Mitochondrial matrix



In the mitochondrion, the matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules.

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. 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. Prior to the citric acid 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) to begin the citric acid cycle. In total, it loses 2 CO2 molecules and 8 electrons, of which 6 are accepted by three NAD+ molecules which get reduced to NADH, and the last two electrons are accepted by FAD to reduce to FADH2 in redox reactions. In the end, oxaloacetate is regenerated to continue the citric acid cycle. In addition, a single ATP molecule is created from the combination of ADP and a phosphate group via substrate-level phosphorylation.

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 ATP 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. Protons are pulled into the intermembrane space by the energy of the electrons going through the electron transport chain. Two electrons are finally accepted by oxygen in the matrix to complete the electron transport chain. The protons return to the mitochondrial matrix through the process of chemiosmosis through the protein ATP synthase.bhjlgh