If you break apart the word glycolysis, you get "sugar" from glyco- and "splitting" from -lysis. Glycolysis is a metabolic process in which glucose is split into two three-carbon molecules known as pyruvate. There are ten enzyme-mediated reactions that occur as part of glycolysis, which we will not go into detail in here. The ten steps can be split into the "preparatory phase," which required two ATP, and the "payoff phase," which produces two NADH, four ATP, and two water molecules.

Glycolysis occurs in the cytoplasm of almost every cell, both prokaryotic and eukaryotic, which makes it a strong piece of evidence for common ancestry.

In the second step of cellular respiration, the pyruvate previously made is oxidized. The pyruvate moves into the mitochondrial matrix through the use of pores and transport proteins known as pyruvate transporters.

Inside the matrix. there is something known as the pyruvate dehydrogenase complex, which uses three enzymes and five coenzymes. In this process, oxidative decarboxylation occurs, which removes the carboxyl group from pyruvate and releases it as a molecule of carbon dioxide. The remaining two carbons from the pyruvate become an acetyl group, which will be bound to Coenzyme A, to create acetyl-CoA. This oxidation, which centers around the transfer of electrons, produces a molecule of NADH.

This reaction will occur twice for each glucose molecule that we started with, as each glucose molecule produces two pyruvates.

The acetyl-CoA that was created in the last step will be used in a cycle of reactions known as the Krebs cycle, or the citric acid cycle, which also occurs in the mitochondrial matrix. There are eight enzyme-mediated steps to this metabolic process, four of which are oxidations. Through these oxidations, three molecules of NADH and one molecule of FADH2 are produced.

With each turn of this cycle, two carbons are added via the acetyl-CoA. In turn, two molecules of carbon dioxide are released with each cycle. In addition, one molecule of ATP, or GTP, is produced as a product of the cycle.

Isozymes are enzymes that have different structures, but complete the same function. Succinyl-CoA synthetase is an enzyme with isozymes that mediates the step of the cycle in which ATP or GTP is produced, depending on which isozyme is functioning as the catalyst. GTP, or guanosine triphosphate, is an analog to ATP that has guanine rather than adenine, but still functions as cellular energy.