Glycolysis is the process by which the carbohydrate monomer glucose (6-carbon sugar) is catabolized (broken down) into two 3-carbon pyruvates. This process takes place in the cytosol of the cell as opposed to the mitochondria where oxygen-dependent catabolic processes take place in the cells. Glycolysis does not require oxygen.
Step 1: Glucose is phosphorylated (a phosphate group is added) to form glucose 6-phosphate. The reaction is catalyzed by hexokinase and requires 1 ATP.
Step 2: Glucose 6-phosphate is isomerized (phosphoglucose isomerase) to form fructose 6-phosphate.
Step 3: A phosphate group is added to fructose 6-phosphate to form fructose 1,6-bisphosphate. The reaction is catalyzed by phosphofructokinase and requires 1 ATP.
Step 4: Aldolase splits each fructose 1,6-bisphosphate into a glyceraldehyde 3-phosphate and a dihydroxyacetone phosphate. The dihydroxyacetone phosphate can be converted to form another glyceraldehyde 3-phosphate.
Step 5: Each glyceraldehyde 3-phosphate is then converted into 1,3-bisphosphoglycerate (remember this happens twice because there are two glyceraldehyde 3-phosphates formed per glucose molecule). This reaction is catalyzed by glyceraldehyde phosphate dehydrogenase and results in the capture of energy (electrons) as NAD+ (an electron carrier) picks up 2 electrons and a hydrogen to from NADH. This energy is used in a separate step of cellular respiration, the electron transport chain.
Step 6: This is the first step to generate ATP. Each 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by phosphoglycerate kinase. This happens twice per glucose molecule, generating 2 ATP. Overall, at this point there has been no net gain of ATP because 2 ATP were used in earlier steps.
Step 7: Each 3-phosphoglycerate is converted to 2-phosphoglycerate with phosphoglyceromutase.
Step 8: A water molecule is lost as each 2-phosphoglycerate is converted into phosphoenolpyruvate with enolase.
Step 9: The phosphate group on each of the two phosphoenolpyruates is used to produce an ATP (2 total). Each phosphoenolpyruvate is converted to a pyruvate by pyruvate kinase. This leads to a net gain of 2 ATP per glucose in glycolysis. Although 4 are produced during the later steps of the process, 2 ATP are used in early steps.
Each pyruvate can then be used to make an acetyl CoA molecule that can enter the next step of cellular respiration, the Kreb's cycle. Again, the 2 NADH (electron carriers) that are formed will enter the mitochondria and deliver electrons to the electron transport chain where additional ATP will be generated.