The Krebs cycle takes place in the mitochondrial matrix. The mitochondrial matrix is the part of a cell that holds its DNA, ribosomes and the enzymes that catalyze the Krebs cycle.
The Krebs cycle, also known as the citric-acid cycle, is a series of chemical reactions that create energy for all organisms that need oxygen to survive. During the cycle, pyruvate is introduced into the mitochondrial matrix, and, through oxidation, is metabolized into adenosine triphosphate, a chemical form of energy. Carbon dioxide is produced as a by-product of the reaction, as well as other chemicals that are necessary for the formation of amino acids.