- [Instructor] What I think is pretty fascinating
is our body is able to maintain a very narrow
and constant range of blood glucose in our body
so noticeably about 60 to 150 milligrams of glucose
per deciliter of blood and it's not important
that you know this exact number but what I think
is significant is it contrasts something like
free fatty acids for example, which we'll
talk about in fatty acid metabolism.
Fatty acids can range almost tenfold
depending on the needs of the body
so they can be very high or very low
but glucose always stays within a constant range,
blood glucose level here, and it's important
that this is a very constant range because
there are some tissues in our body such
as our brain, some of the cells in our eyes,
and our kidneys, and even our red blood cells
that rely on glucose nearly exclusively to produce ATP.
So remember once glucose is in the blood, it can
be used by any of the cells in our body by process
of cellular respiration to produce ATP.
Remember the three big steps of cellular respiration
are first glycolysis, the breakdown of glucose,
and then the glucose goes to the Krebs cycles
where it undergoes some more oxidation to
release all of that energy in the glucose molecule,
and finally the byproducts of glycolysis and the Krebs
cycle go to the electron transfer chain which is
able to produce ATP in bulk amount.
So how does our body keep this blood glucose
in such a narrow range and constant range in our body?
Our body is able to do this differently
depending on what state the body is in, so
the body can be either in the FED state
or something we call a fasted state.
You can imagine the FED state just
after you've eaten a meal.
So let's say you've eaten a chocolate chip cookie.
The glucose that has been broken up in your GI tract
can then be used to directly contribute to this
blood glucose level and then of course the glucose
can be used by our cells.
Now in the fasted state, this is all the times
your body is not eating, the body has come up
with two different ways to regulate blood glucose levels.
Remember in the fasting state, our body needs way
to pump glucose into the blood to keep it at this level,
essentially to replace the glucose that's being used
by our cells because we don't have this constant
intake from our chocolate chip cookie.
In this case our body has glycogen, which is a polymer,
or a string of glucose molecules that it stores away.
Our body ingeniously makes this glycogen by using
some of that glucose that is dumped into
our body during the FED state, so in anticipation
of knowing it's not always going to get glucose
from eating, it preserves some of it in this glycogen
molecule, and most of this glycogen molecule
is located in your liver which is why your liver
is very important for carbohydrate metabolism.
In times of fasting, our body can actually go ahead
and break down this glycogen into the individual
glucose molecules which then can be used to
keep our blood glucose levels constant.
Unfortunately it turns out that this method
of breaking down glycogen only lasts for about
10 to 18 hours in our body, that is to say
after 10 to 18 hours we've used up our glycogen stores
and we need to eat another meal to build those
glycogen stores back up.
You can imagine during an overnight fast for example,
it's usually about hopefully eight to 10 hours.
You can imagine there is a point during the day
where your body needs another way of producing glucose.
Our body has come up with a second way called
gluconeogenesis, which is indeed the topic of this video.
Gluconeogenesis is exactly what its name implies.
It is the genesis of creation of neo, new, glucose.
It's actually fascinating to think about this for a moment.
What we're saying in gluconeogenesis is our body
is taking precursor molecules that are from
a non-carbohydrate source, so looks at what it has
lying around and most commonly it uses amino acids
in our body as well as a molecule called lactate
which is produced as a byproduct in exercising
muscle cells, and it takes these precursor molecules
and reconfigures them to produce glucose
and it's this glucose that can then be used
to be dumped into our blood to maintain
constant blood glucose concentration
and a constant supply of ATP for our tissues.
Now that you have a big picture of carbohydrate metabolism
and where gluconeogenesis fits in, let's go ahead
and talk about this metabolic process, gluconeogenesis.
In order to do this, it's actually important
to revisit glycolysis briefly so I'm gonna go ahead
and bring up the reaction diagram that was used
to explain glycolysis in a previous video.
Just to orient you, remember that glycolysis
begins with glucose up here and glucose is
broken down in a series of steps.
Most notably it's broken down into this three carbon
molecule, glyceraldehyde three phosphate
and then it is broken down even further
and reconfigured, releasing some ATP
and ADH along the way and ultimately
releasing this molecule pyruvate and pyruvate
is a very important molecule because it can continue
to the Krebs cycle where it can be further
oxidized to produce more NADH that can be used
by the electron transfer change to produce ATP.
Alright, so that was a big mouthful.
Just remember, big picture, glycolysis
breaking down glucose into pyruvate.
Turns out, the way I like to think about
gluconeogenesis is that the goal of gluconeogenesis
is to produce glucose and so, gluconeogenesis
is almost the exact reverse pathway of glycolysis.
We start at this end of the reaction pathway,
we start with pyruvate, and we go funnel back
the opposite direction through all of these reactions
to produce glucose.
Now the key word is that it's almost
the exact reverse of glycolysis and it's almost
the reverse because I want to call attention
to these orange arrows so note that there
are three orange arrows, so one from glucose
to this molecule glucose six phosphate,
another one here, and then one at the very end
which converts the last molecule to pyruvate.
What's important to note about these reactions
in glycolysis is that, unlike the other bidirectional
black arrows that are used in most of the reactions,
these orange arrows are unidirectional.
What they're trying to indicate is that these
three reactions are irreversible.
In other words, they have a negative, if we pull
out a fancy term from chemistry, they have a
negative delta g value, or a negative Gibbs free energy
which means that if we were to reverse
these particular reactions we would have
to flip the sign, so these negative delta g values
would be become positive and that's problematic
because we know that for any biological reaction
to occur we must have a negative delta g value.
So our body has come up with a compromise.
Our body has said we have one, two, three, four,
five, six, seven reactions which these
bidirectional black arrows which are essentially
reversible, that is to say they have a delta g value
that is near zero and so they can go either direction.
Our body says we'll keep those seven reactions
but I'm going from pyruvate back to glucose
we have to come up with a different reaction
pathway for the three steps that are irreversible
so that's exactly what our body did.
In fact, I'll go ahead and review in the remainder
of this video but you can say, with those three steps
in mind, we're just performing the reverse of glycolysis.