10 Steps of Glycolysis

Glycolysis
Glycolysis, the dissolution of sugar, is carried out in the cytoplasm of cells. Thomas Shafee via Wikimedia Commons [CC BY 4.0]

Glycolysis literally means "splitting sugars" and is the process of releasing energy within sugars. In glycolysis, glucose (a six carbon sugar) is split into two molecules of the three-carbon sugar pyruvate. This multi-step process yields two molecules of ATP (free energy containing molecule), two molecules of pyruvate, and two "high energy" electron carrying molecules of NADH. Glycolysis can occur with or without oxygen.

In the presence of oxygen, glycolysis is the first stage of cellular respiration. In the abscence of oxygen, glycolysis allows cells to make small amounts of ATP through the process of fermentation. Glycolysis takes place in the cytosol of the cell's cytoplasm. However, the next stage of cellular respiration known as the citric acid cycle, occurs in the matrix of cell mitochondria.

Below are the 10 steps of glycolysis: 

Step 1

The enzyme hexokinase phosphorylates (adds a phosphate group to) glucose in the cell's cytoplasm. In the process, a phosphate group from ATP is transferred to glucose producing glucose 6-phosphate.

Glucose (C6H12O6) + hexokinase + ATP → ADP + Glucose 6-phosphate (C6H13O9P)

Step 2

The enzyme phosphoglucoisomerase converts glucose 6-phosphate into its isomer fructose 6-phosphate. Isomers have the same molecular formula, but the atoms of each molecule are arranged differently.

Glucose 6-phosphate (C6H13O9P) + Phosphoglucoisomerase → Fructose 6-phosphate (C6H13O9P)

Step 3

The enzyme phosphofructokinase uses another ATP molecule to transfer a phosphate group to fructose 6-phosphate to form fructose 1, 6-bisphosphate.

Fructose 6-phosphate (C6H13O9P) + phosphofructokinase + ATP → ADP + Fructose 1, 6-bisphosphate (C6H14O12P2)

Step 4

The enzyme aldolase splits fructose 1, 6-bisphosphate into two sugars that are isomers of each other. These two sugars are dihydroxyacetone phosphate and glyceraldehyde phosphate.

Fructose 1, 6-bisphosphate (C6H14O12P2) + aldolase → Dihydroxyacetone phosphate (C3H7O6P) + Glyceraldehyde phosphate (C3H7O6P)

Step 5

The enzyme triose phosphate isomerase rapidly inter-converts the molecules dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Glyceraldehyde 3-phosphate is removed as soon as it is formed to be used in the next step of glycolysis.

Dihydroxyacetone phosphate (C3H7O6P) → Glyceraldehyde 3-phosphate (C3H7O6P)

Net result for steps 4 and 5: Fructose 1, 6-bisphosphate (C6H14O12P2) ↔ 2 molecules of glyceraldehyde 3-phosphate (C3H7O6P)

Step 6

The enzyme triose phosphate dehydrogenase serves two functions in this step. First the enzyme transfers a hydrogen (H-) from glyceraldehyde phosphate to the oxidizing agent nicotinamide adenine dinucleotide (NAD+) to form NADH. Next triose phosphate dehydrogenase adds a phosphate (P) from the cytosol to the oxidized glyceraldehyde phosphate to form 1, 3-bisphosphoglycerate. This occurs for both molecules of glyceraldehyde 3-phosphate produced in step 5.

A. Triose phosphate dehydrogenase + 2 H- + 2 NAD+ → 2 NADH + 2 H+

B. Triose phosphate dehydrogenase + 2 P + 2 glyceraldehyde 3-phosphate (C3H7O6P) → 2 molecules of 1,3-bisphosphoglycerate (C3H8O10P2)

Step 7

The enzyme phosphoglycerokinase transfers a P from 1,3-bisphosphoglycerate to a molecule of ADP to form ATP. This happens for each molecule of 1,3-bisphosphoglycerate. The process yields two 3-phosphoglycerate molecules and two ATP molecules.

2 molecules of 1,3-bisphoshoglycerate (C3H8O10P2) + phosphoglycerokinase + 2 ADP → 2 molecules of 3-phosphoglycerate (C3H7O7P) + 2 ATP

Step 8

The enzyme phosphoglyceromutase relocates the P from 3-phosphoglycerate from the third carbon to the second carbon to form 2-phosphoglycerate.

2 molecules of 3-Phosphoglycerate (C3H7O7P) + phosphoglyceromutase → 2 molecules of 2-Phosphoglycerate (C3H7O7P)

Step 9

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

2 molecules of 2-Phosphoglycerate (C3H7O7P) + enolase → 2 molecules of phosphoenolpyruvate (PEP) (C3H5O6P)

Step 10

The enzyme pyruvate kinase transfers a P from PEP to ADP to form pyruvate and ATP. This happens for each molecule of phosphoenolpyruvate. This reaction yields 2 molecules of pyruvate and 2 ATP molecules.

2 molecules of phosphoenolpyruvate (C3H5O6P) + pyruvate kinase + 2 ADP → 2 molecules of pyruvate (C3H3O3-) + 2 ATP

Summary

In summary, a single glucose molecule in glycolysis produces a total of 2 molecules of pyruvate, 2 molecules of ATP, 2 molecules of NADH and 2 molecules of water.

Although 2 ATP molecules are used in steps 1-3, 2 ATP molecules are generated in step 7 and 2 more in step 10. This gives a total of 4 ATP molecules produced. If you subtract the 2 ATP molecules used in steps 1-3 from the 4 generated at the end of step 10, you end up with a net total of 2 ATP molecules produced.