fatty acids, or even to make sugars depending
upon the cell.
Now as I said cells must control reactions,
but they don't have the tools available to
them to control reactions with respect to
energy because that's one of those principles
of the universe that the cell can't alter.
So instead, I want to talk a little bit about
mechanisms that cells have to control pathways,
but before I do that I want to give some examples
about the importance of doing that. In cells,
enzymes catalyze the reactions that are occurring.
And enzymes are incredibly powerful tools.
These enzymes can speed up reactions by enormous
amounts compared to the same reactions being
catalyzed without the enzyme. Some enzymes
for example, can catalyze a reaction that
is faster but on the order of 180
quadrillion fold faster than the same reaction
that's uncatalyzed. Now that 180 quadrillion,
a quadrillion is 10 to the 15th times faster,
is a mind-boggling amount of speed that an
enzyme is giving to a reaction. Now to think
of a real world analogy that we have for this,
I'd like to think of going to the grocery store.
We can walk to the grocery store, we can take
a bicycle to the grocery store, or we can
take an Indianapolis racecar to the grocery
store. We will get there a lot faster if we
take an Indianapolis racecar, but the likelihood
that we're going to have an accident or run
into something if we drive at full speed in
that racecar to the grocery store increases
as the speed increases. So controlling things
with respect to speed is very, very important
and that's really what cells are trying to
So cells have available to them, four mechanisms
for controlling enzymatic reactions. The first
of these is called allosterism. Allosterism
occurs for some enzymes, not all enzymes.
But these enzymes that are controlled allosterically
will bind small molecules and the small molecule
will affect the enzyme. It will actually help
the enzyme in some cases to be faster, that
is, to activate the enzyme, or in some cases
to inhibit or slow down the enzyme. In some
cases it may completely physically turn it
off. A second mechanism that cells have for
them is the covalent modification of enzymes.
Now in covalently modifying enzymes, it might
seem like what is happening is you just simply
stopping the enzyme from functioning, but
the reality is actually the opposite of that.
If you go to the store and you buy something
it has been sealed for your protection, the
only way that you can get to that and use
it is if you open the package, and so some
enzymes are synthesized in a form called zymogens,
an inactive form of the enzyme. And to open
the package, peptide bonds have to be broken
in strategic places in order for the enzyme
to become active. Other covalent modifications
that are performed to enzymes include the
addition of phosphate, called phosphorylation,
or the removal of that same phosphate called
dephosphorylation. Now depending upon the
enzyme, phosphorylation may activate or inactivate
the enzyme and dephosphorylation will have
the opposite effect.
Cells also have available to them the ability
to control whether or not an enzyme is made.
We've seen in other modules, the importance
of gene regulation, that is controlling the
transcription and translation of specific
proteins and if cells do that with enzymes,
they are able to physically start or stop. So a
good example for us, is that of the synthesis
of glucose. In our bodies, only specialized
cells in our bodies, will make glucose. They
occur mostly in our liver and kidney. The
other cells of our body will not make glucose.
And the reason that they don't make glucose,
is because those other cells of our body control
the synthesis of a key enzyme that's necessary
for making glucose. By not making the enzyme,
glucose can't be made. The last tool available
to cells to control metabolic pathways is
that of organelle sequestration. This is a
tool in eukaryotic cells which have of course,
organelles like mitochondria. Mitochondria
are membrane enclosed and so to get things
in and out of there is not as easy as if the
reactants are floating around in the soup,
the soup of the cytoplasm of course. In the
mitochondrion, there are reactions occur that
are the citric acid cycle, the breakdown, the
oxidation of molecules, whereas the reactions
of glycolysis are sequestered in the cytoplasm.
These two pathways don't easily cross and
the molecules to be used in each pathway must
cross the lipid bilayer of the mitochondrion
in order to be used.