Mitochondria are organelles within eukaryotic cells that produce adenosine triphosphate (ATP), the main energy molecule used by the cell. For this reason, the mitochondrion is sometimes referred to as “the powerhouse of the cell”. Mitochondria are found in all eukaryotes, which are all living things that are not bacteria or archaea. It is thought that mitochondria arose from once free-living bacteria that were incorporated into cells.
Every living thing is made of cells: tiny compartments contained by a membrane. Cells are the smallest things that can reproduce themselves. When we look inside cells, we see that they have sub-compartments that are smaller still, known as “Organelles” which perform different functions that are essential for the cell to live.
Mitochondria are organelles found in the cells of every complex organism. They produce about 90% of the chemical energy that cells need to survive. No energy; no life! So it’s easy to see why when mitochondria go wrong, serious diseases are the result, and why it is important we understand how mitochondria work.
However, mitochondria do much more than just produce energy. They also produce chemicals that your body needs for other purposes, break down waste products so they’re less harmful, and recycle some of those waste products to save energy.
Mitochondria also have a special role in making cells die (apoptosis). This may sound strange, but it is vital for the processes of growth and development. Sometimes cells don’t die when they should, and start to grow uncontrollably. This is how a tumor starts to grow, so you shouldn’t be surprised that mitochondria play an important part in cancer and are seen as targets for anti-cancer drugs.
To produce all of that energy, mitochondria require oxygen. Mitochondria effectively burn your food in a carefully controlled way to produce that chemical energy by a process called “oxidative phosphorylation”. And just as a fire goes out without oxygen, if mitochondria lack oxygen, they also stop working => No energy; − No life!
During a heart attack, or a stroke, the blood stops delivering oxygen to the heart and brain. These two organs do a lot of work and need a lot of energy. Without oxygen, the mitochondria stop working, and the cells in the brain or heart are damaged or even die. Perversely, if the oxygen does return, then the mitochondria get overwhelmed and produce a lot of “free radicals”. These are very reactive chemicals which cause a lot of additional damage – called “Reperfusion injury”.
Where did the Mitochondria Come From?
If you look at mitochondria in detail, they look a lot like miniature cells themselves, so how did they arise? We know that mitochondria were originally bacteria. About 1,500,000,000 years ago, a bacterial cell was engulfed by another cell, but rather than killing each other, the two cells worked together, probably because it was beneficial to each cell.
Mitochondria have their own DNA
One reason we know that mitochondria came from bacteria is that they still contain a tiny amount of DNA that is similar to bacterial DNA. Mitochondrial DNA is about 16,000 bases long and has 37 genes (in humans). The DNA in the nucleus – sequenced during the human genome project – is 3,000,000,000 bases long and has about 25,000 genes. So only about 0.1% of your genes are in your mitochondria, but the mitochondrion needs more than the 37 genes on the mitochondrial genome to work. We think about another 1,500 genes are needed, and they are on the nuclear genome.
Here’s another strange fact about mitochondria – You only get them from your mother. This is because when sperm fertilize an egg, they only pass on the DNA from their nucleus, not their mitochondria. The embryo has all its mitochondria from the mother’s egg. This means that mitochondrial diseases due to mutations on the mitochondrial DNA are only passed on by the mother – they can affect both her sons and daughters – but it will be only her affected daughters who may pass the disease on to their children. However, if the mutations are on the nuclear DNA, then they can be inherited from both the mother and the father.
Who discovered mitochondria?
Robert Altmann discovered mitochondria in 1886, thinking that he was seeing parasites.
Altmann was a German pathologist who theorized that these bio blasts were actually fundamental, living parts of the cell, but his ideas were criticized at first. He believed that bio blasts were cells that had, through a process called endosymbiosis, been absorbed into eukaryotic cells. With the invention of the electron microscope in 1962, scientists were able to study mitochondria further.
The future of the mitochondria
The past few years have seen a tremendous surge of interest in the biology of mitochondria, partly with the re-emergence of metabolism as a focus of topical interest as well as fundamental importance, and partly through the changing perception of their cell biology, and their recognition as a regulated dynamic network engaging with the other membrane systems of the cell, transmitting and receiving signals on its metabolic status, and serving as a launching-pad for programmed cell death.
Reflecting this explosion of new interest and changing perspective, BMC Biology, Cancer & Metabolism, Extreme Physiology & Medicine, and Longevity & Healthspan are launching a cross-journal series of commissioned articles and research papers that will cover every aspect of mitochondrial biology, from the still-contentious origins of this ancient organelle to the current understanding of its activities as a metabolic and signaling hub.
The special adaptations required to allow the unrestrained growth of cancer cells, and in response to the extremes of hypoxia, starvation and endurance exercise, are the territory of Cancer & Metabolism and Extreme Physiology & Medicine. Articles in Longevity & Healthspan tackle the important and often contentious issues of the part played by mitochondria in maintaining health and lifespan. BMC Biology, with a scope that extends across all of biology, aspires to provide overview reviews for non-specialists on the topics examined in detail in its sister journals, and to extend the series beyond these topics and our own species, to embrace everything from human prehistory to the structural biology of the respiratory chain.
In South Korea, researchers have developed a simple way to deliver healthy mitochondria to cells where it is dysfunctional, rescuing the cells’ energy levels and metabolic function. The study, “Delivery of exogenous mitochondria via centrifugation enhances cellular metabolic function,” was published in the Journal Scientific Reports. Defects in mitochondria, the small organelles that are the cell’s powerhouses, contribute to several health problems, including aging, cancer, metabolic disorders and neurodegenerative diseases.
Scientists have developed therapies aimed at rescuing mitochondria’s function. But they are limited to cases where a gene mutation underlies the defects. Researchers have begun exploring a new strategy — replacing damaged mitochondria with healthy ones. But a problem scientist has faced is that “mitochondrial transfer methods are inefficient and time-consuming,” the South Korean researchers wrote.
They came up with a new way to deliver healthy mitochondria. The first step is to mix healthy mitochondria with the cells where they need to go. The next step is spinning the mixture — a process called centrifugation — to combine the mitochondria and cells. The team spun a mixture of mitochondria from human umbilical cord-derived mesenchymal stem cells and cells that needed healthy mitochondria.
They tried the process with more than one type of cell. One experiment involved mixing mitochondria with the same human cells that had donated the mitochondria. Another was mixing human mitochondria with rat muscle cells. An important finding was that human mitochondria ended up in the rat muscle cells, but rat mitochondria did not increase in the muscle cells. This meant that mitochondria from outside the muscle cells was being successful delivered inside the cells.
When the team compared the efficiency of its mitochondria delivery method with a method called passive transfer, they discovered that their method was much more effective. “The results from centrifugal transfer indicate that mitochondria can pass through the cell membrane more easily than using the passive transfer approach,” they wrote.
The team maintained the higher efficiency level when they used three other types of cells, including stem cells and cancer cells. The key finding was that the transferred mitochondria were able to restore lost mitochondrial function, including generating energy and the reactive oxygen species that take part in metabolism. “This simple and rapid mitochondrial transfer method can be used to treat mitochondrial dysfunction-related diseases,” the team concluded.
Mitochondrial disease research
Mitochondrial diseases affect patients of all ages – the more severe the mitochondrial defect the earlier the onset is likely to be. The disease can start immediately after birth with severe muscle weakness, heart involvement and impaired overall brain function. Unfortunately, these children die in the first few days of life.
More commonly mitochondrial disease presents later in childhood, in adolescence and in adulthood. In the vast majority of patients this is a progressive illness with major disability and often leading to early death (certainly in those patients who present in childhood with severe symptoms).
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