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Tissue specific metabolism and the metabolic states | Chemical Processes | MCAT | Khan Academy

October 14, 2019


– [Voiceover] When I look at this image, I’m not exactly sure what I see. Am I looking at a diagram that shows all the metabolic pathways
in the human body, or am I looking at a map of
the subway in New York City? Now, in order to describe
how the body balances the storing and utilization of energy through different nutrients
in all the tissues of the body, I’m gonna just go ahead and get rid of this diagram. And I’m gonna bring up one
that’s a little bit more simple. Now, this isn’t exactly an
anatomically correct diagram, but it’ll work to describe
how these different tissues function metabolically. First, let me bring in some
metabolic intermediates. And, I don’t want you to get overwhelmed by all of these intermediates, as there are quite a few of them, ’cause we’re gonna be going
through each one of them individually in order to make
sense of this entire diagram. Now the first metabolic
state I’m gonna talk about is the Absorptive State. Well, the Absorptive State is a series of metabolic reactions that your body does when food is in plenty, you
have food in your intestines, and you’re absorbing that food, and so you don’t need to
utilize all this energy. Instead, you want to store
it, so you can use it later. So what is this series
of metabolic reactions look like in the Liver? So imagine the Carbohydrates
are broken down in the intestines, and
you’re absorbing the di- and monosaccharides like glucose. So glucose is then
transported to the Liver, and it can have one or two, kinda go in one or two pathways. The first pathway is that it
can be stored as glycogen. I circled and filled in
glycogen here to remind you that this is the storage
form of Carbohydrates. The other way glucose can
go, is to be converted into Triglycerides, which
are the storage form of Fat. Now in order to do so,
the glucose must first be converted into glycerol,
as well as Fatty Acids. And the process of converting
glucose into Fatty Acids involves going through
Pyruvate and Acetyl CoA, to form the Fatty Acid known as Palmitate, which is the only Fatty Acid that the body can synthesize on its own. And then these glycerol and Fatty Acids can be combined to form Triglycerides. Now in the Liver, we
don’t store Triglycerides. It’s stored mostly in Adipose tissues, and Triglycerides can’t be
transported in the blood. So these Triglycerides
have to be converted into very low-density
lipoprotein, or VLDL, which can then be exported out of the Liver, and into the blood. Next we have the Amino Acids
that are contained in Protein, so the meat we ate in our
cheeseburger is broken down into Proteins that are then broken down to their component Amino Acids, and these Amino Acids are
taken up by the Liver. The Liver will break them
down into Keto Acids. And these Alpha-keto Acids,
when we break them down they’re gonna give off ammonia, which is then excreted as
waste, in the form of Urea. So these, the energy that
was in the Amino Acids is now in our Alpha-keto Acids. Now where can this go? And it can also go one of two ways. If we want to store the energy
that’s in these Keto Acids, we’re gonna convert them to Fatty Acids, and those Fatty Acids
can then be converted into Tryglycerides, and
those Tryglycerides would be once again exported into
the blood, and then later, we’ll store them in the Adipose tissue. But if the Liver needs energy
during this Absorptive State, these Keto Acids can also be
broken down into Acetyl CoA, and that Acetyl CoA can
go through the kreb cycle and electron transport chain, to produce ATP, or usable energy. Now let’s move on to Adipose tissue. So Adipose is Fat, and Fat
can kinda be thought of as just a vast, a warehouse
of energy storage. So when we eat, the
carbohydrates are broken down, and the glucose is
transported in our blood to Adipose tissue, and it’s taken up. And in the Absorptive State, this glucose in Adipose tissue is gonna
be turned into Triglycerides, but first, similar to the Liver, it needs to be converted to glycerol, and Fatty Acids, and then
the glycerol and Fatty Acids are going to be converted
into Triglycerides. However, this isn’t the
only thing that’s happening in the Adipose tissue
during the Absorptive State, remember we exported those
very low-density lipoproteins from the Liver, well those
are now gonna come down through the blood, and they’re gonna enter the Adipose tissue and be
turned into Fatty Acids, and these Fatty Acids can then be combined with glycerol to form Triglycerides. And similar to above where I
circled and filled in glycogen, Triglycerides are now
circled and filled in because this is now the
storage form of our Fats, and it’s stored in the Adipose tissue. So let’s move on to the Muscle. What’s going on in the Muscle? Well, once again, just like in the Liver and in Adipose tissue, glucose is taken up by the Muscle cell during
the Absorptive State, and glucose can now have
one of two pathways. Similar to the Liver, we can
convert it to it’s storage form in glycogen, but also, the
Muscle may need energy, even though, in general
our body is storing energy in our storage forms, it sometimes, during the Absorptive State,
it still needs to utilize some of this energy that we’ve eaten, so that glucose can be
converted to Pyruvate, and through cellular
respiration, we can create ATP, and the Muscle will have usable energy. And in the Absorptive
State, Muscle also takes up the Amino Acids from Proteins that we eat, and these Amino Acids are just stored as Protein in our Muscle. Now let’s talk about the Brain. So, in the Liver, Fat and Muscle cells, the primary actions of
the metabolic pathways during the Absorptive
State was to store energy. To store energy as glycogen,
Triglycerides, and Protein. But in the Brain, we don’t store energy, the brain consumes a
vast amount of energy. So during this Absorptive
State, even though generally, the body is tending to store energy, the Brain is just gonna use it. So the glucose from the blood is taken up, and it’s converted to Pyruvate, and just like in the
Muscle we talked about, it goes through cellular
respiration to produce ATP, so that the Brain can continue to do work. So as you look at all of
these metabolic pathways during the Absorptive
State, it makes sense that, when the body has abundant energy, because it’s just eaten that
we’re gonna store that energy. But what happens when we
haven’t eaten in a while? When our body needs to take
