- [Instructor] Now, a very simple premise that we've built
our discussion of metabolism on,
is that we extract energy from food.
And of course, this comes from the fact that we know that
ATP is our body's main source of chemical energy,
and the way we can produce this ATP is by breaking down
nutrients, such as glucose, and fatty acids and proteins,
which are all found in food.
But of course, the question that you might wonder,
is while we don't constantly eat food, right?
So, how does our body produce a constant flow of ATP
even though we only have food very intermittently?
And of course the answer to that question is that
our body has evolved to store fuel as well,
so that we're not reliant on a immediate reflux of
nutrients right after a meal.
So in the remainder of the video, I actually wanna go ahead
and compare and contrast the three main types of fuel
that our body has evolved to be able to store.
And then, touch on why one of these fuels is actually
a much better storage fuel than the rest of them.
So let's start off with glycogen.
And glycogen is our body's way of storing carbohydrates.
And essentially, it's just a long chain, or a polymer,
of glucose molecules that are all attached to each other.
And our body stores this mostly in the liver.
But also there's some in our muscles, as well.
And, if we were to tally up how much glycogen,
how many grams of glycogen that we had, let's say,
in an average 70 kilogram male, an average healthy
70 kilogram male, we would calculate that there would be
approximately 480 grams of glycogen.
And, just to give you some idea of how much energy we can
extract from glycogen, we can extract approximately
four kilocalories of energy per gram or glycogen.
And kilocalories is just a unit of energy, and it's
something that you might see on cereal boxes, or any type
of food really, when you look at the nutrition label.
And just to give you some perspective, the recommended
average intake of energy for humans is somewhere around
2000 kilocalories per day, and of course that's a huge
ballpark number.
The exact number might fall above or below this,
and really depend on how old you are, what your sex is,
as well as how active you are during the day.
Now a second type of fuel that our body stores is proteins.
And, remember that proteins are nothing more than a long
chain of amino acids.
And, most of the protein in our body is in our muscles.
And again, in an average 70 kilogram man, if we were to
tally up how many grams of proteins, we would get around
6000 grams.
And again, similar to glycogen, we would be able to extract
about four kilocalories of energy per gram of protein.
Now, finally, the third type of fuel that our body stores
is in the form of fats.
And these fats are actually stored up in specialized
tissue in our body called adipose tissue.
Now, if we were to tally up how many grams of adipose tissue
a 70 kilogram man had, it would actually amount to a much
higher amount than both proteins and glycogen combined.
In fact, it's somewhere around 12,000 grams of fat in an
average 70 kilogram healthy male.
And, moreover, unlike glycogen and protein, we can actually
extract a lot more energy per gram of fat.
In fact, that comes out to be somewhere around nine
kilocalories per gram of fat.
Now to put this in perspective, let's kind of do a fun,
simple math problem here.
Let's assume that this 70 kilogram guy requires an intake
of 2000 kilocalories per day.
Now if this is the case, let's kind of ask ourselves a very
theoretical question, because of course it would be
unhealthy to starve oneself.
But if one didn't have a intake of food, how long would this
man be able to survive on each type of fuel?
So, if we round this to about 500 grams of glycogen,
times four kilocalories per gram, that would actually
amount to about 2000 kilocalories that we could extract
from glycogen, and so that would last him about a day,
right?
Now, let's move down to protein.
So, protein, so we have about 6000 grams in our body, times
4 kilocalories per gram, which amounts to about 24,000
kilocalories, divided by 2000, that would last us about 12
days, right?
So you're getting a little bit better.
But now let's actually take a look at fats.
We have substantially more amount of fats in the body than
proteins and glycogen, right?
So 12,000 times, let's round this up to ten, since we're
just approximating anyways, so if we multiply 12,000 times
ten, that's bout 120,000, right?
Divided by 2000, that's about 60 days that one could go,
theoretically of course, and probably may not even make it
that far, but, theoretically we could make it 60 days on
just our fat storage alone.
Now that's pretty impressive, I think.
