The mitochondrial matrix also contains the mitochondria's DNA and ribosomes. The word "matrix" stems from the fact that this space is viscous, compared to the relatively aqueous cytoplasm. The cytosolic compartment has a water content of 3.8 μl/mg protein, while the mitochondrial matrix 0.8 μl/mg protein (Soboll S et al, in "Use of of Isolated Liver Cells and Kidney Tubules in Metabolic Studies" pg 29-40, Academic Press, New York and London). It is not known how mitochondria maintain osmotic balance across the inner mitochondrial membrane, although the membrane contains aquaporins that are believed to be conduits for regulated water transport.
Citric acid cycle
The citric acid cycle (or Krebs cycle or TCA cycle) takes place within the mitochondrial matrix. In this cycle, pyruvic acid generated from glycolysis is converted into acetyl coenzyme A (acetyl CoA) by losing a carbon dioxide molecule. It then combines with oxaloacetic acid to form citric acid, a six-carbon molecule. In total, it loses 2 CO2 molecules and 4 electrons, of which 3 are accepted by NAD+ to reduce it to NADH, and the last electron accepted by FAD+ to reduce to FADH2 in redox reactions. In the end, it regenerates oxaloacetic acid to continue the citric acid cycle. In addition, a single GTP molecule is created from the combination of GDP and a phosphate group.
Since 2 pyruvic acid molecules are formed by glycolysis, each time a cell undergoes glycolysis two turns of the citric acid cycle will occur. That means that the citric acid cycle produces a total of 6 NADH, 2 FADH2, and 2 GTP molecules.
Electron transport chain
The electron transport chain is located in the cristae of the inner mitochondrial membrane. The NADH and FADH2 produced by the citric acid cycle in the matrix release a proton and electron to regenerate NAD+ and FAD+. The proton is pulled into the intermembrane space by the energy of the electrons going through the electron transport chain. The electron is finally accepted by oxygen in the matrix. The protons return to the mitochondrial matrix through the process of chemiosmosis through the protein ATP synthase.