The acquisition of mitochondria was a prerequisite for the evolution of complex animals. Without mitochondria, present-day animal cells would have had to generate all of their ATP through anaerobic glycolysis. When glycolysis converts glucose to pyruvate, it releases only a small fraction of the total free energy that is potentially available from glucose oxidation (see Chapter 2). In mitochondria, the metabolism of sugars is complete: pyruvate is imported into the mitochondrion and ultimately oxidized by O2 to CO2 and H2O, which allows 15 times more ATP to be made from a sugar than by glycolysis alone. As explained later, this became possible only when enough molecular oxygen accumulated in the Earth’s atmosphere to allow organisms to take full advantage, via respiration, of the large amounts of energy potentially available from the oxidation of organic compounds.
Mitochondria are large enough to be seen in the light microscope, and they were first identified in the nineteenth century. Real progress in understanding their internal structure and function, however, depended on biochemical procedures developed in 1948 for isolating intact mitochondria, and on electron microscopy, which was first used to look at cells at about the same time.
Figure 14–5 The relationship between mitochondria and microtubules. (a) a light micrograph of chains of elongated mitochondria in a living mammalian cell in culture. the cell was stained with a fluorescent dye (rhodamine 123) that specifically labels mitochondria in living cells. (B) an immunofluorescence micrograph of the same cell stained (after fixation) with fluorescent antibodies that bind to microtubules. Note that the mitochondria tend to be aligned along microtubules. (Courtesy of Lan Bo Chen.)
Figure 14–6 Localization of mitochondria near sites of high ATP demand in cardiac muscle and a sperm tail. (a) Cardiac muscle in the wall of the heart is the most heavily used muscle in the body, and its continual contractions require a reliable energy supply. It has limited built-in energy stores and has to depend on a steady supply of atp from the copious mitochondria aligned close to the contractile myofibrils (see Figure 16–32). (B) During sperm development, microtubules wind helically around the flagellar axoneme, where they are thought to help localize the mitochondria in the tail to produce the structure shown.
Figure 14–7 Interaction of mitochondria with the endoplasmic reticulum. 0 sec 20 sec 30 sec (a) Fluorescence light microscopy shows that tubules of the Er (green) wrap around parts of the mitochondrial network (red) in mammalian cells. the mitochondria then divide at the contact sites. after contact is established, fission occurs within less than a minute, as indicated by time-lapse microscopy. (B) Schematic drawing of an Er tubule wrapped around part of the mitochondrial reticulum. It is thought that Er–mitochondrial contacts also mediate the exchange of lipids between the two membrane systems. (a, adapted from J.r. Friedman et al., Science 334:358–362, 2011.) the Mitochondrion has an Outer Membrane and an Inner Membrane