Mitochondrion

A mitochondrion (pl. mitochondria) is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. These tiny structures have a special job: they create a type of energy called adenosine triphosphate (ATP) that cells use to power their activities. Because of this important role, mitochondria are often called the “powerhouse of the cell.” They were first discovered in 1857 by a scientist named Albert von Kölliker and later named by Carl Benda in 1898.

Mitochondria have a unique double membrane and contain their own small amount of DNA, which is different from the DNA in the cell’s nucleus. This has led scientists to believe that long ago, mitochondria were once free-living bacteria that formed a partnership with early eukaryotic cells. This idea is known as symbiogenesis. Today, mitochondria are found in almost every cell in your body, though some cells, like mature red blood cells, do not have them.

Besides making energy, mitochondria help with many other important cell tasks, such as sending signals, helping cells grow, and even controlling when a cell dies. The number of mitochondria in a cell can vary a lot — a liver cell can have over 2000, while a red blood cell has none. These tiny structures play a vital role in keeping all living things alive and healthy.

Structure

Mitochondria can have many different shapes. They have two membranes and five main parts: the outer membrane, the space between the membranes, the inner membrane, the folds of the inner membrane called cristae, and the fluid inside called the matrix.

The inner membrane has many folds called cristae, which help it make more ATP — the energy coin of the cell. The matrix inside the inner membrane holds special enzymes and DNA that help create this energy.

Function

Mitochondria are tiny structures inside cells that help create energy. They make a special kind of energy called ATP, which powers many activities in the cell. This process, called respiration, uses oxygen and sugar to produce ATP.

Besides making energy, mitochondria have other important jobs. They help control how the cell uses its fuel, store and release calcium, and even play a role in signaling between cells. They are vital for life and help cells stay healthy and function properly.

Organization and distribution

Mitochondria are tiny structures found inside most cells, including those in animals, plants, and fungi. They often look like small beans but can change shape and move around inside the cell. Some cells have just one mitochondrion, while others, like liver cells, can have thousands. They help the cell by making energy.

The mitochondria-associated ER membrane (MAM) is an important area where mitochondria connect with another part of the cell called the endoplasmic reticulum (ER). This connection helps the cell manage its energy and materials. The MAM plays a role in handling important cell signals and materials, showing that mitochondria work closely with other parts of the cell.

Main article: Mitochondria associated membranes

Origin and evolution

Main article: Symbiogenesis

Scientists have two main ideas about how mitochondria began. One idea is that they were once simple bacteria that lived inside larger cells, helping them produce energy. This is called the endosymbiotic theory. The other idea is that mitochondria formed from a piece of DNA that split off from the cell's nucleus and got surrounded by membranes.

Mitochondria have their own DNA, which is circular like bacterial DNA. They also have small structures called ribosomes that are similar to those in bacteria. These features support the idea that mitochondria came from bacteria. The endosymbiotic theory was popularized by scientist Lynn Margulis, who suggested that mitochondria came from aerobic bacteria that lived inside other cells, giving those cells an advantage by helping them produce energy more efficiently. This relationship likely developed over a billion years ago.

Some simple organisms today have very small or changed versions of mitochondria, showing that even when mitochondria are lost or changed, some of their functions are still important.

Mitochondrial genetics

Mitochondria have their own tiny circle of DNA, different from the DNA in the nucleus of the cell. This DNA helps make important parts of the cell’s energy-making machinery. In humans, this mitochondrial DNA is a small loop with just 37 genes that help create energy and handle other cell tasks.

Scientists study this DNA to learn about family history and how species, including humans, have changed over time. It is passed down only from mothers to their children. Even though it doesn’t change much, scientists have found ways it can mix between cells, helping keep the cell’s energy factories working well.

Dysfunction and disease

Mitochondrial diseases

Main article: Mitochondrial disease

When mitochondria don't work well, it can cause many health problems. These problems often affect the brain and muscles, and can also lead to issues like diabetes and disorders of the hormone system. Some diseases happen because of changes in the DNA inside mitochondria, which are passed from mothers to their children. Examples include Kearns-Sayre syndrome and MELAS syndrome.

Mitochondrial problems can also come from changes in the DNA inside the cell's nucleus. This is seen in diseases like Friedreich's ataxia and Wilson's disease. Environmental factors, like exposure to certain chemicals, might also play a role in some mitochondrial-related diseases. Scientists are studying treatments, such as mitochondrial replacement therapy, to help prevent these diseases, though this area is still new and raises many questions.

History

The first observations of structures inside cells that are now known as mitochondria were made in 1857 by Albert von Kolliker. Later, in 1890, Richard Altmann named these structures "bioblasts." The term "mitochondria" was introduced in 1898 by Carl Benda, using words from the Greek language meaning "thread" and "granule."

Over the years, scientists learned more about mitochondria. In 1900, Leonor Michaelis found a special dye that could show mitochondria clearly. By the 1940s, researchers discovered that mitochondria help cells create energy. In 1957, mitochondria were nicknamed the "powerhouse of the cell" by Philip Siekevitz. Later discoveries showed that mitochondria have their own tiny structures for making proteins and even change shape when cells need less food.