Transverse section stained to show succinic dehydrogenase activity. This mitochondrial enzyme catalyses one stage of the Krebs' cycle. Aerobic muscle fibers are intensely stained; the anaerobic fibers are poorly stained. The citric acid cycle, also known as the tricarboxylic acid cycle (TCA cycle) or the Krebs cycle, is a series of enzyme-catalysed chemical reactions central to all living cells that use oxygen. In eukaryotes, the citric acid cycle occurs in the matrix of the mitochondrion.