Glycolysis: The First Stage in Cellular Respiration

Diagram showing the process of glycolysis

Thomas Shafee / CC BY 4.0 / Wikimedia Commons

Glycolysis, which translates to "splitting sugars", is the process of releasing energy within sugars. In glycolysis, a six-carbon sugar known as glucose is split into two molecules of a three-carbon sugar called pyruvate. This multistep process yields two ATP molecules containing free energy, two pyruvate molecules, two high energy, electron-carrying molecules of NADH, and two molecules of water.


  • Glycolysis is the process of breaking down glucose.
  • Glycolysis can take place with or without oxygen.
  • Glycolysis produces two molecules of pyruvate, two molecules of ATP, two molecules of NADH, and two molecules of water.
  • Glycolysis takes place in the cytoplasm.
  • There are 10 enzymes involved in breaking down sugar. The 10 steps of glycolysis are organized by the order in which specific enzymes act upon the system.

Glycolysis can occur with or without oxygen. In the presence of oxygen, glycolysis is the first stage of cellular respiration. In the absence of oxygen, glycolysis allows cells to make small amounts of ATP through a process of fermentation.

Glycolysis takes place in the cytosol of the cell's cytoplasm. A net of two ATP molecules are produced through glycolysis (two are used during the process and four are produced.) Learn more about the 10 steps of glycolysis below.

Step 1

The enzyme hexokinase phosphorylates or adds a phosphate group to glucose in a cell's cytoplasm. In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate or G6P. One molecule of ATP is consumed during this phase.

Step 2

The enzyme phosphoglucomutase isomerizes G6P into its isomer fructose 6-phosphate or F6P. Isomers have the same molecular formula as each other but different atomic arrangements.

Step 3

The kinase phosphofructokinase uses another ATP molecule to transfer a phosphate group to F6P in order to form fructose 1,6-bisphosphate or FBP. Two ATP molecules have been used so far.

Step 4

The enzyme aldolase splits fructose 1,6-bisphosphate into a ketone and an aldehyde molecule. These sugars, dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP), are isomers of each other.

Step 5

The enzyme triose-phosphate isomerase rapidly converts DHAP into GAP (these isomers can inter-convert). GAP is the substrate needed for the next step of glycolysis.

Step 6

The enzyme glyceraldehyde 3-phosphate dehydrogenase (GAPDH) serves two functions in this reaction. First, it dehydrogenates GAP by transferring one of its hydrogen (H⁺) molecules to the oxidizing agent nicotinamide adenine dinucleotide (NAD⁺) to form NADH + H⁺.

Next, GAPDH adds a phosphate from the cytosol to the oxidized GAP to form 1,3-bisphosphoglycerate (BPG). Both molecules of GAP produced in the previous step undergo this process of dehydrogenation and phosphorylation.

Step 7

The enzyme phosphoglycerokinase transfers a phosphate from BPG to a molecule of ADP to form ATP. This happens to each molecule of BPG. This reaction yields two 3-phosphoglycerate (3 PGA) molecules and two ATP molecules.

Step 8

The enzyme phosphoglyceromutase relocates the P of the two 3 PGA molecules from the third to the second carbon to form two 2-phosphoglycerate (2 PGA) molecules.

Step 9

The enzyme enolase removes a molecule of water from 2-phosphoglycerate to form phosphoenolpyruvate (PEP). This happens for each molecule of 2 PGA from Step 8.

Step 10

The enzyme pyruvate kinase transfers a P from PEP to ADP to form pyruvate and ATP. This happens for each molecule of PEP. This reaction yields two molecules of pyruvate and two ATP molecules.

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Your Citation
Bailey, Regina. "Glycolysis." ThoughtCo, Aug. 27, 2020, Bailey, Regina. (2020, August 27). Glycolysis. Retrieved from Bailey, Regina. "Glycolysis." ThoughtCo. (accessed January 23, 2021).