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 1 glucose molecule
into 2 pyruvate molecules.
First, a kinase reaction adds a phosphate onto glucose to form glucose-6-phosphate.
This is one of 2 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 2nd energy consumption step,
and an irreversible reaction.
In the 4th step of glycolysis,
a lyase reaction splits the 6-carbon fructose-1,6-bisphosphate
into two 3-carbon sugars,
dihydroxyacetone phosphate,
and glyceraldehyde-3-phosphate.
The dihydroxyacetone phosphate is rearranged by another isomerase
to form a 2nd glyceraldehyde-3-phosphate.
At this point in glycolysis,
glucose has been metabolized into two glyceraldehyde-3-phosphates,
and 2 ATPs have been consumed.
The next five steps of glycolysis are the energy producing phase.
In step 6,
both glyceraldehyde-3-phosphates are oxidized
to 1,3-bisphosphoglycerate by a dehydrogenase.
This step produces 1 NADH
for each oxidized glyceraldehyde-3-phosphate
for a total of 2 NADHs.
These NADHs are later used to produce more ATP for the cell.
In step 7,
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 3rd carbon of 3-phosphoglycerate,
to the 2nd carbon position to form 2-phosphoglycerate.
In step 9,
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 1 and 3, this step is irreversible.
At this point, 2 pyruvate molecules,
4 ATPs
and 2 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 2nd step.
Importantly, this means one less ATP is required for the pathway
because the 1st 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.