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The number of mitochondria found in different types of cells varies widely. At one end of the spectrum, the Trypanosome protozoan has one large mitochondrion; by contrast, human liver cells normally have between one and two thousand each. Mitochondria can occupy up to 25% of cell cytosol.

1 Structure

Mitochondria have two functionally distinct membrane systems separated by a space: the outer membrane, which surrounds the whole organelle; and the inner membrane, which is thrown into folds or shelves that project inward. These inward folds are called cristae. The number and shape of cristae in mitochondria differ, depending on the tissue and organism in which they are found, and serve to increase the surface area of the membrane.

• The outer membrane encloses the entire organelle and contains channels made of protein complexes called porinsPorins are transmembrane proteins that are large enough to facilitate passive diffusion. They are prevelant in the outter membrane of the mitochondria. through which moleculeIn science, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. A molecule consists of multiple atoms joined by shared pairs of electrons in a covalent bond''. It may consist of atoms and ions can move in and out of the mitochondrion. It is composed of about 50% lipids and 50% proteins. Large molecules are excluded from traversing this membrane.
• The inner membrane, folded into cristaCristae are the infoldings of the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes, and function in cellular respiration. They provide more surface area for cellular respiration to happe, encloses the matrixIn biology, the word matrix is used for the material between animal or plant cells, or generally the material (or "tissue") in which more specialized structures are embedded, and also specifically for one part of the mitochondrion. The internal structure (the internal fluid of the mitochondrion). It contains several protein complexes, and is about 20% lipid and 80% protein. Stalked particles are found on the cristae: these are the ATP synthaseAn ATP synthase is a general term for an enzyme that can synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate by utilizing some form of energy. The overall reaction sequence is: :ADP + P → ATP These enzymes enzyme molecules, which produce ATP.
• The intermembrane space between the two membranes contains enzymes that use ATP to phosphorylateIn biochemistry, phosphorylation is the addition of a phosphate (PO) group to a protein or a small molecule. Its prominent role in biochemistry is the subject of a very large body of research (the Medline database returns over 100,000 articles on the subj other nucleotides and that catalyze other reactions.

"Mitochondrion" literally means 'thread granule', which is what they look like under a light microscope: tiny rod-like structures present in the cytoplasm of all cells. The matrix contains soluble enzymes that catalyze the oxidation of pyruvate and other small organic molecules. Parts of the Krebs cycle occur within mitochondria. The matrix also contains several copies of the mitochondrial DNA (usually 5-10 circular DNA molecules per mitochondrion), as well as special mitochondrial ribosomes, tRNAs, and proteins needed for DNA replication.

When the cell divides, mitochondria replicate by fission. They also replicate if the long-term energy demands of a cell increase. For example, fat storage cells, which require little energy, have very few mitochondria, but energy-demanding muscle cells tend to have many. Mitochondria are generally theorised to be highly adapted symbiotic bacteria, probably belonging to the alpha-proteo bacteria (with the closest known candidate being Rickettsia, the causative agent of typhus), and are believed to have been incorporated only once (compare chloroplast).

The mitochondrial proteins are found on the outer membrane, the inner membrane, or the intermembrane space. Stop-transfer sequences anchor proteins to the outer membrane. Matrix-targeting sequences target the protein for the mitochondrial matrix.