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The mitochondrial matrix is the compartment enclosed by the inner mitochondrial membrane, containing mitochondrial DNA, ribosomes and many enzymes. Most citric acid cycle steps occur here; succinate dehydrogenase is membrane-associated. The citric acid cycle supplies NADH for the mitochondrial respiratory chain.[ref]
Molecular Biology of the Cell, 4th edition by Bruce Alberts and others. Chapter 14, The Mitochondrion Contains an Outer Membrane, an Inner Membrane, and Two Internal Compartments; Figure 14-8, matrix enzymes and inner-membrane succinate dehydrogenase
Distinguish the location of a chemical reaction from the location of its detection. An external oxygen electrode can report consumption by an intact preparation without entering the mitochondrial matrix. That signal does not locate every intermediate, establish protein targeting or directly count newly formed ATP.