Citric acid cycle

Definition

The citric acid cycle, also known as the TCA cycle (TCA = tricarboxylic citric acid) or the Krebs cycle, is a series of chemical reactions that occur inside the cell that is part of the process of respiration to generate energy (both aerobic respiration and anaerobic respiration). In eukaryotic cells, this cycle occurs inside the mitochondrion, whereas in prokaryotic cells, this cycle occurs directly within the cytoplasm.

Summary

Item	Value
Type of organisms in which the citric acid cycle occurs	All organisms that carry out cellular respiration, whether aerobic or anaerobic; in particular this includes all eukaryotes and many prokaryotes.
Type of cells within the organism that undergo the citric acid cycle	Prokaryotes are unicellular, so if the organism supports the citric acid cycle, its only cell will. In eukaryotes, all cells that contain mitochondria support the citric acid cycle. This is almost all cells; the main exception is red blood cells in mammals.
Places in the cell where the citric acid cycle occurs	In eukaryotic cells, this cycle occurs inside the mitochondrion (and specifically, within the mitochondrial matrix, that is on the inside of the inner mitochondrial membrane), whereas in prokaryotic cells, this cycle occurs directly within the cytoplasm. The locations for the two kinds of cells correspond to each other per the endosymbiotic theory of mitochondrial origin.
Substrate (input or reactant) for the process	Acetyl-CoA, that in turn is obtained from pyruvic acid via pyruvate decarboxylation; the pyruvic acid in turn is generated as an output of glycolysis in the cytosol. There are also various other intermediates that mainly change in terms of oxidation states, protons, or phosphates. The intermediates are produced through glycolysis.
Products of the process	Coenzyme A, carbon dioxide, and updated versions of the intermediates. For the most part this sort of cancels out the changes made during glycolysis.
Energy change	The process releases energy (how much?). The released energy is stored chemically by converting ADP to ATP or by converting GDP into GTP.