The mitochondria are organelles found in all eukaryotic cells in which perform a very important function: they transform energy into useable ways to boost cellular reactions. This said in other words, is that they form ATP ( what is ATP? ) From other molecules such as fatty acids and sugars or carbohydrates.
This ability to form ATP in large quantities (much greater than those produced in glycolysis) means that its entire structure is adapted to this function.
Aerobic cellular respiration occurs in the mitochondria, which is the set of biochemical reactions by which, from glucose, other carbohydrates and fatty acids, the mitochondria generates ATP in the presence of oxygen.
Up to 38 ATP molecules can be generated from one glucose molecule, unlike glycolysis, which is an anaerobic and much less efficient process that produces only 2 ATP molecules without the presence of oxygen.
The chemical reactions that occur to generate ATP molecules in the mitochondria are known as the electron transport chain.
Morphology and structure of mitochondria
Mitochondria are traditionally described (and drawn) as elongated, rigid cylinders with a diameter between 0.5 and 1 microns, with a very bacterial-like appearance.
Nothing is further from the truth, mitochondria are very mobile and plastic constantly changing shape. Mitochondria can fuse and separate from each other and move through the cytoplasm via microtubules.
Depending on the type of cell, the distribution of mitochondria can vary, some can move in certain cells or remain fixed in others to more efficiently fulfill their function. For example, mitochondria in cardiac cells are clustered and packed between myofibrils adjacent to the heart muscle and can also be found coiled around the sperm flagellum.
Structurally (and functionally), what first catches the attention of mitochondria is its membrane. They have a double membrane: external and internal.
The outer membrane of the mitochondria is quite permeable for molecules of up to 10,000 daltons, so that even small proteins can pass through it due to the presence of large amounts of the transport protein called porin. Furthermore, in the outer membrane there are enzymes that transform lipid substrates into others that can be used in the matrix of the mitochondria.
The inner membrane is abundant and impermeable (thanks to the presence of the phospholipid cardiolipin) so that many of the molecules that cross the outer membrane cannot cross the inner one.
This membrane has a large number of ridges where the electron transport processes that will generate ATP take place. The number of crests of mitochondria depends on their location, for example, the number of crests of mitochondria in cardiac cells is three times higher than in liver cells. The ridges also have different morphologies depending on the cell type.
The inner membrane also serves to compartmentalize the internal environment that has enzymes that catalyze certain cellular reactions and therefore, the content of the internal environment is very specific. Three types of proteins are located in this membrane:
- The proteins that catalyze the reactions of the respiratory chain (oxidative phosphorylation): NADH dehydrogenase complex, bc complex and cytochrome oxidase complex, among others.
- An enzyme complex called ATP synthetase that catalyzes the production of ATP in the matrix.
- Electron transport proteins: cytochromes, quinone (or coenzyme Q, ubiquinone in mammals and plastoquinone in plants) and iron-sulfur proteins.
The impermeability of the inner membrane and the way of transporting metabolites and ions is key to maintaining the electrochemical gradient that allows the generation of ATP, but this will be seen in another article later on cellular respiration and the electron transport chain.
In the matrix of the mitochondria there is a highly concentrated mix of hundreds of different enzymes and molecules. Within these we find identical copies of the DNA of the mitochondria, mitochondrial ribosomes (mitorribosomes), tRNA, enzymes necessary for the expression of genes and the enzymes necessary for the oxidation of pyruvate and fatty acids.
Division of mitochondria
Mitochondria (as with chloroplasts) always arise from the growth and division of previous mitochondria and are never synthesized again.
As we saw previously, mitochondria can fuse with each other but they also divide to generate new mitochondria with a process similar to how bacteria do, through invaginations of the membrane.
Origin of mitochondria
The discovery of mitochondria cannot be attributed to just one person. Many were those who observed them (Cowdry, Flemming, Kolliker, Otto Heinrich Warburg, Maggi, Altman …) and gave a large number of names that Lenhinger tried to unify: blepharoplast, chondriocont, chondrimites, chondroblasts, … among many others.
The name that finally triumphed, mitochondria, is due to Carl Benda in 1889, although they were not isolated for the first time until 1934 from liver cells. It was not until 1948 that it was determined that they were the place where cellular respiration occurred thanks to Hogeboon, Schneider and Palade.
The evolutionary origin of mitochondria has been established within the framework of the endosymbiotic theory proposed by Lynn Margulis in 1967 from previous hypotheses of other scientists from the late nineteenth and early twentieth centuries whose hypotheses, although some of them are wrong in terms of details, are the basis of Margulis’s theory. At the time, these hypotheses were widely attacked and taken for nonsense, costing the prestige of many scientists of the time.
The endosymbiotic theory establishes that eukaryotic cells appeared by incorporating bacteria into them, and adapting the functions of different organelles. Margulis proposes that these incorporations were serialized and the full name of the theory is serial endosymbiotic theory. This theory is widely accepted by the scientific community although some discrepancies are still considered.
According to Margulis’ theory, mitochondria were incorporated in the second stage, from an aerobic bacterium , giving the ability to metabolize oxygen to the first protoequariotic organism that was anaerobic (the result of the fusion between an archaea and a spirochete). This was a very important evolutionary advantage that allowed other energy sources to be exploited in an environment very rich in oxygen. In a third stage, the bacteria that gave rise to the chloroplasts would be incorporated.
These fusions are produced by the phagocytosis of bacteria without them being digested, which is a frequent phenomenon in nature. The advantage provided by the aerobic bacteria by producing ATP for the functioning of the cell that it phagocytes and the advantage provided by the larger cell in terms of a medium rich in nutrients, represents a perfect symbiosis.
One of the points on which this theory is based is the similarity that exists between mitochondria and bacteria such as structure, size, function … in addition to the fact that mitochondria have their own DNA although joint evolution has achieved that part of this DNA passes into cellular DNA, and that eukaryotic cells without mitochondria are not viable and vice versa.
- In most cells, during cell interface, mitochondria divide in preparation for mitosis.
- Organelle DNA replication occurs throughout the cell cycle, apparently randomly choosing the DNA molecules to be replicated.
- A cell is capable of increasing or decreasing the number of mitochondria according to its needs, for example, a muscle that begins to be stimulated sees how the number of mitochondria increases.
- The genome of mitochondria is usually circular and its size depends on the type of cell. There are several copies of mitochondrial DNA molecules.
- In mammals, mitochondrial DNA accounts for less than 1% of the total body DNA, but in plants this number can vary significantly up to 15% in corn leaves.
- Mitochondria can carry out DNA replication and protein synthesis with proteins unique to mitochondria that are curiously encoded in the cell nucleus and not in mitochondrial DNA.
- In mitochondria, 4 of the 64 codons have a different meaning than they have in other genomes, for example, UGA is not a stop signal but rather it means tryptophan.
- The translation of RNA in mitochondria is more relaxed, tolerating more errors than in the cell nucleus.
- Due to the above, there is a high rate of evolution of mitochondrial genes that allows comparison of sequences to establish dates of evolutionary events.
- The mitochondrial genome in plants is much larger than in animals.
- The inheritance of mitochondrial genes does not follow a Mendelian pattern since they are inherited by segregation in the cytoplasm and do not follow the pattern of nuclear genes.
- In general, in humans and in many organisms, mitochondria are maternally inherited, although there are documented exceptions . This is because the egg contributes much more cytoplasm to the egg than the sperm.
- Chloroplasts have more similarities to bacteria than mitochondria.
- Mitochondria do not usually synthesize their phospholipids but rather import them from those manufactured in the endoplasmic reticulum.