People often refer to mitochondria as the powerhouses of the cell. Their main function is to generate the energy necessary to power cells, but they are also involved in cell signaling and death.
Present in nearly all types of human cells, mitochondria are vital to survival. They generate the majority of our adenosine triphosphate (ATP), the cell’s energy currency.
Mitochondria are also involved in other tasks, such as signaling between cells and cell death, otherwise known as apoptosis.
Mitochondria are small, often between
Unlike other organelles (miniature organs within the cell), mitochondria have
Mitochondria have different compartments or regions, each carrying out distinct roles. These include the following:
- Outer membrane: Small molecules can pass freely through this. This outer portion includes proteins called porins, which form channels that allow proteins to cross. It also hosts several enzymes with a wide variety of functions.
- Intermembrane space: This is the area between the inner and outer membranes.
- Inner membrane: This membrane holds proteins that have several roles. There are no porins in the inner membrane, so molecules can only cross it in special membrane transporters. This is where most ATP is created.
- Cristae: These are the folds of the inner membrane. They increase the membrane’s surface area, therefore increasing the space available for chemical reactions.
- Matrix: This is the space within the inner membrane. Containing hundreds of enzymes, it is important in the production of ATP. Mitochondrial DNA (mtDNA) is here.
Different cell types have different numbers of mitochondria. Cells with a high demand for energy tend to have greater numbers of mitochondria. For instance, cells in the liver, kidneys, and muscles tend to have lots of mitochondria.
Mitochondria take up around 40% of the cytoplasm in heart muscle cells.
Although pictures often depict mitochondria as oval-shaped organelles, they constantly divide (fission) and bond together (fusion). So, in reality, these organelles are linked together in ever-changing networks.
Also, in sperm cells, the mitochondria spiral in the midpiece and provide energy for tail motion.
Although most of our DNA is in the nucleus of each cell, mitochondria have their own set of DNA. Interestingly, mtDNA is more similar to bacterial DNA.
The mtDNA holds the instructions for several proteins and other cellular support equipment across 37 genes.
The human genome, stored in the nuclei of our cells, contains around
During reproduction, half of a child’s DNA comes from their father and half from their mother. However, the child
Although the best-known role of mitochondria is energy production, they also carry out other important tasks.
In fact, only about 3% of the genes needed to make a mitochondrion go into its energy production equipment. The vast majority are involved in other jobs that are specific to the cell type where they occur.
Below are some of the mitochondria’s roles:
Producing energy
ATP, a complex organic chemical found in all life forms, is often referred to as the molecular unit of currency because it powers metabolic processes. Most ATP is produced in mitochondria through a series of reactions, known as the citric acid cycle or the Krebs cycle.
Energy production mostly takes place on the folds or cristae of the inner membrane.
Mitochondria convert chemical energy from food into a form of energy the cell can use. This process is called oxidative phosphorylation.
The Krebs cycle produces a chemical called NADH. Enzymes in the cristae use NADH to produce ATP. ATP molecules store energy in the form of chemical bonds. When these chemical bonds break, the energy can be used.
Cell death
Cell death, also called apoptosis, is an essential part of life. As cells age or break, they are cleared away and destroyed. Mitochondria help decide which cells are destroyed.
Mitochondria release cytochrome C, which activates caspase, one of the chief enzymes involved in cell destruction during apoptosis.
Because certain diseases, such as cancer, involve a breakdown in regular apoptosis, some research suggests that mitochondrial dysfunction plays a role in the disease.
Storing calcium
Calcium is
Calcium is also necessary for muscle function and blood clotting, among other things.
Because calcium is so critical, the cell regulates it tightly. Mitochondria play a part in this by absorbing calcium ions and holding them until they are necessary.
Heat production
When we are cold, we shiver to keep warm. However, the body can also generate heat in other ways, one of which is by using a tissue called brown fat.
During a process called proton leak, mitochondria can generate heat. This is known as non-shivering thermogenesis.
The mtDNA is more prone to damage than the rest of the genome. This is because free radicals, which can cause DNA damage, are
However, the
When mitochondria stop functioning, the cell they are in is starved of energy. So, depending on the type of cell, symptoms can vary widely.
As a general rule, cells that need the largest amounts of energy, such as heart muscle cells and nerves, are
Symptoms of mitochondrial disease
Diseases that generate different symptoms but are due to the same mutation are called genocopies.
Conversely, diseases with the same symptoms but that are due to mutations in different genes are called phenocopies. An example of a phenocopy is Leigh syndrome, which can be due to several different mutations.
Although symptoms of a mitochondrial disease vary greatly, they might include:
- loss of muscle coordination and weakness
- problems with vision or hearing
- heart, liver, or kidney disease
- gastrointestinal problems
- neurological problems, including dementia
Examples of mitochondrial disease
Some of the
- Barth syndrome, a rare genetic condition that affects the metabolism of lipids (fatty compounds)
- chronic progressive external ophthalmoplegia, a chronic, progressive condition that can cause weakness and an inability to move the eyes
- Kearns-Sayre syndrome, which affects the retina
- Leigh syndrome
- mitochondrial DNA depletion syndromes, which cause progressive muscle weakness
- Pearson syndrome, which causes anemia and pancreatic issues
Over recent years, researchers have investigated a link between mitochondrial dysfunction and aging. There are various theories surrounding aging, and the mitochondrial free radical theory of aging has become popular over the last decade or so.
The theory is that mitochondria produce reactive oxygen species (ROS) as a byproduct of energy production. These highly charged particles damage DNA, fats, and proteins.
ROS can damage the functional parts of mitochondria. When the mitochondria can no longer function so well, more ROS are produced, worsening the damage further.
Although researchers associate aging with a deterioration of mitochondrial functions and components, more research is necessary to fully understand this link and how scientists can use the information to support aging.
Mitochondria are, quite possibly, the best-known organelle. And, although people often refer to them as the powerhouse of the cell, they carry out a wide range of actions that are much less known about.
From calcium storage to heat generation, mitochondria are hugely important to our cells’ everyday functions.
Mutations or damage to mitochondrial DNA can also lead to certain disorders, such as Leigh syndrome.