What is Glycolysis? Video Lecture - NEET

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.
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