tymoczko3e

20.1 Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria

Like the citric acid cycle, the respiratory chain and ATP synthesis take place in mitochondria. Whereas the citric acid cycle takes place in the mitochondrial matrix, the flow of electrons through the respiratory chain and the process of ATP synthesis take place in the mitochondrial inner membrane, a location, as we will see, that is crucial to the process of oxidative phosphorylation.

Mitochondria Are Bounded by a Double Membrane

Electron microscopic studies reveal that mitochondria have two membrane systems: an outer membrane and an extensive, highly folded inner membrane. The inner membrane is folded into a series of internal ridges called cristae. Hence, there are two compartments in mitochondria: (1) the intermembrane space between the outer and the inner membranes and (2) the matrix, which is bounded by the inner membrane (Figure 20.2). The mitochondrial matrix is the site of the reactions of the citric acid cycle and fatty acid oxidation. In contrast, oxidative phosphorylation takes place in the inner mitochondrial membrane. The increase in surface area of the inner mitochondrial membrane provided by the cristae creates more sites for oxidative phosphorylation than would be the case with a simple, unfolded membrane. Human beings contain an estimated 14,000 m² of inner mitochondrial membrane, which is the approximate equivalent to the area of three football fields in the United States.

Figure 20.2 Electron micrograph (A) and diagram (B) of a mitochondrion.

Page 365

The outer membrane is quite permeable to most small molecules and ions because it contains many copies of mitochondrial porin, a 30- to 35-kDa poreforming protein also known as VDAC, for voltage-dependent anion channel. VDAC regulates the flux of molecules crucial to the function of cellular respiration—usually anionic species such as phosphate, components of the citric acid cycle, and the adenine nucleotides—across the outer membrane. In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttle metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the matrix side and the cytoplasmic side (the latter because it is freely accessible to most small molecules in the cytoplasm). In bacteria, the electron-driven proton pumps and ATP-synthesizing complex are located in the plasma membrane.

BIOLOGICAL INSIGHT

Mitochondria Are the Result of an Endosymbiotic Event

Mitochondria are semiautonomous organelles that live in an endosymbiotic relationship with the host cell. These organelles contain their own DNA, which encodes a variety of different proteins and RNAs. However, mitochondria also contain many proteins encoded by nuclear DNA. Cells that contain mitochondria depend on these organelles for oxidative phosphorylation, and the mitochondria in turn depend on the cell for their very existence. How did this intimate symbiotic relationship come to exist?

Figure 20.3 Overlapping gene complements of mitochondria. The genes within each oval are those present within the organism represented by the oval. The genome of Reclinomonas contains all the protein-coding genes found in all the sequenced mitochondrial genomes.

An endosymbiotic event is thought to have taken place whereby a free-living organism capable of oxidative phosphorylation was engulfed by another cell. The double-membrane, circular DNA, and mitochondrial-specific transcription and translation machinery all point to this conclusion. Thanks to the rapid accumulation of sequence data for mitochondrial and bacterial genomes, speculation on the origin of the “original” mitochondrion with some authority is now possible. The most mitochondrial-like bacterial genome is that of Rickettsia prowazekii, the cause of louse-borne typhus. Sequence data suggest that all extant mitochondria are derived from an ancestor of R. prowazekii as the result of a single endosymbiotic event.

The evidence that modern mitochondria result from a single event comes from examination of the most bacteria-like mitochondrial genome, that of the protozoan Reclinomonas americana. Its genome contains 97 genes, of which 62 specify proteins. The genes encoding these proteins include all of the protein-coding genes found in all of the sequenced mitochondrial genomes (Figure 20.3). It seems unlikely that mitochondrial genomes resulting from several endosymbiotic events could have been independently reduced to the same set of genes found in R. americana.

Page 366

Note that transient engulfment of prokaryotic cells by larger cells is not uncommon in the microbial world. In regard to mitochondria, such a transient relationship became permanent as the bacterial cell lost most of its DNA, making it incapable of independent living, and the host cell became dependent on the ATP generated by its tenant.

●

◌

▣