Cell structure Mitochondria

Subsurface cisternae are a system of smooth, membrane-bound, flattened cisternae that can be found in many neurons. These structures, referred to as hypolemmal cisternae by Palay and Chan-Palay [1], abut the plasmalemma of the neuron and constitute a secondary membranous boundary within the cell. The distance between these cisternae and the plasmalemma is usually 10-12 nm and, in some neurons, such as the Purkinje cells, a mitochondrion may be found in close association with the innermost leaflet. Similar cisternae have been described beneath synaptic complexes, but their functional significance is not... 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

The theory has been advanced that there exists in the cell a particulate structure somewhat smaller than the mitochondrion, the lyso-some, that contains certain autolytic enzymes in a latent situation. The lysosome theory55,56 is very largely based upon measurements made in sucrose homogenates of rodent liver. Although the results for a-D-mannosidase in this tissue (see Table IV) are not incompatible with the theory, the results for other tissues do not always conform to it. In particular, the contrast between mouse and rat spleen argues against a universal single particle to which a-D-mannosidase is confined. Apart from the results quoted in Table IV, not much work has been done on the intracellular location of a-D-mannosidase. 

Because Rlil homologs have been found only in archaebacteria (reviewed in Tachezy and Dolezal 2007), it is hypothesized that Rlil and other components of cytosolic translation machinery are derived from them. Therefore, we propose that the intimate association of the relic mitochondrion, of eubacterial origin, with its associated membranes is a structural reflection of the cell s attempt to facilitate efficient ribosome biogenesis and translation initiation following reductive evolution of the organelle (Keeling 2004 ... 

Mitochondria are intracellular centers for aerobic metabolism. They are cell organelles that are identified by well-defined structural and biochemical properties. In morphological terms, mitochondria are relatively large particles that are characterized by the presence of two membranes, a smooth outer membrane that is permeable to most important metabolites and an inner membrane that has unique transport properties. The inner membrane is highly folded, which serves to increase its surface area. Figure E10.1, which shows the structure of a typical mitochondrion, divides the organelle into four major components inner membrane, outer membrane, intermembrane space, and the matrix. These regions are associated with different and... 

Figure 7.1 The mitochondrion is found in all cells and contains many structures. This includes the cristae, where most ATP is produced in the final stage of cellular respiration. Being the site of ATP production, the mitochondrion is called the powerhouse of the cell.

Organelle A subcellular structure such as the mitochondrion or nucleus of a cell. 

In addition to the foregoing more general concerns are questions concerning the localization of an enzyme activity. The location of an enzyme can determine the type of cell lysis, since it could be more advantageous to lyse the cell completely or in such a manner that the organelles are left intact. For example, some lysis methods such as sonication completely disrupt mitochondria, nuclei, and Golgi systems. If an activity is localized in an organelle such as a mitochondrion, it would seem sensible to adopt a method that leaves these structures intact, to facilitate their separation from the rest of the cellular debris. Thus, for the isolation of mitochondrial enzymes, sonication is not the method of choice for cell lysis. 

The other phases are less exotic. The mesh phases consists of lamellae with ordered holes, while ribbon phases are deformed cylinders on a rectangular lattice (see Fig. 12-22). These phases can are usually type I phases with the tails inside the deformed cylinders or inside the hole-filled lamellae, but they could also be inverse, type II, phases. Type II mesh and ribbon phases seem not to have been reported much type II strut phases are common for two-tailed lipids, such as those in cell membranes. In fact, type II strut phases evidently serve biological functions, since they have been found to exist in cellular structures such as the endoplasmic reticulum and the mitochondrion (Seddon 1996). 

The complex tetrapyrrole ring structure of heme is built up in a stepwise fashion from the very simple precursors sue-cinyl-CoA and glycine (Figure 32-2). The pathway is present in all nucleated cells. From measurements of total bilirubin production, it has been estimated that daily synthesis of heme in humans is 5 to 8mmol/kg body weight. Of this, 70% to 80% occurs in the bone marrow and is used for hemoglobin synthesis. Approximately 15% is synthesized in the liver and is used to produce cytochrome P-450, mitochondrial cytochromes, and other hemoproteins. The pathway is compartmentalized, with some steps occurring in the mitochondrion and others in the cytoplasm. Little is known about the transport of intermediates across the mitochondrial membrane, and no transport defect has yet been reported in the porphyrias. 

In part, this membrane structure serves the same purpose for the mitochondrion as does the cell membrane for the cytoplasm, thal is, of regulating the traffic of chemicals between inside and out ... 

Aerobic respiration, a process by which cells use 02 to generate energy, takes place in mitochondria. Each mitochondrion is bounded by two membranes. The smooth outer membrane is permeable to most molecules with masses less than 10,000 D. The inner membrane, which is impermeable to ions and a variety of organic molecules, projects inward into folds that are called cristae. Embedded in this membrane are structures called respiratory assemblies that are responsible for the synthesis of ATP. 

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