Textbooks define metabolism,
a topic in biochemistry,
as a series of
chemical reactions
that take place inside of
our bodies to sustain life.
Now, this is a pretty broad
definition of metabolism.
So in this video, I really
want to break this definition
down to a more workable
understanding of what
metabolism really is.
So first, I'm going to introduce
another arrow in this diagram,
like this, and say that really,
the requirements of life,
let's say in a human
being, such as maintaining
a constant internal temperature,
reproducing, growing, and all
that jazz, all of that
ultimately boils down
to the body's ability to utilize
four essential biomolecules.
And these four essential
biomolecules, or as
they're sometimes known
as macromolecules,
are proteins, fats,
carbohydrates,
or carbs, and nucleic
acids, like DNA and RNA.
And ultimately, all
of these biomolecules
perform different
life-sustaining reactions
inside of all of the
cells in our body
to ultimately promote life.
So as you can see,
we've already begun
to break down this
definition of metabolism.
Essentially what
we're saying here
is that metabolism is really
the study of how we're
able to obtain these important
biomolecules to sustain life.
So how do we obtain
these biomolecules?
Now, a simple answer
to this question
is, of course, that
we eat food to obtain
all of these important
biomolecules.
But there is an
important word of caution
here, which is that
since most food comes
from living organisms
like plants and animals,
these plants and animals also
contain an array of proteins,
fats, carbohydrates, nucleic
acids, but not necessarily
in the same flavor
or configuration
that our bodies would prefer.
So what do our
bodies do instead?
Well, in our bodies, we
go ahead and eat the food.
That's a very large head
there, but you get the idea.
And in our bodies, we
break down this food
through a process
called digestion
into the component parts of
all of these biomolecules.
So what do I mean
by component parts?
Well, the smallest
subunit of proteins
is called an amino acid.
And our body breaks down
all the different types
of proteins that we digest
into individual amino acids.
And the same pattern
continues for the rest
of the biomolecules.
So in the case of
fats, we're talking
about fatty acids, which are
the smallest subunits of fats.
And then for
carbohydrates, which
are long chains of sugars.
One of the most common
subunits of carbohydrates
that our body loves
is called glucose.
So I'll go ahead
and write that here,
since you'll be seeing it
a lot in the discussion
of metabolism.
And then finally,
for nucleic acids
were talking about nucleotides.
So at this point, you're
probably thinking, well, OK.
I understand that our body can't
use the same macromolecules
found in food
because maybe they're
not in the right configuration.
But how does breaking them
down do anything for us?
Now the key here is to
recognize that in our body
there is actually a
delicate balance going
on between the processes
of breaking down molecules,
such as in the
process of digestion,
and then taking these products
and building them back up.
So essentially, you can see all
of these subunits, or monomers,
as LEGO pieces that we're
essentially reconstructing
to build the right
configurations of proteins,
fats, carbs, and nucleic
acids that our body needs.
So that's really
the key idea here,
which is that metabolism is a
balance between breaking things
down and building them
back up in our body
so that we can
customize, so to say,
what type of macromolecules
that we create.
And just to throw
in some vocab words,
biochemists call the
process of breaking down
molecules in our
body catabolism.
And similar sounding
word called anabolism
is used to describe the process
of building molecules back up.
And the way I like to
remember this is looking
at the first letter of each
of these words, I think of C,
I think of cutting molecules
up into tiny pieces,
so breaking them down.
And then for anabolism,
A, I think of as
like the apex of a
building, for example.
So we're building something up.
Now this seems all
fine and elegant,
but there's one more issue
that we need to contend with,
which is a consequence of having
to balance breaking things
down and building them back up.
And that is that this
process of building molecules
back up requires energy.
Which I'm kind of
indicating here
by these yellow
lightening bolt stars.
So the question I want to answer
in this last part of the video
is where does this
energy come from?
Now, the answer to this
question is that, well, we
also get this energy
by eating food.
So how does that work?
So first, recall that the energy
currency of the cell-- and I'm
going to go ahead
and erase this just
to give us some more space.
The energy currency
of our bodies
is a molecule called ATP,
or adenosine triphosphate.
And this high energy
molecule, as it's often
referred to, when it is broken
down into ADP, so it loses
a phosphate group, it releases
usable chemical energy
that can fuel energy requiring
processes in our body,
such as the building up
process of anabolism.
Now, in order for this
process to continue non-stop
in our bodies, ADP must
be regenerated into ATP.
And that is where food comes in.
So remember that
we digest our food
into all of these subunits.
And some of these subunits,
such as glucose and fatty acids
mainly, but occasionally
amino acids--
I'm going to put
that in parentheses--
can essentially be used
as fuels in our body.
So just like wood,
for example, is
a fuel for a burning
fire, which produces heat,
these fuels in our
body can essentially
be broken down even further
to produce the energy that's
necessary to convert
ADP back into ATP
and thus allowing this
cycle to continue.
And just to throw in
another vocabulary word
that you'll probably
see, this process
of taking these
fuels, which I've
indicated with this
asterisk, and breaking them
down into usable
energy is a process
that's referred to as
cellular respiration.
And recall that because
cellular respiration involves
breaking down
things even further,
it's also a catabolic process.
So it falls under this
category of catabolism.
And just to tie everything
here together at the end,
notice here that
another way to interpret
this cycling between
ATP and ADP is
to say that catabolism
fuels anabolism.
So what do I mean by this?
Well, essentially, catabolism,
such as the process
of breaking things down
and extracting energy
through processes of
cellular respiration,
is coupled with this process
of building things back up.
And so in essence, one
relies on the other.
And as you can probably
guess, these processes
are really tightly
regulated in our bodies.
Because obviously
you wouldn't want
to be breaking down something
while you're building something
back up.
And in fact, just to give
you a preview forward,
catabolism and anabolism are
often regulated, so controlled,
through the use of hormones.
So I'm going to write
here that hormones
are a form of regulation, and
tell the body whether it should
be in a catabolic
or anabolic state.
