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This is a colored transmission electron micrograph (TEM) of a mitochondrion. Mitochondria are a type of organelle found in the cytoplasm of eukaryotic cells. They oxidize sugars and fats to produce energy in a process called respiration. A mitochondrion has two membranes, a smooth outer membrane and a folded inner membrane. The folds of the inner membrane are called cristae, and it is here that the chemical reactions to produce energy take place. CNRI/Science Photo Library/Getty Images


There are two major types of cells: prokaryotic and eukaryotic cells. The "power houses" of a eukaryotic cell are mitochondria. What does it mean to say that mitochondria are the cell's power producers? These organelles generate power by converting energy into forms that are usable by the cell. Located in the cytoplasm, mitochondria are the sites of cellular respiration. Cellular respiration is a process that ultimately generates fuel for the cell's activities. Mitochondria are also involved in other cell processes, such as cell division, growth, and cell death.

Mitochondria: Distinguishing Characteristics

Mitochondria have a distinctive oblong or oval shape and are bounded by a double membrane. They are found in both animal and plant cells. The number of mitochondria within a cell varies depending on the type and function of the cell. Some cells, such as mature red blood cells, do not contain mitochondria at all. The absence of mitochondria and other organelles leaves room for the millions of hemoglobin molecules needed in order to transport oxygen throughout the body. Muscle cells, on the other hand, may contain thousands of mitochondria needed to provide the energy required for muscle activity. Mitochondria are also abundant in fat cells and liver cells.

Mitochondrial DNA

Mitochondria have their own DNA, ribosomes and can make their own proteins. Mitochondrial DNA (mtDNA) encodes for proteins that are involved in electron transport and oxidative phosphorylation, which occur in cellular respiration. In oxidative phosphorylation, energy in the form of ATP is generated within the mitochondrial matrix. Proteins synthesized from mtDNA also encode for the production of the RNA molecules transfer RNA and ribosomal RNA.

Mitochondrial DNA differs from DNA found in the cell nucleus in that it does not possess the DNA repair mechanisms that help prevent mutations in nuclear DNA. As a result, mtDNA has a much higher mutation rate than nuclear DNA. Exposure to reactive oxygen produced during oxidative phosphorylation also damages mtDNA.

Next > Mitochondrion Anatomy


  • Encyclopædia Britannica Online, s. v. "mitochondrion", accessed December 07, 2015,
  • Cooper GM. The Cell: A Molecular Approach. 2nd edition. Sunderland (MA): Sinauer Associates; 2000. Mitochondria. Available from:

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Mitochondria: Mitochondrion Anatomy

Animal Mitochondrion
Animal Mitochondrion. Mariana Ruiz Villarreal

Mitochondrion Anatomy

Mitochondria are bounded by a double membrane. Each of these membranes is a phospholipid bilayer with embedded proteins. The outermost membrane is smooth while the inner membrane has many folds. These folds are called cristae. The folds enhance the "productivity" of cellular respiration by increasing the available surface area.

The double membranes divide the mitochondrion into two distinct parts: the intermembrane space and the mitochondrial matrix. The intermembrane space is the narrow part between the two membranes, while the mitochondrial matrix is the part enclosed by the innermost membrane.

The mitochondrial matrix contains mitochondrial DNA (mtDNA), ribosomes, and enzymes. Several of the steps in cellular respiration, including the Citric Acid Cycle and oxidative phosphorylation, occur in the matrix due to its high concentration of enzymes.

Mitochondria are semi-autonomous in that they are only partially dependent on the cell to replicate and grow. They have their own DNAribosomes, make their own proteins, and have some control over their reproduction. Similar to bacteria, mitochondria have circular DNA and replicate by a reproductive process called binary fission. Prior to replication, mitochondria merge together in a process called fusion. Fusion is needed in order to maintain stability, as without it, mitochondria will get smaller as they divide. These smaller mitochondria are not able to produce sufficient amounts of energy needed for proper cell function.

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