Outer membraneWraps the mitochondrion; porins in it let small molecules through.
Intermembrane spaceThe gap between the membranes; hydrogen ions are pumped into it to power ATP synthase.
Inner membraneHolds the electron transport chain and ATP synthase; it lets very little through.
CristaeFolds of the inner membrane that give it far more surface for making ATP.
Inside the matrix: 4 4
MatrixHolds the enzymes of the citric acid cycle, and the organelle's own DNA and ribosomes.
RibosomesMitochondria have their own ribosomes, which make some of their proteins.
Mitochondrial DNA (mtDNA)The organelle's own small loop of DNA, passed down from the mother.
ATP synthaseA protein in the inner membrane that makes ATP as hydrogen ions flow back through it.
How this list was compiled: Read 2026-09-26. OpenStax Biology 2e, 4.3 Eukaryotic Cells, https://openstax.org/books/biology-2e/pages/4-3-eukaryotic-cells : mitochondria are oval, double-membrane organelles with their own ribosomes and DNA; the inner membrane has folds called cristae; the area surrounded by the folds is the mitochondrial matrix; the space between the membranes is the intermembrane space; ATP synthesis takes place on the inner membrane. OpenStax Biology 2e, 7.4 Oxidative Phosphorylation, https://openstax.org/books/biology-2e/pages/7-4-oxidative-phosphorylation : hydrogen ions pass from the intermembrane space into the matrix through ATP synthase, an integral protein of the inner membrane, which makes ATP. Wikipedia, Mitochondrion, https://en.wikipedia.org/wiki/Mitochondrion : compartments are the outer membrane, intermembrane space, inner membrane, cristae and matrix; the outer membrane holds porins; the matrix holds mitochondrial ribosomes and copies of the mitochondrial DNA; the inner membrane holds ATP synthase. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
This mitochondria diagram labeled part by part shows one mitochondrion cut lengthwise, with both membranes, the folds of the inner one and what floats in the matrix. The rings split into the membranes and the inside of the matrix.
Read the full guide
Two membranes, one gap
The outer membrane is smooth and lets small molecules through. Inside it is the narrow intermembrane space, then the inner membrane, folded into cristae that reach into the matrix. The folds give it far more surface for making ATP, which is why a mitochondrion is called the powerhouse of the cell.
Inside the matrix
The matrix holds the enzymes of the citric acid cycle. It also holds the organelle's own small loops of DNA and its own ribosomes, which make some of its proteins.
The pale core inside each crista is the intermembrane space running into the fold; the matrix is only the tinted space round the folds.
Questions and answers
What is the difference between the cristae and the matrix?
Cristae are the folds of the inner membrane. The matrix is the fluid-filled space the inner membrane encloses, round the folds. The citric acid cycle runs in the matrix; ATP is made on the cristae.
Why does a mitochondrion have its own DNA?
Mitochondria are thought to descend from bacteria that were taken into another cell and never destroyed. They kept their own small loop of DNA and ribosomes, like a bacterium's.
Where is ATP made in a mitochondrion?
On the inner membrane. Hydrogen ions pumped into the intermembrane space flow back into the matrix through ATP synthase, which uses the flow to make ATP.