Mitochondria definition
Mitochondria refers to a cellular organelle responsible for production of energy (ATP) necessary for cell optimal functioning.
Mitochondria origin and synthesis
The existing theory suggests that mitochondria is a semiautonomous organelle, which was originated in endocytosis of a bacterium by an ancestral nucleated cell. This theory, not yet fully confirmed, is called the Endosymbiotic Theory.
In cell, mitochondria is not synthesized again with each cell division, on the contrary it increases in size (usually to double) and then divides into two equal halves (approximately) to daughter cells at cell division.
Mitochondrial growth is a complicated process because its proteins are encoded in both nuclear and mitochondrial genome (genome of the organelle itself). These proteins are synthesized in free ribosomes in cell cytosol and, due to a signal sequence added to protein, they are transported into mitochondria. It is known that a large percentage of proteins transport between cytosol and mitochondria occurs only in this direction. Mitochondria transport to outside only occurs in case of apoptosis, i.e., when it is initiated programmed cell death.
Mitochondrial genome
Mitochondria have a characteristic that makes it very different from all other organelles present in animal cell: it contains its own genome – the mitochondrial DNA. This particularity of mitochondria is shared only by chloroplasts, which exist in plant cells.
Mitochondrial DNA is small, containing only 6000 nucleotides in protozoa and reaching 300,000 nucleotides in plants. In mammals, mitochondrial DNA makes up less than 1% of the cell’s genome (about 10,500 nucleotides), encoding 13 genes that give rise to proteins and RNAs involved in oxidative phosphorylation and electron transport chain. Being a genome of its own, it has some characteristics that make it different from cellular genome, such as:
- it is independent and separate from the nuclear genome;
- it is circular;
- it has a high content of genes, that is, almost all nucleotides are part of a coding sequence;
- pairing codons with respective anticodons is very flexible in the third position;
- codons have different meanings from those described in genetic code, giving rise to different amino acids and
- it is passed on to succeeding generations by a pattern of non-Mendelian inheritance, being passed down from mother to daughter.
Mitochondria Features and Functions
Mitochondria exist as an organelle in cell’s cytosol and their number and shape varies greatly within same cell and between different cell types. This organelle is very important in cellular respiration and consequent production of ATP (form of energy used by cell to stay in action). For example, myocardial muscle cells (smooth muscle cells that make up the heart wall itself) contain large amounts of mitochondria when compared to cells in intestinal mucosa. This is because cells in heart have a greater need for ATP, so they need more mitochondria in their cytosol.
Normally, this organelle has a form of elongated cylinder and presents a great mobility and plasticity in cellular cytoplasm. Because of this great mobility, mitochondria are thought to be associated with cytoplasm microtubules.
In its constitution (figure 1), mitochondria contain two membranes: the inner membrane (full of ridges that extend into the organelle) and the outer membrane. These two membranes are separated by intermembrane space. Inside, inner membrane delimits the matrix. The outer membrane consists of many transport proteins (porins), however, most of transported molecules remain in intermembrane space, not being able to pass through inner membrane. Enzymes present in mitochondrial matrix are those involved in ‘citric acid cycle’, name given to the mechanism by which this organelle metabolizes pyruvate and/or fatty acids in NADH and CO2. These electrons then pass to the inner membrane, where they enter the electron transport chain forming NAD+ and FADH2, a mechanism called by ‘oxidative phosphorylation’. At the end, there is production of ATP by an enzyme complex present on the inner membrane. All these processes are part of respiratory chain that leads to release of O2 and ATP to intermembrane space and, when necessary, to cell cytoplasm.
Figure 1 – Schematic image of mitochondria constituents, indicating the different components names.
References:
Alberts B., Johnson A., Lewis J., Raff M., Keith R., Walter P. (2007). Molecular Biology of the Cell (5th edition). Garland Science, New York.
Berg J.M., Tymoczko J.L., Stryer L. (2002). Biochemistry (5th edition). W. H. Freeman, New York.
Cooper G.M. (2000). The Cell: A Molecular Approach (2th edition). Sinauer Associates, Sunderland (MA).
