So let's talk about one of
the most famous molecules in all of biochemistry, ATP. So why is ATP so famous? Well, it's the energy
currency of the cell. And the reason it's called the
energy currency of the cell is because it powers, it
essentially fuels, many life sustaining reactions
inside of our body. So some examples
of these include biosynthesis of biomolecules. So remember fats,
proteins, carbohydrates, and nucleic acids are
all essential to life, and building these
molecules requires energy in the form of ATP. In addition, ATP is also
used to contract our muscles. And this is very
important in order to allow living
organisms to move. And additionally,
ATP is also involved in some ion movement
across cell membranes. And of course, moving
ions across membranes is really important to
maintain a comfortable internal environment within the cell. Now, you could
take my word for it that somehow ATP
magically powers all of these life-sustaining
processes, but you really don't have to. In fact, in this
video, we're going to review some topics
from general chemistry to really understand how
ATP, on a chemical level, really fuels these reactions. Now, the topic we want to
review in introductory chemistry is a thermodynamic parameter
called Gibbs free energy, or as it's more often written
as just simply delta G. So now recall that delta G
is a quantitative number. And it's a number
that's measured in units of joules, which
is a measurement of energy. And depending on whether this
value is positive or negative, it tells us whether or not
a reaction requires energy, or whether a reaction
releases energy. Now remember that delta G
is equal to the free energy of the products of a reaction,
minus the free energy of the reactants in a reaction. So if the change in
Gibbs free energy is negative, which means
that the products have a much smaller free
energy than the reactant, we say that this
reaction releases energy. On the other hand, if
we have a positive value of delta G, which means
that our products are at a much higher energy
level than our reactants, we say that that reaction
requires an input of energy. And I know I've been kind of
nebulous about this term energy here. And so, briefly, I
want to remind you that the change in free
energy going from reactants to products of a reaction
takes into account both the change in enthalpy, as
well as the change in entropy, which are two topics
that you might be familiar with from
general chemistry. So how does this
all relate to ATP? Well, it turns out that
there is a reaction involving ATP that has a very large
negative delta G value. That is to say it releases
a lot of free energy. Specifically, this
reaction involves ATP combining with water, and
when it combines with water, we call this a
hydrolysis reaction. So I'll just write
that here to remind us. And the products
of this reaction are a molecule called ADP
and a free phosphate group. And like I mentioned before,
the change in Gibbs free energy is very negative. So what's going on here? So ATP starts out
with triphosphate, three phosphate groups,
loses a phosphate group because it becomes diphosphate. And then it forms a
free phosphate group that it cleaved off. So on first glance, it might
seem that this reaction is not balanced because we
don't have this hydrogen or oxygen on the right
side of our equation. But I just want to note here
that the negatively charged hydroxyl group becomes a
part of the phosphate group. And the remaining
hydrogen ion of the water combines with another
molecule of water in solution to become a positively
charged hydronium ion. And usually, these two
things are left out just for the sake of convenience. But I wanted to point them
out here so that you wouldn't be confused by
the stoichiometry. On the other hand, many
biosynthesis reactions in the body have a
positive delta G value. So remember, a
positive delta G value means it requires
an input of energy. And an example of
this type of reaction is when we take a monomer, such
as an amino acid for example, and we string them together
covalently to form a polymer. So in the case of an amino
acid, that would mean we're forming a
long peptide chain. Now here's where our knowledge
of introductory chemistry comes in. So in thermodynamics, the
study of energy changes, there's an important
principle that states that the overall
delta G for a reaction-- I'm going to scroll down here
to give us some more space. So the overall delta
G for a reaction is equal to the
sum of the delta G values for the individual
steps of a reaction. So let's actually go ahead
and add these two reactions together and see what happens. So let's write that out. So we have ATP as a
reactant, as well as water, as well as
our monomer subunits. And we are producing ADP, a free
phosphate group, and a polymer. Now what is the delta G
for our overall reaction? Well, we just simply
have to add the delta G values for each step. Now I didn't give you
actual numerical values for each of these steps. But in general, the
hydrolysis of ATP produces energy in excess
of the energy needed for biosynthesis reactions,
such as this one. So essentially
what I'm saying is that if we add a very large
negative number to a smaller positive number, we will get an
overall negative delta G value. In other words,
we have just taken a previously energetically
unfavorable reaction with a positive delta
G value and turned it into an energetically
favorable reaction with a negative delta G value. And we have done this by what
we call coupling a reaction that has a favorable delta G value,
such as the ATP hydrolysis, with a reaction that has an
unfavorable delta G value. I want to mention that the
ability to add these delta G values tells us
nothing about the path that the reaction
actually takes. And in fact, generally
speaking, almost never does a reaction
proceed in two discrete steps like it's written here. Instead, this coupled process
often occurs simultaneously. But as you can see, it's
still beneficial to separate these two reactions
into two discrete steps, so you can prove to
yourself essentially why ATP, with its negative
delta G value, is able to fuel energetically
unfavorable processes.