Mitochondria

Mitochondria (singular: mitochondrion) are important organelles involved in aerobic respiration. They are absent from prokaryotic cells and from red blood corpuscles, and their shape may be oval, spherical, or a spiral, strip-like form.

Structure

A mitochondrion is a double-membrane-bound organelle. Its outer membrane is permeable to a wide range of metabolites, thanks to a protein called porin, or Parson's particles, embedded in it; the inner membrane, by contrast, is selectively permeable to only a few substances. The two membranes are separated by a space called the outer chamber. The inner membrane is thrown into numerous finger-like or plate-like folds, called cristae, which greatly increase its surface area, and it carries large numbers of stalked particles called oxysomes as well as cytochromes and other electron-carrier molecules. The cavity enclosed by the inner membrane, called the inner chamber, is filled with a fluid called the matrix, which contains a few coils of circular DNA, RNA, ribosomes of the 70S type, lipids, and the various enzymes of the Krebs cycle and related metabolic pathways. This mitochondrial DNA is naked (not bound to histones), circular, and double-stranded.

Oxysomes and ATP synthesis

The particles studding the inner membrane, called oxysomes -- also known as F1-F0 particles, Fernandez-Moran particles, elementary particles or mitochondrial particles -- each consist of a head and a stalk (or foot). The head (F1), sometimes described as 'lollipop-shaped', faces into the matrix and functions as the enzyme ATP synthase, while the foot (F0) is embedded in the inner membrane and functions as a proton channel. Together, these oxysomes carry out the pumping of protons and the synthesis of ATP. It is this machinery -- the electron-carrier proteins along the folded cristae feeding a proton gradient that drives ATP synthase in the oxysomes -- that makes the mitochondrion the site where the energy released during aerobic respiration is converted into ATP, the cell's usable energy currency, which is exactly why mitochondria are commonly called the 'power house of the cell'.

The endosymbiont theory …