Mitochondria double membrane

Figure 17.1. Mitochondrion. The double membrane of the mitochondrion is evident in this electron micrograph. The numerous invaginations of the inner mitochondrial membrane are called cristae. The oxidative decarboxylation of pyruvate and the sequence of reactions in the citric acid cycle take place within the matrix. [(Left) Omikron/Photo Researchers.]...

The mitochondria are aerobic cell organelles that are responsible for most of the ATP production in eukaryotic cells. They are enclosed by a double membrane. The outer membrane permits low-molecular-weight molecules to pass through. The inner mitochondrial membrane, by contrast, is almost completely impermeable to most molecules. The inner mitochondrial membrane is the site where oxidative phosphorylation occurs. The enzymes of the citric acid cycle, of amino acid catabolism, and of fatty acid oxidation are located in the matrix space of the mitochondrion. 

These initial studies indicate that the 6 kb molecule is the mitochondrial genome. If borne out, it would be the most compact mitochondrial genome described to date. The subcellular location of the 35 kb molecule remains a mystery. One possibility is that it is within the spherical body , a double-membrane enclosed organelle which is closely apposed to the mitochondrion in electron micrographs (68). The function of the spherical body is unknown. 

Three of the most important organelles in eukaryotic cells are the nucleus, the mitochondrion, and the chloroplast. Each is separated from the rest of the cell by a double membrane. The nucleus contains most of the DNA of the cell... 

A second very important eukaryotic organelle is the mitochondrion, which, like the nucleus, has a double membrane (Figure 1.13). The outer membrane has a fairly smooth surface, but the inner membrane exhibits many folds called cristae. The space within the inner membrane is called the matrix. Oxidation processes that occur in mitochondria yield energy for the cell. Most of the enzymes responsible for these important reactions are associated with the inner mitochondrial membrane. Other enzymes needed for oxidation reactions, as well as DNA that differs from that found in the nucleus, are found in the internal mitochondrial matrix. Mitochondria also contain ribosomes similar to those found in bacteria. Mitochondria are approximately the size of many bacteria, typically about 1 pm in diameter and 2 to 8 pm in length. In theory, they may have arisen from the absorption of aerobic bacteria by larger host cells. 

Oxidative phosphorylation occurs on membranes. In bacteria, chemiosmotic ATP synthesis occurs at the cytoplasmic membrane. In plant and animal cells, these reactions occur in the mitochondrion, a double-membraned organelle (Figure 11-1). The ancestor of mitochondria was a bacterial cell incorporated into a nucleated cell, which subsequently lost much (although not all) of its DNA. Most mitochondrial proteins are encoded by nuclear DNA. Some respiratory proteins, along with mitochondrial ribosomal RNA and transfer RNAs, are encoded by mitochondrial DNA. 

This, in a sense, creates a problem because CoA carries several negative charges and therefore we have now made a molecule (stearoyl CoA) that is much too hydrophilic to be able to cross the double membrane of the mitochondrion without help. The solution lies in a specific carrier system for shuttling the fatty acid chains across the membrane barriers. It uses a molecule called carnitine, which, unlike CoA, carries no net charge. There is a system of three membrane-boimd enzymes that (1)... 

Mitochondria are intracellular organelles with a double-membrane structure. Both the number and size of mitochondria vary in different cells - for example, a liver cell contains some 800 mitochondria, a renal tubule cell some 300 and a sperm about 20. The outer mitochondrial membrane is permeable to a great many substrates, while the inner membrane provides a barrier to regulate the uptake of substrates and output of products (see, for example, the regulation of palmitoyl CoA uptake into the mitochondrion for oxidation in section 5.5.1). 

The most likely deficiency is a lack of 2,4-dienoyl CoA reductase, an enzyme that is essential for the degradation of unsaturated fatty acids with double bonds at even-numbered carbons. Such fatty acids include linoleate (9-ds,12-ds 18 2). Four rounds of oxidation of linoleoyl CoA generate a 10-carbon acyl CoA that contains a trans-A and a cis-A double bond. This intermediate is a substrate for the reductase, which converts the 2,4-dienoyl CoA to ds-A -enoyl CoA. A dehciency of 2,4-dienoyl reductase leads to an accumulation of trans-A, ds-A -decadienoyl CoA molecules in the mitochondrion. The observation that carnitine derivatives of the 2,4-dienoyl CoA are found in blood and urine provides evidence that these molecules accumulate in the mitochondrion and are then attached to carnitine. Formation of carnitine decadienoate allows the acyl molecules to be transported across the inner mitochondrial membrane into the cytosol, and then into the circulation. 

Yeast mitochondrial DNA occurs as double-stranded 26-/im closed circles, a molecular size corresponding to about 50 x 10 daltons. The number of circles per mitochondrion may range from zero to about five the total amount of cellular mtDNA per wild-type cell varies with the strain and accounts for 10-25% of the total cellular DNA. RNA-DNA hybridization studies indicate that yeast mtDNA contains one cistron of each of the 15 S and 21 S RNA species and probably 20 tRNA cis-trons. It has been reported that mitochondria from HeLa cells contain only 12 tRNA cistrons, 9 on the heavy DNA strand and 3 on the light strand. These authors suggested that since the proteins formed by mitochondrial ribosomes are enriched in hydrophobic amino acids, an array of 12 tRNAs may be sufficient for the complete synthesis of the inner-membrane proteins by mitochondria. Alternatively, some nuclear coded tRNAs may be available to the mitochondrial protein-synthesizing system. 

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