So now the question I want to answer is, why has that
become the major source of storage fuel in our body?
And to do that, let's first actually remind ourselves
what the chemical structure of fats are.
So I've kind of, to save us some time, drawn out the
chemical structure of a triacylglyceride.
And I'll actually go ahead and write that out here,
so tri acyl glyceride.
And just a point of clarification, I am using the word
fat and triacylglyceride interchangeably.
And that's because fat is just kind of an everyday term
that we use to refer to the type of fat,
the triacylglyceride, that we store in our body.
And since we're talking about the chemical structure,
it probably makes sense in this case, to refer to it
from its chemical name, which is triacylglyceride.
And in fact, its chemical name tells us a lot about its
structure, so let's take a look.
So this tri acyl refers to these three acyl groups,
these acyl side chains.
So what is an acyl side chain?
So, you know, an acyl is just a reference to a type
of organic chemistry functional group.
So, there are some functional groups you might be familiar
with, like hydroxyl groups, or phosphate groups,
and in this case, the acyl group is anything that has
a carbon double bond oxygen attached to a long chain
of carbons and hydrogens.
And actually, I should say, the chain doesn't have to be
long, it just has to be some type of organic functional
group, but in this case, the chain happens to be very
very long.
And of course, I've kind of just gone ahead and drawn three
acyl groups that have come pretty much randomly to my mind,
because the idea here is that these chains can vary
immensely, depending on the type of triacylglyceride
in our body, depending on the type of fats that we ingest.
And so if some of them might have single bonds,
which we refer to as them being saturated with hydrogens,
and some of them might have double bonds, in which case
we refer to these triacylglycerides, or these side chains,
as being unsaturated.
So that's kind of just some nomenclature that you might see.
And then, finally, this glyceride refers to the backbone
of this molecule, these kind of three carbons that are
hanging out down here.
And, they also have a oxygen attached to them as well,
and of course they link with these acyl side chains
through this ester linkage, that I'm kind of highlighting
in green here.
So, that's the big picture of this molecule.
Now, the reason why I think it's important to be familiar
with this structure of a fat, when trying to understand
why fat is such a prominent type of storage fuel in
our body, is because now you can visually see where all
of that energy, where all of that nine kilocalories
per gram of energy, is coming from.
Because looking at this molecule, you can see the bulk of it
is formed by these long carbon hydrogen chains.
And, these carbon hydrogen chains are referred to as being
very high energy, because they have a lot of electrons
kind of stored up in these bonds.
And we know, if we remind ourselves, back to kind of our
general principle, is that if you have a reduced organic
molecule, like this, like we can see here, we are able to
extract energy by oxidizing it in kind of subsequent steps,
and this flow of electrons can be harnessed by something
like the electron transfer chain to allow us to produce ATP.
So that's kind of my first point here, and I'll scroll down
here, and I'll go ahead and write that,
which is that the triacylglyceride is a very
energy-rich molecule,
has lots of these carbon-hydrogen bonds,
that can be oxidized to produce ATP.
Now a second reason why these triacylglycerides are such
a good form of storage energy is because they are
relatively chemically inert.
So, what I mean by this, is that they're unlikely to react
with other things in the body.
And this is of course in contrast to things like glucose
and proteins, which are quite polar, they have many
polar functional groups, like hydroxyl groups,
for example, and they can react with a lot of things in the
aqueous environment of the body.
But triacylglycerides are very, remember, they're not
soluble in water, and just remind yourself,
if you've ever made salad dressing, it's so hard to mix
the oil and water together, right?
And so because this isn't going to dissolve in water,
and it has a lot of these carbon-hydrogen chains that are
considered not to be very reactive, it serves as a great
form of storage energy, because it won't randomly react,
or be wasted in side reactions.
A third reason why fats are good for energy storage is
because they have no large or prominent functional role
inside the body, in contrast to proteins, for example,
which are used to make enzymes.
