Glycolysis illustration


Take a look at the opening stages of cellular respiration

The reactions of glycolysis

A diagram showing the stages of glycolysis, taken from our cellular respiration poster.


‘Big Picture: Exercise, Energy and Movement’ (2012)

Glycolysis is the first stage of all the reactions of respiration, regardless of whether oxygen is available. A complex process occurring in the cytoplasm of the cell, it’s essentially the oxidation of glucose to form pyruvate and adenosine triphosphate (ATP).

There are many steps involved in glycolysis, but they can be summarised as three simple reactions:

  1. Glucose (a six-carbon molecule) is phosphorylated twice, producing a six-carbon sugar phosphate (hexose bisphosphate) that is more reactive. Two molecules of ATP are converted to adenosine diphosphate (ADP) to provide the phosphate.
  2. The six-carbon sugar phosphate then breaks down into two triose phosphates, which each contain three carbon atoms.
  3. Triose phosphate is then converted to pyruvate. For each triose phosphate molecule, hydrogen is removed and passed to an NAD molecule (ie, the triose is oxidised and the NAD reduced). The phosphate groups are also removed from the triose molecules, converting two ADP molecules to ATP per triose.

Summary: what goes in and what comes out

Into glycolysis

Out of glycolysis

Net products

1 x glucose

2 x NAD

2 x ATP

2 x pyruvate

2 x reduced NAD (NADH + H+)

4 x ATP

2 x pyruvate

2 x reduced NAD (NADH + H+)

2 x ATP

After glycolysis

What happens next depends on whether oxygen is present. If it is, aerobic respiration can take place. However, oxygen may not always be available in the quantity that is required, and not all organisms are capable of respiring aerobically. To solve this issue, many organisms are instead able to respire anaerobically – without oxygen.

To see how these two respiration processes differ, check out our articles on aerobic and anaerobic respiration. You can also use our cellular respiration poster to help visualise the steps of glycolysis and respiration. 

Lead image:

An illustration of pyruvate kinase, an enzyme that regulates the final step of glycolysis, at work. 

Grace Hsu/Wellcome Images CC BY NC ND


Further reading

About this resource

This resource was first published in ‘Exercise, Energy and Movement’ in August 2016.

Cell biology
Exercise, Energy and Movement
Education levels:
16–19, Continuing professional development