lets us use oxygen.

  • Pyruvate itself has three carbon atoms.

  • Along comes a chemical that bumps one of them off, turning it into a molecule with two carbon

  • atoms.

  • This new product is called Acetyl CoA and it's a big deal in the Krebs Cycle.

  • Next, we add a four-carbon molecule to Acetyl CoA to make a molecule with six carbons called

  • citrate.

  • The Krebs cycle is also called the citric acid cycle because of this molecule.

  • So by this point, we have a modest four ATP — 2 from glycolysis, and two more from the

  • Krebs Cycle.

  • But the more interesting products are the ten molecules of NADH and the newly created

  • two molecules of FADH2.

  • These things are really gonna pay off in the next step.

  • Manipulating these leftover ingredients will get us a lot of ATP.

  • It's a process called oxidative phosphorylation.

  • Again, big science words, but it means exactly what it says.

  • We're going to shuffle around phosphate and use oxygen.

  • In order to contain and process that energy, the NADH and FADH2 molecules transfer their

  • electrons along a series of steps called the electron transport chain.

  • They present their electrons to the mitochondria's inner membrane where electron transporters

  • move them towards the inside of the mitochondria.

  • This process releases some energy, which sets up a smooth gradient of Hydrogen ions across

  • the mitochondrial membrane.

  • This gradient can be used to power ATP synthase, an enzyme that helps put a phosphate on ADP,

  • turning it into ATP.

  • After you tally everything up, you realize how much energy you generated.

  • You gained two ATP directly from glycolysis and two more from the Krebs cycle.

  • Each of those ten NADH molecules can get us up to 3 ATP, and each of the two FADH2 can

  • yield two ATP.

  • That's a total of 38 molecules of ATP for every molecule of glucose under prime conditions.

  • You also ended up with water and carbon dioxide as byproducts.

  • How cool is that?!

  • We went from 2 ATP per molecule of glucose to 38 by adding oxygen and a few enzymes.

  • Now, we can extract ATP from a few other molecules.

  • Glucose itself is a simple carbohydrate, but we can also utilize more complex carbohydrates

  • by breaking them into simpler versions.

  • Then they go through a similar cycle to beforeglycolysis, Kreb's cycle, and oxidative

  • phosphorylation.

  • Of course, it would be great if we just always had a batch of glucose on hand to use whenever

  • we needed it.

  • Like, we always have some dissolved glucose in our blood that our cells can use, but it's

  • constantly increasing or decreasing depending on things like food, exercise, time of day,

  • or whether or not you have diabetes.

  • And low blood glucose can get dangerous, especially if your brain doesn't have enough glucose

  • since that's the only fuel source it can use.

  • Well, unless you're starving.

  • As a workaround, our bodies convert spare glucose into an easily useable storage form

  • of glucose called glycogen.

  • This starchy substance is kept mostly in the liver and skeletal muscle, and can be tapped

  • whenever your body needs some quick energy.

  • Your liver can sense when overall blood glucose is low and chip off some glycogen to get used

  • as fuel, and your muscles can use glycogen if they need more energy during exercise.

  • Now, stored fat is a more energy-dense molecule than glucose.

  • However, fat is more than just energy storage, it's a whole organ unto itself.

  • But for now, we'll focus on how we extract so much ATP from it.

  • When our bodies want to use some of our stored fat, it first needs to break it into fatty

  • acids and glycerol.

  • By the end of its processing, we end up with some familiar ingredientsacetyl CoA,

  • NADH, and FADH2.

  • Then that acetyl CoA goes into the Kreb's Cycle, just like if it came from a glucose

  • molecule.

  • However, what makes using a molecule of fat different than a molecule of glucose is the

  • total ATP payoff.

  • For a typical molecule of fat with sixteen carbons, we'll end up with twenty one molecules

  • of ATP from NADH, fourteen from FADH2, and ninety six from Acetyl CoA for a whopping

  • total of a hundred and thirty one molecules of ATP.

  • Although, caveat here, this isn't a concrete number since fat comes in multiple carbon

  • combinations.

  • But either way, it's a lot of ATP.

  • Now, molecules of fat are bigger than molecules of glucose, but a gram of fat still nets you

  • two and a half times more ATP than a gram of glucose.

  • That's what makes it such a great way of storing energy.

  • But that's only one of the amazing functions of our fat.

  • In the next episode, we're gonna talk about fat as an organ and learn about all the other

  • things it does aside from storing energy.

  • Thanks for watching this episode of Seeker Human, I'm Patrick Kelly.

It may seem counterintuitive, but every day that you've barely been able to get out

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你的身體是如何將食物轉化為能量的? (This Is How Your Body Turns Food Into Energy)

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    Summer 發佈於 2021 年 01 月 14 日
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