- [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.