Mitochondrion
Mitochondria run aerobic respiration, converting the energy in glucose and fats into ATP the cell can spend. Their folded inner membrane and their own small genome are relics of free-living bacteria.
- 1NucleusOrganelle · double membrane
- 2MitochondrionOrganelle · double membrane
- 3RibosomeMolecular machine · no membrane
- 4Endoplasmic reticulumOrganelle · single membrane network
- 5Golgi apparatusOrganelle · single membrane stacks
- 6LysosomeOrganelle · single membrane
- 7Cell membraneBoundary · phospholipid bilayer
- 8CentriolesMicrotubule pair · organizes the spindle
Animal cell: what you are looking at
A flexible, roughly rounded cell 10–30 µm across, bounded only by its membrane. It has a pair of centrioles near the nucleus and many lysosomes, but no wall, no chloroplasts, and no large central vacuole.
Centrioles. A pair of short microtubule cylinders at right angles; they organize the spindle during cell division.
The mitochondrion is outlined in coral; it occurs in animal cells, plant cells, fungi, protists. Hover or tab through the key to light up the other structures, and click any of them to open its page.
Compare all eight organelles ↗Know its structure
| Found in | Animal cells, Plant cells, Fungi, Protists |
| Membranes | Two: a smooth outer membrane and a deeply folded inner membrane (the folds are cristae) |
| Typical size | About 0.5–1 µm wide and 1–10 µm long, roughly the size of a bacterium |
| Number per cell | From a handful to thousands; a liver cell holds about 1,000–2,000 |
| Contains | A matrix with the Krebs-cycle enzymes, its own circular DNA (16,569 base pairs in humans), and bacterial-type ribosomes |
| Discovered | Named by Carl Benda in 1898; the endosymbiotic origin was argued convincingly by Lynn Margulis in 1967 |
What it does
- Makes ATP by aerobic respiration. Oxidative phosphorylation across the cristae yields about 30–32 ATP per glucose (older textbooks quote up to 38)
- Runs the Krebs cycle. Pyruvate from glycolysis is oxidized to CO₂ in the matrix, loading NADH and FADH₂ for the electron transport chain
- Signals and stores. Buffers Ca²⁺ and releases cytochrome c to start apoptosis when a cell is beyond repair
- Produces heat. Brown-fat mitochondria let protons leak through UCP1, turning the gradient into warmth for newborns and hibernators
Mitochondrion: common questions
Wondering about the why? Start here.
What does the mitochondrion do?
Mitochondria carry out aerobic respiration: the Krebs cycle in the matrix strips electrons from pyruvate and loads them onto NADH and FADH₂, and the electron transport chain on the folded inner membrane (the cristae) uses them to pump protons and drive ATP synthase. In total, C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O yields about 30 to 32 ATP per glucose, compared with just 2 from glycolysis alone.
Read: cellular respirationWhy do mitochondria have their own DNA?
Because they descend from free-living bacteria that were engulfed by an ancestral cell around two billion years ago, the endosymbiotic theory. Human mitochondrial DNA is a circle of 16,569 base pairs carrying 37 genes, translated on its own small bacterial-style ribosomes, and it is inherited almost entirely from the mother through the egg. The double membrane is another relic: the inner one is the old bacterial membrane.
Compare with the chloroplastWhy do muscle cells have so many mitochondria?
Because they spend ATP faster than almost any other cell. Mitochondria fill about a third of a heart muscle cell’s volume, and a liver cell holds 1,000 to 2,000 of them. Each glucose gives only 2 ATP in the cytoplasm but about 28 more inside a mitochondrion, so a cell cut off from oxygen loses over 90% of its energy income within minutes, which is why a blocked coronary artery damages heart muscle so fast.
Practice cell questions