Mitochondria: Form, Function, And Disease - Medical News Today

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Medical News TodaySubscribeWhat are mitochondria?

Medically reviewed by Jillian Foglesong Stabile MD, FAAFP, DABOM — Written by Tim Newman — Updated on May 27, 2025

Structure
DNA
Functions
Disease
Aging
Summary

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.

The structure of mitochondria

Mitochondria illustration. — Mitochondria structure. Illustrated by Jason Hoffman.

Mitochondria are small, often between 0.5 and 3 micrometers, and are not visible under a microscope unless they are stained.

Unlike other organelles (miniature organs within the cell), mitochondria have two membranes, an outer one and an inner one. Each membrane has different functions.

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.

Mitochondrial DNA

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 3 billion base pairs, whereas mtDNA consists of less than 17,000 base pairs.

During reproduction, half of a child’s DNA comes from their father and half from their mother. However, the child always receives their mtDNA from their mother. Because of this, mtDNA has proven very useful for tracing genetic lines.

What do mitochondria do?

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 vital for various cellular processes. For instance, releasing calcium back into a cell can initiate the release of a neurotransmitter from a nerve cell or hormones from endocrine cells.

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.

Mitochondrial disease

The mtDNA is more prone to damage than the rest of the genome. This is because free radicals, which can cause DNA damage, are produced during ATP synthesis. Mitochondria also lack the same protective mechanisms found in the cell nucleus.

However, the majority of mitochondrial diseases are due to mutations in nuclear DNA that affect products that end up in the mitochondria. These mutations can either be inherited or spontaneous.

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 affected the most by faulty mitochondria. However, there are many types of mitochondrial disease.

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 most common examples of mitochondrial disease include:

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

»Learn more:Mitochondrial diseases

Mitochondria and aging

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.

Summary

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.

Biology / Biochemistry

How we reviewed this article:

SourcesMedical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Casanova A, et al. (2023). Mitochondria: It is all about energy.https://pmc.ncbi.nlm.nih.gov/articles/PMC10167337/
Chandimali N, et al. (2025). Free radicals and their impact on health and antioxidant defenses: A review.https://www.nature.com/articles/s41420-024-02278-8
Cooper D, et al. (2023). Biochemistry, calcium channels.https://www.ncbi.nlm.nih.gov/books/NBK562198/
El-Gammal Z, et al. (2022). Regulation of mitochondrial temperature in health and disease.https://pmc.ncbi.nlm.nih.gov/articles/PMC9492600/
Hubbane M, et al. (2021). Human mitochondrial DNA: Particularities and diseases.https://pmc.ncbi.nlm.nih.gov/articles/PMC8533111/
Lettieri-Barbato D. (2019). Redox control of non-shivering thermogenesis.https://pmc.ncbi.nlm.nih.gov/articles/PMC6599457/
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Rossmann MP, et al. (2021). Mitochondrial function in development and disease.https://pmc.ncbi.nlm.nih.gov/articles/PMC8214736/
Shami GJ, et al. (2021). Three-dimensional ultrastructure of giant mitochondria in human non-alcoholic fatty liver disease.https://www.nature.com/articles/s41598-021-82884-z
Somasundaram I, et al. (2024). Mitochondrial dysfunction and its association with age-related disorders.https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2024.1384966/full
Talking glossary of genomic and genetic terms. (n.d.).https://www.genome.gov/genetics-glossary
The biology behind mitochondrial disease. (2020).https://umdf.org/the-biology-behind-mitochondrial-disease/
The mitochondrion. (2002).https://www.ncbi.nlm.nih.gov/books/NBK26894/
Types of mitochondrial disease. (n.d.).https://umdf.org/what-is-mitochondrial-disease-2/types-of-mitochondrial-disease/

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Medically reviewed by Jillian Foglesong Stabile MD, FAAFP, DABOM — Written by Tim Newman — Updated on May 27, 2025

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