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.