Acetyl CoA though, we can see is at the center
of it regardless of what we are metabolizing.
So straight up glucose, we end up with Acetyl CoA.
Fatty acids, the fatty acid tails come in as Acetyl CoA.
And then even portions of proteins will come in
as Acetyl CoA. So Acetyl CoA is a pivotal
molecule in the whole process of cell respiration
no matter which fuels we metabolize.
Before we move on, I want to remind you of the idea
that cellular mechanisms go both in a forward direction
and in a backward direction. So we can build things and
we can break them down. So the process of anabolism
is building things. So anabolic steroids means building
muscles. And then catabolism is breaking things down.
So whether we are considering glucose or fats or
proteins, all of these pathways go in both directions.
So looking at glucose pathways first, we could either be
breaking down glucose in the process of glycolysis,
or we could be making glucose in the
process of gluconeogenesis.
So, on the other hand if we want to take glucose and store
it in the form of glycogen, we would have glycogenesis.
So that is the packing of glucose together, storage molecule
of glycogen, but when we are short on blood sugar
and we need to have sugar for fuel, we will have
glycogenolysis or the breaking down of glycogens.
So glycogen lysis to break glycogen and to form glucose
and it can go all the way through and make tons and tons of ATP.
Either way, the central unit here is going to be Acetyl CoA.
Similar processes occur when we consider the breakdown,
catabolism or buildup anabolism of fatty acids. Bi-directional,
so we can store them or we can break them down and use them.
And everything in the system is always influx. So, if we need
a lot of fuel, we need to make more ATP, then fatty acids
will be catabolised and broken down and enter as Acetyl CoA.
So the common theme here I think you see is Acetyl CoA
as a pivotal molecule. When we look at proteins, we can have
protein catabolism, protein anabolism but, in general
not such an easy pathway to get into because
there are so many different places for entry
but we still have the possibility to use proteins as
fuel and some of them will come in as Acetyl CoA.
Many others as we saw will come in at different places
throughout the Krebs cycle or even in
through the breakdown of pyruvate and pyruvate oxidation.
So in general, you see the scheme here is that all of these
fuels that we consume can enter the cellular respiration
process and be used as fuels. Again primarily we will use
glucose. Other sugars can enter this pathway through
simple modifications and then we'll use fats which
have high energy yields but require the use of oxygen.
So we cannot breakdown fats until we have oxygen present
because you can see where they come into the scheme of things,
below glycolysis we need oxygen to breakdown fatty acids.
So now that we have a good idea of how each of the types of
fuel we consume are broken down, let's look at how
the process of metabolism can be controlled or regulated.