A mitochondrion is a tiny structure inside a cell that makes ATP, the main molecule cells use to carry energy. A typical mitochondrion is shaped like a rounded rod, about 1–2 micrometers long and about half as wide. A micrometer is a millionth of a meter. A single cell can hold hundreds or thousands of mitochondria.

Size

Mitochondria come in many shapes and sizes, so their size is given as a typical range. In most cells they are rods or grains about 0.5 micrometers wide and 1–2 micrometers long, roughly the size of a bacterium. Textbook drawings often show a larger one, about 2 micrometers long and 1 micrometer wide. In some cells, mitochondria are several micrometers wide and up to 10 micrometers long.

Mitochondria also change shape. Under a light microscope they can be seen to grow, branch, divide and join together. In some cells they link up into a branching network that spreads over tens of micrometers. That makes it hard to say where one mitochondrion ends and the next begins, and hard to count them.

Size comparisons

A mitochondrion is about the same size as an E. coli bacterium, which is about 2 micrometers long and 1 micrometer wide.
Laid end to end, about four to eight mitochondria would span a red blood cell, which is about 7.5 micrometers across.
A mitochondrion's width, 0.5–1 micrometer, is close to the wavelength of deep red light, 0.75 micrometers.
If a typical human cell, about 10 micrometers across, were enlarged to the size of a basketball, its mitochondria would be about 2.4–4.8 centimeters (1–2 inches) long.

What mitochondria do

Mitochondria release the chemical energy stored in glucose and other nutrients and use it to make ATP. This process, called cellular respiration, uses oxygen and gives off carbon dioxide as a waste product. The carbon dioxide a person breathes out comes from these reactions.

Each mitochondrion has two membranes. The inner one is folded into ridges called cristae, which give it more surface for making ATP. Cells that need a lot of energy, such as muscle cells, have many mitochondria.

Where mitochondria came from

The name comes from the Greek words for "thread" and "granule", the two shapes that mitochondria take. The German scientist Carl Benda coined it in 1898. In the 1960s, scientists found that mitochondria carry their own DNA and their own ribosomes, the machines that build proteins.

When the DNA of mitochondria was sequenced in 1976, it turned out to resemble the DNA of bacteria. This supported the endosymbiotic theory, proposed by Lynn Margulis. It says that mitochondria began as bacteria that were swallowed by another cell and went on living inside it.

Sources

How big are mitochondria?. In Cell Biology by the Numbers. Ron Milo and Rob Phillips, Garland Science.
Mitochondria. Stephen L. Wolfe and Bryan Ness, EBSCO Research Starters, 2021.
How big is an E. coli cell and what is its mass?. In Cell Biology by the Numbers. Ron Milo and Rob Phillips, Garland Science.
How big is a human cell?. In Cell Biology by the Numbers. Ron Milo and Rob Phillips, Garland Science.
Eukaryotic Cells. In Biology 2e. OpenStax, 2018.
Cell Division. In Biology 2e. OpenStax, 2018.
Unique Characteristics of Eukaryotic Cells. In Microbiology. OpenStax, 2016.
The Electromagnetic Spectrum. In College Physics 2e. OpenStax, 2022.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.

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