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ATP and respiration | Crash Course biology| Khan Academy

November 4, 2019

– Oh, hello there. Um, I’m at the gym, I
don’t know why you’re here, but I’m going to do some push-ups. You can join me on the floor if you want. I’m not doing this just
to show off or anything, I’m actually doing this for science, okay. (pained grunt) 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, it’s pretty neat, huh? Well, it turns out that
how we make and use energy is a lot like sports or
other kinds of exercise. Can be hard work and a
little bit complicated, but if you do it right, it comes 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 your best friends: ATP. (upbeat music) 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 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 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 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 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 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’s 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. Now 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 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 H2Os 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 ATPs, 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 ATPs 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 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 six carbon ring into two
three-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 like this. If you wanna make money,
you gotta spend money. Glycolysis needs the
investment of two ATPs in order to work, and in the
end it generates four ATPs, for a net profit, if
you will, of two ATPs. In addition to those four ATPs, glycolysis also results in two pyruvates 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. To help us keep track of
all of the awesome stuff we’re making here, 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, 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 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 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 tissues. (grunts) Back to the score. Now we’ve made two molecules
of ATP through glycolysis, but your cells really need
the oxygen in order to make the other 30-some
molecules that 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 that they require oxygen. And so we find ourselves at the next step in cellular respiration. After glycolosis comes 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 two
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 two molecules of NADH to be used later. As in glycolysis, and really all life, enzymes are essential here. They are 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, for example, bring together a phosphate with an ADP, to create another ATP
molecule for each pyruvate. Enzymes also help join the acetyl coA and a four-carbon molecule
called oxaloacetic acid. I think that’s how you pronounce it. Together they form a six-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. (jazzy piano music) 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 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. (inhales and exhales) This video, by the way, I’m
using a lot of ATP making it. Now, each time a 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 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 yields three
NADHs and one FADH2 per cycle, and since each glucose
has been broken down into two pyruvates, that means
that each glucose molecule can produce six NADHs and two 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 here? “Let’s make it happen, Capt’n! “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. In a very efficient cell, it
can net a whopping 34 ATPs. So, remember all those NADHs and FADH2s 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 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 come out of the Krebs Cycle have just enough energy to produce roughly three ATPs each. And we can’t forget our
friends, the FADH2s. 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 ATPs for each
pyruvate during glycolysis. We made two in the Krebs Cycle. And then during the
electron transport chain, we made about 34. And that’s just for one
molecule of glucose. Imagine how much your body
makes and uses every single day.

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