Metabolism Video Lecture - NEET

Voiceover: Let's talk about
this thing called metabolism.
I remember being told in medical
school that metabolism was
the sum total of all of the chemical
reactions in the human body,
but I never actually
knnew what that meant.
I like to think of metabolism
as kind of like the balance
in the body between the
reactions that build things up,
and another way of
saying that is anabolism.
On the other side of the scale,
the reactions that break things down.
The medical way of saying
that is catabolism,
or catabolism.
So, metabolism is kind
of the balance between
building things up, or repairing
or storing inside the body
and breaking things down,
usually for energy needs.
How are the things that we eat
broken down for energy or
stored within the body?
Let's start by looking at carbohydrates.
So, we eat our carbohydrates,
and they are in the form
of starches or sugars,
and we saw in our last
video that those could get
broken down into their
kind of component parts.
The major component, or the
major single unit currency
of carbohydrates is called glucose.
That glucose, and this
is very big-picture,
can get broken down further
into a compound called pyruvate.
The pyruvate then gets broken down into
a very important molecule
that's kind of the center
of making energy in the body,
and that molecule is called acetyl-CoA.
Now, I think of acetyl-CoA
kind of like one of those
little trampolines you see in the gym,
and basically acetyl-CoA
can be moved around
to different parts.
We'll put the trampoline there,
and then we'll put it over there,
and then we'll put it over there.
Basically, different
chemcals bounce on and off
of that little portable trampoline
as they're going around this cycle.
This cycle is called the TCA cycle.
That stands for tricarboxylic acid cycle,
or it's sometimes called the Krebs cycle.
Essentially, what you need
to know about this cycle
is that as the acetyl-CoA
moves around the cycle
to form all of these
slightly different compounds,
things are bounced off,
and the things that are bounced off
are these high-energy carriers.
I'll just write high-enegy carriers.
What happens to the
high-energy carriers next
is that they basically
bounce down a staircase
called the electron transport chain.
As they go down that staircase,
they give off a chemical called ATP.
ATP is the currency of
energy in the human body.
ATP is the same as
energy in the human body.
Now, just to add some detail here,
water is also given off,
and I'll write the chemical
formula for water, which is H2O,
and in order for this cycle,
this TCA cycle to move around,
oxygen is required, and
carbon dioxide, CO2,
and water, again, are given off.
Oxygen is also necessary
for this step to happen,
and this is really neat
because this is essentially
the reason why we need to breathe.
Now let's look at what happens to
the protein that's in our diet.
Protein was one of the 3 macro nutrients
that we looked at in the last video,
and the proteins in our
diet are broken down
by our digestive tract as we saw
into unit parts called amino acids.
Depending on what kind of
amino acid you're looking at,
the amino acid can either
enter the system here,
at this level, or it can enter here,
depending on which kind
of amino acid it is.
When the amino acid makes this transition,
something is given off,
and that something is the amino group.
So the amino group, or
the amine, is given off,
and that amine becomes
one of the waste products
in our urine.
That's how we get rid
of those amine groups.
Let's look now at what happens to the fats
that we eat in our diet.
Fats, basically, are made up
of a little glycerol backbone,
so I'm gonna write here "glycerol",
and long chains of what
we call fatty acids.
There are three of them attached
to each glycerol backbone.
What happens when the body
needs to use those for energy
is that it breaks them down
into units of 2 carbons each,
and I'll write here fatty acids,
and those 2 carbon units can
only enter the system over here,
at the level of acetyl-CoA.
Whereas this small glycerol
backbone goes in over here.
Those are the breaking down reactions.
That's this side of the
equation of metabolism,
but what about the building up reactions?
Well, the amino acids that come
from the protein in our diet
can be used to make things like muscle
and other lean tissue in our bodies.
They can be used for repair.
They can be used as protein
carriers in our blood,
and the carbohydrate that we eat,
and the glucose in our body,
can also be stored as
something called glycogen,
which is the storage form of
carbohydrates in our bodies.
The fats that we've eaten,
if there are too many
of those in our diet,
they can also be stored as adipose tissue,
so they're put back together
and stored in our body.
The balance between the
breaking down reactions,
or the catabolic reactions,
and the building up reactions,
the balance is controlled
by hormones in our body.
There are several hormones that lead to
breaking down of tissues.
Things like glucagon
leads to the breakdown of
the stores of glycogen in our body.
Other hormones are the kind
of fight or flight hormones.
Epinephrine is one of
them and glucocorticoids.
These are sort of the
stress hormones in our body.
These guys all favor the
breaking down, or catabolism,
of the tissues.
The hormones that lead to the building up,
or the anabolism, in our bodies,
the major one is insulin.
I'm sure you've all heard of it.
Insulin leads to storage,
or things being put away.
There's also a part to be
played by the sex hormones,
or the sex steroids,
and then there are some
hormones that are anabolic,
or favor the building up of proteins,
and those are the hormones
thyroxin and growth hormone,
I'll just write GH.
Those guys favor the
anabolism or the building up
of protein stores, so protein
synthesis is favored by these guys.
But, they also work on this side.
they favor the breakdown of
fats and carbohydrate stores.
Thyroxin and GH, growth hormone,
those guys favor the
breaking down of our fat
and carbohydrate stores.
The last thing I want to add here
is that some of these
reactions are reversible,
so glucose going to
pyruvate can be reversed.
It can go back up in the other direction,
and that's how we can make glycogen
from, let's say, these amino
acids that make pyruvate.
This kind of amino acid can
be used to make glycogen.
The backbone of the fatty
acids called glycerol
could be used to make glycogen
or to make glucose to feed the brain,
but this reaction, this
particular one here,
is an irreversible, one-way reaction.
That means that fatty
acids can never go up
to make pyruvate and be used
as glucose for the brain,
or they can never be used
to replenish glycogen stores
in the body.
Another important thing
that we have to understand
is that this cycle will only go around
if ATP is needed by the body.
If we're using energy,
we're doing exercise,
we're demanding ATP and
energy from our system,
then this cycle will
continue going around.
But if we don't,
let's say we're sedentary
and we don't use energy,
then acetyl-CoA is going to build up,
and it's gonna favor the
return in this direction
towards adipose tissue.
This is how excess carbohydrate,
if it's taken in in the diet,
can lead to excess adipose
tissue, excess fat,
if we eat too much fat in our diet,
that can lead to adipose tissue,
and even if we eat too much protein,
it'll just be deaminated
and converted to acetyl-CoA,
which can be converted to adipose tissue.