What Is Mitochondria? - Definition & Functions

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Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

Expert Contributor
Amanda Robb

Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Mitochondria are organelles in eukaryotic cells that produce the chemical energy necessary to support biochemical reactions. Explore the definition and functions of mitochondria, discover how they began, and understand their importance in the survival of eukaryotic cells. Updated: 09/22/2021


Once upon a time, long before you were born, there was a little bacterium who was christened Mitochondrion, or Mito for short. Mito was mighty small and was always running away from bigger and badder cells. One day, he was eaten by a big cell. Poor, Mito! You might be wondering; did Mito die?

Actually no, something else happened. But what is it? Who was this little Mito? And what does he do for a living now? It's a mystery we're about to solve.

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  • 0:05 Mitochondrion
  • 0:36 What are Mitochondria?
  • 2:21 Mitochondria: Functions
  • 3:57 Lesson Summary
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What Are Mitochondria?

In the annals of history, scribes have theorized that Mito and his kind, the mitochondria, were likely once free-living bacteria that eventually came to inhabit larger and more complex cells called eukaryotic cells in a mutualistic relationship that gave mitochondria food and protection and the host eukaryotic cells extra energy for their own functions and survival.

The mitochondria are now double-membraned organelles that exist within a eukaryotic cell's cytoplasm, the types of cells that make up our body. The cytoplasm is gel-like and located inside a cell's plasma membrane but outside of the nucleus. And an organelle is simply a membrane-bound structure responsible for a specific function within a cell.

Remember, Mito was mighty small and lots of things, like bigger bacteria, could easily kill him. One day, his luck ran out, or so he thought, and a bigger cell engulfed him. Mito should have died. But, by golly, Mito survived! The bigger cell could not digest him. By a stroke of luck, instead of dying, Mito realized he was stuck inside a big cell that had a lot of food floating around; food that he munched on voraciously. It was also a bigger cell that protected him from some predators that would have killed him for sure. So, Mito found a new home and a new source of food. Who would try and leave that?

In exchange for all that food and protection, Mito repaid his host cell with money like we would any nice bed and breakfast. No, really. Mito exchanged cold hard energy currency for food and board. Once inside the eukaryotic cells, he began to repay his landlord and restaurant, the eukaryotic cell, with a specific kind of energy currency.

Mitochondria: Functions

This energy currency, called adenosine triphosphate (ATP), is produced by the mitochondria in our body's cells. Think of a mitochondrion, single for mitochondria, as the Department of the Treasury in the U.S. that designs and prints money, as a well as a power plant, all rolled into one little organelle. This energy currency helps pay for and drive our body's many functions including our movement, our heartbeat, and the synthesis of many molecules necessary for our survival.

The way it works, in a jiffy, is that parts of the food we eat are converted by the mitochondrion into ATP much like parts of a tree are converted into paper money. The process by which mitochondria produce ATP is called alternatively the citric acid cycle, Krebs cycle, or tricarboxylic acid (TCA) cycle; take your pick.

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Additional Activities

Mitochondria Research

In this research project, students will be learning about additional important roles of the mitochondria inside the cell. During a basic biology class, students learn that the mitochondria make energy for the cell. However, as discussed in the lesson, there are also other functions such as cell signaling, apoptosis, and metabolism. In this research activity, students will choose one lesser-known function of the mitochondria and construct either a poster or slideshow presentation about it. Students should only use academic sources from universities, scientists, teachers, or news outlets for their research.


In this activity, you are going to be researching one of the lesser-known functions of the mitochondria. Many of us are familiar with the function of making energy, but the mitochondria do much more inside the cell, such as regulating cell survival, signaling pathways, and other branches of metabolism. Using academic sources such as those from universities, scientists, or news outlets, you will be researching one aspect of mitochondria function. You will compile your research along with citing your sources into either a poster or a slideshow. Before you get started, review the criteria for success below to ensure that your final product has everything it needs.

Criteria for Success

  • One function other than producing ATP is researched
  • At least 3 different academic sources are used
  • Research is compiled into a slideshow or poster
  • Slideshow or poster is colorful, attractive and includes important information from the research
  • The chosen function is fully explained with supporting details as needed
  • Diagrams and pictures are used to further convey the information

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