Citric acid cycle
The citric acid cycle occurs after glycolysis only if oxygen is present (it is an aerobic process).
The pyruvate enters the matrix of the mitochondria and carbon dioxide is removed.
The removal of carbon forms an acetyl group. This acetyl group combines with coenzyme A to form acetyl coenzyme A.
In the citric acid cycle the acetyl from acetyl coenzyme A combines with a molecule called oxaloacetate to form citrate (this is where the citric acid cycle gets its name).
Intermediate molecules are formed because the enzymes remove carbon (in the form of carbon dioxide) and hydrogen/electrons. The carbon dioxide is released as a by-product.
The citric acid cycle also results in the creation of ATP.
These enzyme-controlled intermediate steps gradually convert citrate back into oxaloacetate. This results in the generation of ATP and carbon dioxide is releases. The oxaloacetate is then able to combine with another acetyl group.
Dehydrogenase enzymes remove hydrogen ions and electrons from intermediates, which are passed to coenzymes NAD (forming NADH). The high-energy electrons are passed to the electron transport chain.