And enzymes, are of course, of paramount importance
in our body, and so, we wouldn't want to rely too heavily
on proteins, because it would kind of be a conflict
of interest for our body, right?
Because if we use up too many proteins, we wouldn't
be able to make enzymes, and that's why fats end up being
kind of a good compromise, because we're able to kind of
essentially just store up fats for one major purpose alone,
which is to produce energy.
Of course, that's a blanket statement, fats are important
in other ways as well, but largely speaking, their main role
is to store energy.
So I've touched on why triacylglycerides are a good form
of energy storage and why proteins might not be such a good
form of energy storage.
But you might be wondering, what about carbohydrates?
What about glycogen, why didn't our body evolve to make
glycogen the major storage fuel?
And that brings me to my fourth point, which is that
unlike proteins or carbohydrates, triacylglycerides fats
are very hydrophobic.
And, a benefit of being hydrophobic, as we kind of touched
on earlier, so not only does it make it more inert,
chemically, because it can't react with a lot of things
that water, in an aqueous environment, but it also means
that it won't be kind of weighed down by water.
So you might be wondering, why is that beneficial?
Why is that beneficial not to be weighed down by water?
And to answer that question we can actually just go ahead
and do kind of a quick math problem here,
to kind of give some insight into that.
So the math problem that I want to solve is,
how many grams,
of glycogen,
including, I'm gonna say this, including its water weight,
because we know that all of these polar molecules
attract some water, and that contributes, of course,
to how much weight that they have.
So how many grams of glycogen, including the water weight,
will our body need, let's say,
if we want to have the same number of calories,
so I'm gonna say, if we're equal to the same number
of calories, kilocalories, that,
the 12,000 grams of fat, in the average 70 kilogram
man that we talked about above, can produce.
So in order to answer this question, we have the
information in the table above.
But we also just need to know one more thing,
which is, how many grams of water are associated with
each gram of glycogen?
And the answer to that is, there's about three grams
of water weight, so let's say H2O weight,
associated with one gram of glycogen, or protein
actually, for that matter.
So I'll just put that in parentheses for our reference.
Alright, so to answer this question, we first need to
find out how many kilocalories can 12,000 grams of fat
produce?
And that's simply 12,000 grams of fat, right,
times, from the table above, I'll remind you, we said that,
for every gram of fat, we can burn about nine kilocalories
of energy, and because we're just trying to get a ballpark
number, I'm gonna go ahead and round that up to about
ten kilocalories, okay?
And that is going to be equal to 120,000 kilocalories.
So that's how many calories, kilocalories we need to
be able to produce with glycogen.
Now, with glycogen, we know that for every four
kilocalories, we're able to essentially utilize,
or for every four kilocalories, we're able to get that out
of one gram of glycogen, and that's without accounting
for its water weight.
But if we do account for its water weight, and we know
from this ratio above, right here, that for every gram of
glycogen, there is three grams of water weight.
I'll just put H2O here to remind us, okay?
So, our units are gonna cancel out here, and here,
and altogether, if we scroll down, we end up getting
120,000 divided by four, ends up being
30,000 times three, which ends up being a whopping
90,000 grams of glycogen that would weigh us down,
in order to produce the same number of calories
as only 12,000 grams of fats.
So nearly a seven to eight times fold difference in the
amount of weight that our bodies would have to carry.
And of course, this is not practical, right?
Because, we were talking about a 70 kilogram man,
and here we have 90,000 grams of glycogen.
That's equal to 90 kilograms, which is more than 100%
of body weight just in glycogen alone.
And so, that's not practical because we haven't evolved
a skeletal structure, or enough muscle mass, to be able to
handle that extra weight.
And so that's probably why fats has evolved to be the most
prominent type of energy storage molecule in our body.
So just to kind of summarize, we return to our list
up here, we notice that not only it's that itself,
just by its structure, energy-rich,
it's also chemically inert, it plays no functional role,
unlike proteins which are important in enzymatic function,
and because fats are very hydrophobic, we're able to pack
a lot in our body without carrying any extra water weight.