The mitochondrion (sing.: mitochondria) is a eukaryotic membrane-enclosed cell organelle that generates chemical energy (in the form of ATP) needed for the metabolic activities of the cell. The rod-shaped organelle measures from 0.5 to 10 micrometers and located in the cytosol. It is considered the “power plant” or “power house” of the cell because it produces most of the ATP needed to carry out all the biochemical processes within the cell. It also has its own DNA separate from the nuclear DNA.
The discovery of mitochondria has a long history. Many scientists studied the functions and structures of mitochondria since the 1850s.
The mitochondrion has a complex structure and biochemistry. In terms of structure, the mitochondrion is consists of the outer mitochondrial membrane, the inner mitochondrial membrane, the intermembrane space, the cristae, and the matrix.
The general function of the mitochondrion is energy conversion (food to energy). Its other functions include calcium storage, heme and steroid synthesis, calcium signaling, apoptosis, and regulation of membrane potential, cell proliferation, and cell metabolism. There are mitochondria that have specialized functions, like the liver mitochondria which contain enzymes for metabolic detoxification.
Structure of Mitochondria
Outer Mitochondrial Membrane
This structure is composed of phospholipids and proteins (1:1 mol. weight ratio) just like eukaryotic cell membrane. It encloses the entire organelle and protects all structures found within the mitochondrion.
The outer membrane contains a lot of integral proteins called porins, which form channels where small molecules pass through. The porins are very selective; they only allow the entry of small molecules with molecular weight of 5000 daltons and below. Larger molecules (e.g. protein) are only allowed to pass the outer membrane through the aid of translocases.
There is a structure that connects the outer mitochondrial membrane to the endoplasmic reticulum (ER). This structure is called the mitochondria-associated ER-membrane (MAM) and involved in the transfer of lipids from the ER into the intermembrane space (the space between the outer membrane and the inner mitochondrial membrane). The structure is also needed for calcium signaling.
Inner Mitochondrial Membrane
This membrane is also made up of proteins and phospholipids but the amount of proteins is three times higher than the amount of phospholipids by molecular weight. There are five types of proteins in the inner membrane in terms of function: proteins involved in redox reactions of oxidative phosphorylation; proteins that act as ATP synthases which generate ATP in the matrix; proteins involved in protein import machinery; proteins that regulate the passage of metabolites into and out of the matrix; and lastly, the mitochondria fusion and fission protein.
An unusual plasma membrane in the inner membrane is the cardiolipin which primary function is to make the membrane more impermeable to molecules.
Unlike the outer membrane, there are no porins in the inner membrane and almost all molecules need transporters (translocases) to pass through the membrane and reach the matrix.
Moreover, it is in the inner membrane where the electron transport chain happens. Proteins in the inner membrane create a membrane potential, which when released, generates ATP.
The intermembrane space is the space between the outer and inner mitochondrial membranes. Small molecules from the cytosol such as ions and sugars are transported here through the porins. Proteins with small molecular weight are also transported here through the action of translocases. One important protein that is located in the space is the cytochrome c.
The cristae (sing.: crista) are compartments produced through the invagination or folding of the inner membrane. The cristae increase the surface area of the inner membrane to enhance energy production. The cristae have high density of proteins involved in ATP generation. Cells that have greater demand for ATP such as the muscles (for contraction), have more cristae than ordinary cells with low ATP demand.
This is the space enclosed by the inner membrane and highly important for ATP production through the aid of ATP synthases located in the inner membrane. The matrix contains a high concentration of enzymes involved in the citric acid cycle as well as in the oxidation of pyruvate and fatty acids.
The mitochondrial genome is located in the matrix. It contains the complete instruction for the biosynthesis of mitochondrial protein.
The matrix contains the materials needed for mitochondrial protein synthesis which include ribosomes and RNA (tRNA and rRNA).