In the mitochondrion

Mitochondrion

In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

, the matrix contains soluble enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s that catalyze

Catalysis

Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....

 the oxidation of pyruvate

Pyruvic acid

Pyruvic acid is an organic acid, a ketone, as well as the simplest of the alpha-keto acids. The carboxylate ion of pyruvic acid, CH3COCOO−, is known as pyruvate, and is a key intersection in several metabolic pathways....

 and other small organic molecules.

The mitochondrial matrix also contains the mitochondria's DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 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

Osmosis

Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

 balance across the inner mitochondrial membrane, although the membrane contains aquaporin

Aquaporin

Aquaporins are proteins embedded in the cell membrane that regulate the flow of water.Aquaporins are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells....

s that are believed to be conduits for regulated water transport.

Citric acid cycle

The citric acid cycle

Citric acid cycle

The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...

 (or Krebs cycle or TCA cycle) takes place within the mitochondrial matrix. Prior to the citric acid cycle, pyruvic acid

Pyruvic acid

Pyruvic acid is an organic acid, a ketone, as well as the simplest of the alpha-keto acids. The carboxylate ion of pyruvic acid, CH3COCOO−, is known as pyruvate, and is a key intersection in several metabolic pathways....

 generated from glycolysis

Glycolysis

Glycolysis is the metabolic pathway that converts glucose C6H12O6, into pyruvate, CH3COCOO− + H+...

 is converted into acetyl coenzyme A (acetyl CoA) by losing a carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 molecule. It then combines with oxaloacetic acid

Oxaloacetic acid

Oxaloacetic acid is an organic compound with the chemical formula C4H4O5 or HOOC---COOH. It also has other names ....

 to form citric acid

Citric acid

Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

 (a six-carbon molecule) to begin the citric acid cycle. In total, it loses 2 CO₂ molecules and 8 electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

s, 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

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

. In the end, oxaloacetate is regenerated to continue the citric acid cycle. In addition, a single GTP molecule is created from the combination of GDP 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 GTP molecules.

Electron transport chain

The electron transport chain

Electron transport chain

An electron transport chain couples electron transfer between an electron donor and an electron acceptor with the transfer of H+ ions across a membrane. The resulting electrochemical proton gradient is used to generate chemical energy in the form of adenosine triphosphate...

 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

Intermembrane space

The intermembrane space also known as IMS is the region between the inner membrane and the outer membrane of a mitochondrion or a chloroplast. The main function of the intermembrane space is oxidative phosphorylation....

 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

Chemiosmosis

Chemiosmosis is the movement of ions across a selectively permeable membrane, down their electrochemical gradient. More specifically, it relates to the generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration....

 through the protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 ATP synthase

ATP synthase

right|thumb|300px|Molecular model of ATP synthase by X-ray diffraction methodATP synthase is an important enzyme that provides energy for the cell to use through the synthesis of adenosine triphosphate . ATP is the most commonly used "energy currency" of cells from most organisms...

.