Mitochondria

Mitochondria were first seen by A. Kolliker in 1880, initially named 'Bioplasts' by Richard Altmann in 1894, and finally given their current name, mitochondria, by Carl Benda in 1897-98. They are ovoid, rounded or rod-shaped organelles, and their exact shape can vary considerably from one cell (or even one moment) to the next - a property called pleomorphism. Every mitochondrion is built of two separate membranes. The outer membrane is smooth and highly permeable to small molecules; this permeability comes from proteins called porins, which form open channels that let any molecule smaller than about 1000 daltons diffuse through freely. The inner membrane, by contrast, is far more restrictive - it is essentially impermeable to most ions and small molecules, and this selective barrier is exactly what lets the mitochondrion build up and maintain the proton gradient that powers oxidative phosphorylation. Together, the two membranes divide the mitochondrion's interior into two separate compartments: a narrow outer chamber, sandwiched between the outer and inner membranes, and a much larger inner chamber, filled with a thick, protein-rich matrix. The inner membrane itself is not smooth - it is thrown into numerous folds called cristae (singular: crista), and it is on these cristae that most of the electron-transport-chain enzymes actually sit. Studding the surface of the cristae are small stalked structures - variously called elementary particles, F1 particles, oxysomes, or Fernandez-Moran particles - each built of a base, a stem and a rounded head; the round head is where the enzyme ATP synthase sits and carries out oxidative phosphorylation. The matrix filling the inner chamber holds essentially every enzyme of the Krebs cycle except succinate dehydrogenase (which is instead embedded in the inner membrane itself), alongside the mitochondrion's own circular DNA and its own 70S ribosomes - features that let mitochondria replicate somewhat independently of the rest of the cell (by simple fission, using a strand-displacement replication model), which is exactly why they are described as semi-autonomous organelles. Chemically, mitochondria are roughly 73% protein, 25-30% lipid, 5-7% RNA, with only traces of DNA, and they carry about 60 distinct types of enzyme. Because they generate the cell's ATP …