the energy that’s stored in glycogen and Triglycerides
and Protein, and use it? Well, that’s what happens in
the Post Absorptive State. So, for a moment here
I’m gonna just remove the Absorptive State from our diagram. So now our body has absorbed
all of its nutrients from the gut, see there’s no longer any Fat, Protein or Carbohydrates here, and we’ve stored it in its
respective storage forms, in the Liver we’ve stored
Carbohydrates as glycogen, and in the Fat tissue we’ve
stored Fats as Triglycerides, and in the Muscle we’ve stored Protein. So say a little time has gone by, and now the body needs
to utilize its energy, but it can’t just use Protein
or glycogen or Triglycerides, it has to first break those nutrients down into more usable forms of energy. And the series of metabolic
reactions that occur are known as the Post Absorptive State. In general, during the Absorptive State, we’re building up and storing energy. In the Post Absorptive
State, we’re gonna be breaking down that, those molecules and releasing the energy. So, in general a lot of these reactions are just going to be going
in the opposite direction, but not all of them, so
let’s go through them here. And once again, we’re
gonna start in the Liver. Now, in the Liver, glycogen
is going to be broken down into glucose, and glucose
can then be exported out of the Liver into the blood, and be used for energy all over the body. Now, the Amino Acids are
actually still going to be taken up by the Liver. And just like during the Absorptive State, the Amino Acids are gonna be converted to Keto Acids and give off ammonia, which will produce Urea
as a waste product. Now remember in the Absorptive State, the Keto Acids were
turned into Fatty Acids to make Triglycerides. Well now in the Post Absorptive State, we’re gonna use these
directly to make glucose, and that glucose can
be exported to be used by other parts of the body. But the Liver still
requires energy of its own, so in part, these Keto
Acids are still gonna be broken down into Acetyl
CoA, to produce ATP. So what about glycerol and Fatty Acids? Remember in the Absorptive State, we were combining them into Triglycerides. But this is a storage nutrient, and in the Post Absorptive State we want to create more usable energy. So these are actually
going to be also converted into glucose. Now there’s one other important step in the Liver that’s happening. Now these Fatty Acids can also undergo a second reaction, they can be broken down to form Ketones. Now, Ketones are an important
metabolic intermediate, because they’re one of the
very few types of energy that can be used by the Brain. Now that we have a good
idea of what’s going on in the Liver in the Absorptive State, what’s going on in Adipose? Well I mentioned earlier that Adipose is the storehouse of
energy, and that energy is stored in the form of Triglycerides. Now, in the Post-Absorptive State, we want to use this energy, but we can’t just use a Triglyceride,
we have to break it down. So the Triglycerides are
broken down, into glycerol, and Fatty Acids, and
they can just be exported into the blood and are
brought to the Liver to help make more glucose. Now, let’s move on to Muscle. In Muscle, we have
Proteins, and the Protein is gonna be broken down
into its Amino Acids. And these Amino Acids
are gonna be exported into the blood, and just like the glycerol and the Fatty Acids in Adipose tissue, these Amino Acids are
also gonna make their way to the Liver, so that
they can be converted to Alpha-keto Acids, and then
converted into glucose, which is a more usable form of energy. But Muscles also contain some glycogen. And the glycogen is stored in Muscle, so that they’ll have a little bit more direct access to energy. So in the Post Absorptive
State, this glycogen is gonna be converted into glucose, and that glucose can be
converted into Pyruvate, which can then go on to create Acetyl CoA, and produce energy through
the electron transport chain. And this is all happening
within the Muscle cell. But, I don’t have it drawn in here, but these series of reactions right here, in order to produce this ATP for energy, it requires oxygen. But what happens if the
Muscle cell is low on oxygen? Well luckily glucose
can also be broken down into another product, and that’s lactate. And when it’s broken it down into lactate, we produce ATP, but we
don’t need the oxygen like we do during cellular respiration. Now you may be asking,
“Well if you don’t need “the oxygen, and you
can still get the ATP, “why doesn’t glucose always be broken down “into lactate and produce ATP?” And there’s two reasons for this. This process here isn’t as efficient, it doesn’t produce as
many molecules of ATP as would be produced if we went through cellular respiration. And also, this lactate is an acid, and it disturbs the
pH-balance of the blood. So lastly, let’s move back to the Brain. Now I said in the, during
the Absorptive State, that the Brain just always uses energy. And that’s true. Once again, we’re just taking,
the Brain is taking glucose from the blood, converting
it into Pyruvate and going through cellular
respiration to produce ATP. But also remember, the
ketones from the Liver are also used by the Brain for energy in the Post Absorptive State. Now that we’ve gone through
the Post Absorptive State and you see how the different
tissues are breaking down and using energy, let’s just bring back in the absorptive metabolic pathways, so you can get an idea of
all the different types of metabolic reactions that are going on depending on whether your body
is trying to store nutrients or utilize the nutrients.

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3 Comments

  • Reply Piotr Kaminski October 20, 2015 at 2:03 pm

    I don't like this video, has been it peer revived?
    In absorptive state, you skipped part when adipose tissue pick up FAs from blood (from chylomicrons) and put FA synthesis in it? You missed main point!
    In post absorptive state, you totally skipped ATP generation from FAs! Lipolisys->Beta Oxidation->Acetyl-CoA. In muscles this pathway is dominant over carbs metabolism when you are in low VO2max.

  • Reply spontaneousgrits June 1, 2017 at 9:53 am

    in what world are fatty acids converted into glucose? wtf god attempt though with the pictures

  • Reply Sarah A May 20, 2018 at 12:23 am

    Can someone please edit this video for possible mistakes? Fatty acids cannot become glucose, until they're the odd-numbered chain ones that go through beta oxidation.

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