HAZEL SIVE: Chemical
reactions in the cell
are not only governed by
the laws of thermodynamics
and are not only
catalyzed by enzymes.
They are also often
linked into pathways
where one chemical reaction
will proceed another
and another and another.
And the whole thing forms
a kind of production line
so that starting
from point A, you
land up with a
particular product
that the cell really needs.
This is just like a
factory production line
where you have to make something
from lots of components.
And there are lots of steps
involved in the manufacture.
So we can define
pathways and something
that I'll call feedback control
in the chemical reactions
of the cell.
And the point that
you should get
is that in order to
get somewhere, in order
to manufacture
something in the cell,
there are multistep pathways
comprising linked chemical
reactions.
And these pathways
can be governed
so that you can make more of the
product or less of the product
depending on the need of
the cell, really analogous
again to the factory.
The control of a pathway and
how much of the outcome there is
can be positive in which
case you'll make more
along that particular reaction.
Or it can be negative in
which case you will make less.
These controls are
called feedbacks.
And you can have positive
or negative feedback.
And then finally,
pathways can be
controlled by within the cell.
They can also be controlled
by external influences
just like in a factory.
If one needs more
cars because there's
more demand from the consumers,
the factory production line
will speed up.
In the cell, if
there is something
that happens outside
the cell, this
can give a signal to
a particular pathway.
And that can speed up or
slow down the pathway.
So there can be external
signals from outside the cell
that control pathways.
Let's look at a diagram of this.
I've drawn you here a
pathway that starts off
with reactants labeled A. And
those reactants can go down
one of two pathways.
They can either make B in
which case they will go on
and make C. Or they can
make D and go on to make E.
And those are a
splitting pathway.
Each of those arrows,
I should note,
is governed by a
particular enzyme.
Each of those are
chemical reactions.
So that is your basic pathway
that splits in my diagram.
Here is an example
of positive feedback.
Let us say that when you
get a lot of product C,
you also need a
lot of product E.
And the way that
this is controlled
is that C, whatever
the molecule is,
goes and speeds up the
production of D, which
then gives you the
increased production of E.
You can also have
a negative feedback
where once you've
gotten lots of D,
you might want to turn
off the other pathway.
And so D might inhibit
production of B and C
with this feedback inhibition,
this negative feedback.
This is what it looks
like in a diagram.
And we can add to it
an external signal.
Here I've shown
you a signal coming
from somewhere else
that's going to speed up
the production of B
and C. What does it
look like actually in the cell?
This is the pathway diagram for
phenylalanine, an amino acid.
And these are all
the different ways
that phenylalanine
is metabolized
in the cell, all the different
chemical reactions that
lead to its synthesis and
leads to its breakdown.
And you can see that it
is really complicated.
This is a circuit diagram with
both activation and inhibition
components.
And if we were to
zoom out and look
at the chemical pathways in the
cell, the metabolic pathways
in the cell shown in this
kind of line diagram,
we would fill the space with
very complicated activation
and inhibition pathways.
The circuitry of the cell is
really, really complicated.
Good.
So take a moment now and go
and do an exercise on pathways
so that you can
practice the positive
and the negative aspect
of what pathways mean.
