The mitochondrion (pl. mitochondria) is the 'power house of the eukaryotic cell, performing oxidative phosporylation. Mitochondria have two internal, membrane-bound spaces, unlike chloroplasts, which have three internal spaces.

The outer mitochodrial membrane is similar in constitution to the eukaryotic cell’s plasma membrane, while the inner membrane is similar in chemical composition to bacterial membranes. This difference is one of several lines of evidence for the serial endosymbiotic origin of mitochondria as phagocytozed purple bacteria.

Above left – click to enlarge : simplified diagram of a mitochondrion showing:
1. space between inner and outer membranes
2. matrix
3. christae
4. junction between membranes
5. inner membrane
6. outer membrane

The image at right (click to enlarge) is based on reconstruction of serial tem slices through a mitochondrion. The outer membrane (violet) surrounds the organelle. The inner membrane (pale blue) is contiguous, at membrane junctions (pale blue connecting to green at lower center), with the inner membrane that forms the walls of cristae (green).

The matrix – a soup factory – lies between the cristae, and contains mitochondrial DNA and the components of intermediary metabolism. image - mitochondrion cut : tour mitochondrion :

The outer and inner membranes are composed of phospolipid bilayers studded with proteins, much like the cell membrane. However, the composition of the inner and outer membranes is very different.

The inner mitochondrial membrane contains more than 100 different polypeptides. The protein to phospholipid ratio is very high – more than 3:1 by weight, having about 1 protein for 15 phospholipids. The inner membrane is also rich in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes. This composition, along with other evidence, has led to the assumption that the inner membrane is derived from endosymbiotic prokaryotes. The endosymbiotic theory of eukaryotic evolution is now widely accepted.

In contrast, the outer membrane, which encloses the entire mitochondrion, is similar in composition to the cell membrane and comprises about 50% phospolipids by weight and contains a variety of enzymes. The enzymes carry out activities such as the oxidation of epinephrine (adrenaline), the degradation of tryptophan, and the elongation of fatty acids.

The plant chloroplast is the site of photosynthesis : animation - chloroplast : tour the chloroplast : Virtual Cell Textbook - Cell Biology

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