Biological organisms require energy to survive.
Glycolysis is one of the pathways cells use to transform sugars like
glucose into biochemical energy in the form of ATP.
In the cytosol of the cell,
glycolysis converts glucose into pyruvate,
through a series of 10 enzymatic reactions.
This process produces ATP,
along with other products, such as NADH,
that can be used later to produce even more ATP for the cell.
Let's watch as these enzymes oxidize
one glucose molecule into two pyruvate molecules.
First, a kinase reaction adds a phosphate onto glucose
to form glucose-6-phosphate.
This is one of two energy consumption steps
and is an irreversible reaction.
Next, an isomerase reaction converts glucose-6-phosphate
into fructose-6-phosphate
by rearranging covalent bonds.
Another kinase removes a phosphate group from ATP
and gives it to fructose-6-phosphate
to form fructose-1,6-bisphosphate.
This is the second energy consumption step
and is an irreversible reaction.
In the fourth step of glycolysis,
a lyase reaction splits the 6-carbon
fructose-1,6-bisphosphate
into two 3-carbon sugars,
glyceraldehyde-3-phosphate
and dihydroxyacetone phosphate.
The dihydroxyacetone phosphate is rearranged by another isomerase
to form a second glyceraldehyde-3-phosphate.
At this point in glycolysis,
glucose has been metabolized into two glyceraldehyde-3-phosphates,
and two ATPs have been consumed.
The next five steps of glycolysis are the energy producing phase.
In step six,
both glyceraldehyde-3-phosphates are oxidized
to 1,3-bisphosphoglycerate
by a dehydrogenase.
This step produces one NADH
for each oxidized glyceraldehyde-3-phosphate
for a total of two NADHs.
These NADHs are later used to produce more ATP for the cell.
In step seven, a kinase transfers a phosphate
from 1,3-bisphosphoglycerate to ADP
to form ATP and 3-phosphoglycerate.
This step is reversible even though ATP is formed.
The next step involves a mutase reaction
that moves the phosphate on the
third carbon of 3-phosphoglycerate
to the second carbon position
to form 2-phosphoglycerate.
In step nine, a lyase reaction removes water
from 2-phosphoglycerate
to form phosphoenolpyruvate.
In the final step of glycolysis,
a kinase reaction removes the phosphate group
from phosphoenolpyruvate
and donates it to ADP
to form ATP and pyruvate.
Like reactions one and three,
this step is irreversible.
At this point, two pyruvate molecules,
four ATPs,
and two NADHs are formed
for each glucose
that was broken down in glycolysis.
The pyruvates and NADHs
could be used in aerobic respiration to produce more energy for the cell.
Here we depict glycolysis as a closed process.
But in cells, substrates produced by other reactions
can enter glycolysis at different points.
For example,
when an animal breaks down glycogen,
glucose 6-phosphate is produced
and can then enter the glycolysis pathway at the second step.
Importantly, this means one less ATP is required
for the pathway because the first ATP consuming step is skipped.
Other sugars can also enter the glycolysis
pathway at different points,
each having a different effect
on the net number of ATPs
that are produced by glycolysis.
These ATPs are important energy molecules
required for many biochemical pathways
and ultimately life itself.
Glycolysis is a major contributor
to the pool of ATP used in these pathways,
pathways that are essential
to the survival of biological organisms.