One source of ATP is glycolysis,

a series of reactions that breakdown sugars while producing ATP.

Glycolysis is a series of 10 reactions that converts sugars, like glucose,

into 3-carbon molecules called pyruvate.

Glycolysis is an important pathway for cells because it produces ATP,

and other products such as NADH and pyruvate,

that can be used later to produce more ATP for a cell.

This key process takes place in the cytosol of the cell.

Let’s take a look at an overview of the key steps of glycolysis

and focus on the input and output molecules.

The first five steps of glycolysis convert a 6-carbon sugar (glucose)

into two 3-carbon sugars (glyceraldehyde-3-phosphate).

To do so, ATP is consumed in the first and third steps.

Both of these chemical reactions are catalyzed by an enzyme known as a kinase,

and both reactions are irreversible.

The next five steps of glycolysis produce energy.

One ATP is made per glyceraldhyde-3-phosphate

in both reaction 7 and 10.

This means a total of four ATPs are produced in glycolysis.

Both of these steps are carried out by a kinase reaction.

Note, that step 7 is reversible while step 10 is not.

It is important to remember that the products in steps 6-10

are formed for each of the glyceraldehyde-3-phosphate molecules generated

in steps four and five.

This means that a total of two NADH and four ATPs are made in glycolysis.

Because two ATPs were spent in the early steps of the pathway,

glycolysis results in a net of two ATPs.

There is one more key product of glycolysis to consider called NADH.

NADH is formed in the sixth reaction by an enzyme called a dehydrogenase.

NADH is an electron carrier with the potential to produce more ATPs

through a process called oxidative phosphorylation.

Details of that process are shown in a separate animation.

As you can see,

glycolysis is a process involving multiple enzymatic reactions that breakdown sugars.

Its major contribution to biological organisms

is the production of ATP

which is used to drive many other biochemical pathways.