Mitochondria are the cell powerhouse of eukaryotic cells. The main function of mitochondria is the production of adenosine triphosphate (ATP) via a process known as oxidative phosphorylation. ATP is a nucleotide that stores energy in the form of chemical bonds. Energy is derived from cell nutrients, mostly from glucose and fatty acids, and released whenever it is needed by the energy-requiring functions of cells. These energy-requiring cellular functions are membrane transport, synthesis of compounds for driving metabolic reactions, and mechanical work.
These structures are double membrane-bound cytoplasmic organelles found in most eukaryotic cells. They are flexible and rod shaped, and their diameter ranges from 0.5 to 1.0 micrometer. The membrane system of a mitochondrion consists of a smooth outer mitochondrial membrane and a folded inner mitochondrial membrane, separated by a narrow space called the intermembrane space. Matrix space, or intercristal space, is the large space enclosed by the inner membrane. Each of these components plays significant roles in cellular function and contribute to the main function of mitochondria.
The outer mitochondrial membrane has a large number of porins, which are transmembrane protein channels that permit the free diffusion of large and small molecules. It can allow molecules that are as large as 10 kilodaltons and as small as 6,000 daltons. This membrane is relatively permeable to ions and small molecules, thus the contents of the intermembrane space resemble the cytosol.
Nevertheless, the major function of mitochondria is found in the inner mitochondrial membrane and the matrix space. The inner mitochondrial membrane is folded into cristae, which provide a larger surface area for synthesis of ATP. This membrane possesses a large number of cardiolipins, which are phospholipids that make the latter nearly impermeable to protons, electrons, and ions. ATP synthase and respiratory chains, both protein complexes, are also found in this membrane. The ATP synthase is responsible for ATP generation, whereas the respiratory chains maintain the proton gradient that provides energy for oxidative phosphorylation.
The matrix space is filled with a dense fluid mostly composed of enzymes responsible for the degradation of fatty acids and pyruvate into the metabolic intermediate acetyl coenzyme A, and subsequent oxidation of this intermediate in the Krebs or tricarboxylic acid cycle. Pyruvate is an initial product of glucose metabolism that occurs in the cytosol, which is then transported into the mitochondria. The matrix space also contains the mitochondrial genetic system, the double-stranded mitochondrial circular deoxyribonucleic acid (cDNA) and the enzymes necessary for mitochondrial genome expression. Although it has its own genetic system, the necessary gene-encoded proteins needed for the formation and function of mitochondria to be normal are located in the genome of the cell’s nucleus.