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 products — chemistry
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 question — where 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 before — glycolysis, 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 ingredients — acetyl 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.