Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Cells
Mitochondria are the "power plants" of eukaryotic cells, supplying energy

Cell/35.html">Mitochondria occupy a prominent place in the Cytoplasm of Eukaryotic Cells (Fig. 2-10). Their size, appearance, number, and intracellular localization can vary over a very wide range depending on The Cell type. A single rat Liver cell contains about 1,000 mitochondria. Their diameter is approximately 1 μm, meaning that in size, mitochondria differ very little from bacterial cells. Certain types of eukaryotic cells, such as spermatozoa or Yeast cells, contain only a few very large mitochondria, whereas others, such as a chicken egg, contain many thousands. Sometimes mitochondria form highly branched structures and occupy a significant portion of the cytoplasmic volume.

Each mitochondrion is bounded by two membrane systems. The smooth outer membrane completely encloses the organelle, while the inner membrane folds inward to form cristae. In liver mitochondria, the number of cristae is relatively small, whereas Heart Muscle cells contain a vast number of parallel cristae. The internal space of the mitochondrion is filled with a gel-like substance called the matrix.

Mitochondria function as the cell's "power plants," supplying energy. They contain A large number of Enzymes that jointly catalyze The oxidation of cellular organic nutrients by molecular oxygen into carbon dioxide and Water. Some of these enzymes are located in the matrix, while others reside in the inner membrane. This oxidation process releases a large amount of energy, which is used to synthesize adenosine triphosphate (ATP)—the cell's primary energy-storing molecule. ATP molecules produced in the mitochondria diffuse to all PARTS OF THE cell, where they are utilized to perform essential work.

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Fig. 2-10. Structure of a mitochondrion. The name mitochondria derives from the Greek words "mitos" meaning thread and "chondros" meaning grain or granule.

In some cells, mitochondria are elongated and nearly thread-like, though in most cases they are elliptical or spherical in shape. The electron micrograph shown here, depicting a mitochondrion from a pancreatic cell, clearly reveals the smooth outer membrane and numerous folds of the inner membrane, known as cristae.

Mitochondria contain small amounts of DNA, RNA, and Ribosomes. Mitochondrial DNA encodes the synthesis of several specific Proteins of the inner membrane. One might wonder: why do mitochondria contain their own DNA? This question led to a fascinating hypothesis proposing that mitochondria evolved when larger anaerobic eukaryotic host cells were invaded by smaller Prokaryotic Cells capable of utilizing molecular oxygen to oxidize nutrients (Fig. 2-11). The invading Bacteria thus initially became parasites within the host cells. Over the course of further evolution, this relationship gradually evolved into a mutually beneficial Symbiosis for both host and parasite. Today we know that mitochondria divide alongside the cell during division. Thus, it is quite possible that mitochondrial DNA and ribosomes are the descendants of the DNA and ribosomes of those small bacteria that once invaded the cell.

Fig. 2-11. A plausible hypothesis for the evolutionary origin of mitochondria. This hypothesis is based on the striking biochemical and genetic similarities between bacteria and the mitochondria of eukaryotic cells. During the evolution of eukaryotic cells, a mutually beneficial symbiotic relationship was established between the host cell and the bacterium that penetrated its cytoplasm. Ultimately, these cytoplasmic bacteria transformed into mitochondria.