Hey it's professor Dave, let's talk about glycolysis.
We need energy to do literally anything, from running a race, to simply
breathing. And every cell in your body is furiously producing all this energy all the time.
Where does this energy come from? Well technically it arrives from the sun.
The sun is releasing enormous amounts of energy as a byproduct of nuclear fusion
reactions, and that energy makes its way to earth, where it can be absorbed by
plants through photosynthesis, which we will discuss later.
Plants use sunlight as well as carbon dioxide and water to make glucose, and it
is all of this glucose, among other biomolecules, that becomes the starting
material for metabolic processes in our bodies. This degradation of biomolecules
to generate energy that cells can use is called cellular respiration, or sometimes
more specifically, aerobic respiration. Let's see how this works,
looking at glucose as the substrate. Aerobic respiration requires oxygen so
any organism that breathes in oxygen from the atmosphere is doing so in order to
facilitate aerobic respiration. Glucose, which we can either consume as starch
or break off from glycogen stored in the cell, can be converted through metabolic
pathways in the presence of oxygen into carbon dioxide, which we breathe out,
water, which is most of what we are, and energy, the energy we need to think and move.
This is not unlike the combustion reactions that happen in an engine, so
it's a reasonable analogy to view biological organisms as machines.
The electron exchanges that occur throughout these metabolic pathways
utilize the electron carrier NAD+ and its other form, NADH. This is a
dinucleotide with an interesting base, nicotinamide, that can exist either
as NAD+, with a
positively charged nitrogen atom, or if reduced it can become NADH. This transfer,
facilitated by an enzyme called dehydrogenase, helps catalyze the
breakdown of glucose. Cellular respiration happens over three major
pathways. There's glycolysis, the citric acid cycle, and oxidative phosphorylation.
Let's focus on these one at a time.
Glycolysis comes first, and it happens in the cytoplasm of the cell. This is the
process by which glucose molecules are split into two pieces called pyruvate.
This first pathway is actually anaerobic, meaning it does not require
oxygen, so it is the most evolutionarily ancient metabolic pathway, occurring in
even the simplest cells. In this pathway, one glucose molecule can yield a net of
two ATPs. It requires 10 enzymes to happen which catalyze each of the 10
steps, as well as an investment of two ATP molecules in the preparatory phase to
get four ATPs back over several steps in the payoff phase. The names and
details of each individual reaction may not be of interest to every viewer, but
in case they do interest you,
here is a brief overview of each step. First the hexokinase reaction. The enzyme
hexokinase phosphorylates the oxygen on carbon 6 to make glucose 6-phosphate.
The polar phosphate group traps the molecule inside the cell and also reduces the
concentration of regular glucose inside the cell, which encourages more glucose
to enter by diffusion. This step costs 1 ATP, which provides the necessary
phosphate group for the reaction. Next, glucose-6-phosphate isomerizes to
become fructose-6-phosphate, a process which is catalyzed by phosphoglucoisomerase.
After that is another phosphorylation, this time on the carbon 1
hydroxyl which gives us fructose-1,6-bisphosphate. This step is catalyzed by
phosphofructokinase 1
and it will cost another ATP. Now this molecule is ready to be cleaved
into two smaller ones. Fructose bisphosphate aldolase is a lyase
enzyme that will split fructose-1,6-bisphosphate into a molecule of
glyceraldehyde-3-phosphate, or GADP, and a molecule of dihydroxyacetone phosphate
or DHAP. The DHAP will be converted into another molecule of GADP by the enzyme
triosephosphate isomerase, which leaves us with two molecules of GADP.
That's the end of the five-step preparatory phase, with two ATPs spent to
achieve the two phosphorylations. Now it's time for the payoff phase.
Let's just look at one of our two GADP molecules from the preparatory phase, and
we see that the first thing that will happen is an oxidation to become
1,3-bisphosphoglycerate. This requires NAD+ and a free phosphate, or inorganic
phosphate to occur, and the enzyme involved is called glyceraldehyde
phosphate dehydrogenase. Next, a phosphoglycerate kinase will
catalyze transfer of a phosphate group to ADP to become 3-phosphoglycerate
producing one ATP in the process. Since each of the two GADP molecules will make
one ATP, that's a total of two ATPs, for half the total payoff of glycolysis.
Then, phosphoglycerate mutase transfers the remaining phosphate from this hydroxyl
to the next one over to make 2-phosphoglycerate.
Then, enolase catalyzes a dehydration, resulting in the loss of this hydroxyl group which will
produce phosphoenolpyruvate. And lastly, the remaining phosphate group is
transferred to an ADP by pyruvate kinase, generating another ATP and the pyruvate
we discussed before. So altogether it's a 10-step process. The first five steps comprise
the preparatory phase, which take one molecule of glucose and produce two
molecules of GADP. This will cost two ATP. Then the other five steps make up the
payoff phase, in which each molecule of GADP will be converted into pyruvate,
producing two ATP each in the process, for a total of four, meaning the net
energy production from one molecule of glucose is two ATP. If you are required to
memorize basic facts about glycolysis this should probably suffice, but if you
wish to memorize this process in more detail, here is a table that lists the
necessary enzymes for each step as well as any relevant input or output besides
the actual substrate. Certainly the main thing to remember is that in glycolysis,
glucose in the cytoplasm of the cell is converted into pyruvate, which will then
move on to the next stage of cellular respiration.
Thanks for watching, guys. Subscribe to my channel for more tutorials. and as always, feel free to email me: