What Is Glycolysis? / The Glycolysis Pathway: Step-by-Step

Glycolysis is the cell’s foundational energy-producing pathway. It is the sequence of reactions that breaks down one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate, generating ATP, NADH, and water along the way.

 “Glycolysis is the metabolic process that converts glucose into pyruvic acid.”

A few important points:

• It occurs in the cytoplasm of every living cell.

• It does not require oxygen, which means it functions in both aerobic and anaerobic conditions.

• It represents the first stage of cellular respiration and is essential to life.

During aerobic respiration, glycolysis is followed by the Krebs cycle and electron transport chain. In the absence of oxygen, glycolysis continues as part of fermentation, producing small but vital amounts of ATP.

Historically, glycolysis was mapped out by three scientists—Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas—and is therefore known as the EMP pathway.

The Glycolysis Pathway (Step-by-Step)

To make the process easier to understand, think of glycolysis as a 10-step assembly line, each step driven by a specific enzyme.

Stage 1: Phosphorylation of Glucose

The enzyme hexokinase adds a phosphate group to glucose, creating glucose-6-phosphate. ATP donates that phosphate.

Stage 2: Rearranging the Structure

Glucose-6-phosphate is converted into fructose-6-phosphate by phosphoglucomutase.

Stage 3: Adding a Second Phosphate

Another ATP donates a phosphate, and phosphofructokinase converts the molecule into fructose-1,6-bisphosphate. This is a major regulatory step.

Stage 4: Splitting the Sugar

The enzyme aldolase splits the six-carbon molecule into two three-carbon molecules:

• Glyceraldehyde-3-phosphate (G3P)

• Dihydroxyacetone phosphate (DHAP)

Stage 5: Converting DHAP

Triose-phosphate isomerase converts DHAP into G3P so both molecules can continue in the pathway.

Stage 6: Oxidation and Phosphate Addition

Each G3P is oxidized by glyceraldehyde-3-phosphate dehydrogenase, producing NADH and 1,3-bisphosphoglycerate.

Stage 7: ATP Formation

Phosphoglycerokinase transfers a phosphate to ADP, forming ATP. Two ATP molecules are generated here.

Stage 8: Phosphate Shift

Phosphoglyceromutase rearranges the phosphate, forming 2-phosphoglycerate.

Stage 9: Water Removal

Enolase removes water to produce phosphoenolpyruvate (PEP).

Stage 10: Final ATP Production

Pyruvate kinase transfers a phosphate from PEP to ADP, forming pyruvate and another ATP.

Summary of Products

From one molecule of glucose, glycolysis yields:

• 2 pyruvate

• 2 ATP (net gain)

• 2 NADH

• 2 water molecules

And it all happens without oxygen.

Key Points to Remember

• Glycolysis is universal—it occurs in every cell, from bacteria to humans.

• It is the first step in energy harvesting from glucose.

• The pathway uses 10 enzyme-driven reactions.

• It functions with or without oxygen, making it essential for survival in low-oxygen environments.

• The end products feed into major metabolic pathways, including the Krebs cycle and fermentation.

For more clear explanations, visuals, and biology learning resources, continue exploring HN Series.