What happens in Citric acid cycle?

Cellular respiration generally involves breaking down of large organic molecules to release ATP (energy). Citric Acid cycle, also known as Kreb's cycle or Tricarboxylic acid cycle is the second stage of the cellular respiration (unique to aerobic organisms). Citric acid cycle occurs in the intracellular space or matrix of the mitochondria of eukaryotes.

Glycolysis, which is the first step of cellular respiration, produces pyruvate which is then converted to Acetyl CoA in order to enter the Kreb's cycle by first combining with oxaloacetate. Generally, citric acid cycle involves an eight-steps reaction consisting of series of reduction-oxidation, hydration, dehydration, decarboxylation reactions, with each step catalyzed by different enzymes.

In a nutshell, oxaloacetate is generated back at the completion of the cycle alongside 2 molecules of CO2, one GTP/ATP molecule and electron donors; NADH2 and FADH2. These reduced electron donors enter the third step of aerobic cellular respiration and act as the first electron donor in the Electron transport chain.