The Intracellular Organelles

The importance of the availability of purified mitochondria for Lehninger s early studies of oxidative phosphorylation, and in the 

Studies were also made of mitochondrial physiology. Laird noted that mitochondrial numbers were greater in metabolically active cells like liver (ca. 1000) compared with resting cells like small lymphocytes ( 10). Keith Porter linked the extent of cristal surface with the amount of work done by the cell. Muscle mitochondria had significantly more cristae than those from liver. 

As further tissues were examined it became evident that the details of mitochondrial morphology were very variable. While most cells had rod-or sausage-shaped organelles, some were spherical. Other cells had mitochondria with spiral cristae or with massive crystalline inclusions. In confirmation of earlier suggestions from classical microscopists the position of mitochondria in cells was also seen to be linked with the site in the cell where energy was required. In skeletal muscle the mitochondria were adjacent to the myofibrils in the renal tubules they were close to the inner (non-luminal) surface of the cell which was then found to be the location of the Na/K-ATPase involved in active 

Careful examination of the yellowish sediment obtained after spinning down the crude mitochondrial fraction showed it was frequently overlaid with loosely packed, fluffy material —the fluffy layer. Experiments from de Duve s and, later, Novikoff s laboratories in the 1950s demonstrated that the lighter, lysosomal fraction was enriched in a number of hydrolases including acid phosphatase, aryl sulphatase, B glucuronidase, RNAase, and a peptidase, cathepsin. All the enzymes had optimal pHs in the acid range (pH 5-pH 6). Density 

The microsomal fraction was first obtained by Claude in 1943. In addition to lipid in the fraction, he noted the presence of RNA-rich granules, consistent with reports from Brachet that cytoplasm stained for RNA by the methyl-green/pyronin procedure. Glucose-6-phos-phatase was a prominent enzyme when the fraction was prepared from liver. Since density gradient sedimentation showed G-6-P-ase was absent from mitochondria and lysosomes, it was used as a marker for liver microsomes. 

The nucleus. Except for bacteria all cells have a large, often spherical, nucleus in which one or several nucleoli can often be seen imder the optical microscope. The nucleus is surrounded by an inner and an outer membrane 

The 23 pairs of chromosomes in a human nucleus contain, together, about 200000 genes at the other end of the scale, a virus may have as few as ten. [A gene is a strip of DNA that completely specifies a particular RNA, which itself completely specifies one protein (mRNA) or one amino-acid (tRNA)]. 

A plasmid is an extrachromosomal genetic structure, foimd in most of the bacterial species, but not constantly present. It varies in size from 1000 to 400 000 nucleotide pairs, the latter size corresponding to about 600 genes. Plasmids can pass from bacterium to bacterium, even between members of different species. They are frequently responsible for introducing resistance to drugs (Section 6.5). For a review of plasmids, see Cohen (1976). 

Mitochondria. Mitochondria, the energy generators-and-storers of the living cell, are present in all cells except bacteria. Mitochondria, which are the site of all oxidative phosphorylation, form rods or nearly spherical cylinders, from 0.2 to 3.0 pm in diameter. Often as many as a thousand mitochondria are present in a cell. 

Under aerobic conditions, as most cells grow, mitochondria are the site of (i) the tricarboxylic acid cycle which transforms (to carbon dioxide, water, and energy) the acetyl Co-A which is produced by the metabolism of both carbohydrates and fatty acids (ii) the enzymes that oxidize and convert fatty acids to acetyl Co-A (iii) the respiratory chain enzymes which transmit, to atmospheric oxygen, the electrons removed from all the various metabolic substrates, and store part of the energy, obtained in this way, as adenosine triphosphate. The enzymes of carbohydrate glycolysis, the Meyerhof sequence, are in the cytoplasm. 

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The hypothesis of the participation of those cholesterol transporters (NPCILI and ABCAl) in the carotenoid transport remains to be confirmed, especially at the in vivo human scale. If the mechanism by which carotenoids are transported through the intestinal epithelial membrane seems better understood, the mechanism of intracellular carotenoid transport is yet to be elucidated. The fatty acid binding protein (FABP) responsible for the intracellular transport of fatty acids was proposed earlier as a potential transporter for carotenoids. FABP would transport carotenoids from the epithelial cell membrane to the intracellular organelles such as the Golgi apparatus where CMs are formed and assembled, but no data have illustrated this hypothesis yet. 

Lucas WJ, Wolf, S. Plasmodesmata the intracellular organelles of green plants. Trends Cell Biol 1993 3 308-315. 

Because disrupted tissue preparations were unsatisfactory, attempts were made to work either with more organized systems such as tissue slices (liver-Krebs) or to identify and isolate the intracellular organelles involved in the reactions. Cytochemical procedures were developed in the 1930s and 1940s to locate sites of reaction in situ in cells (Chapter 9). Examination of cell ultrastructure became possible when the electron microscope was introduced after 1945. Techniques for the isolation of cell organelles, notably mitochondria, were developed about this time (Chapter 9). 

Mitochondria The intracellular organelle in which respiration and other important metabolic reactions take place. 

Ribosomes The intracellular organelles attached to the rough endoplasmic reticulum and involved with protein synthesis. 

Membrane structure and function are discussed in some detail in Chapter 9 and in the discussion of the intracellular organelles that follows. It is worthwhile to note here that biologic membranes, whether the cytoplasmic membrane or those of intracellular organelles, play active and unique roles in the integrated metabolism and function of cells. 

As has been extensively discussed elsewhere (Schurer and Elias, 1991 Wertz, Kremer, and Squier, 1992 Gray et al., 1982), the lipids of the stratum comeum intercellular spaces are unusual in mammalian biology, in that ceramides make up a substantial mole fraction of the total. Furthermore, the final lipid composition of the stratum comeum ilitercellular membranes is considerably different from that found in the intracellular organelles in which these lipids are first synthesized (Squier et al., 1991b Grayson et al., 1985 Wertz et al., 1984), and extensive lipid modifications occur during epidermal differentiation. 

Whereas DNA is mostly located in the nucleus of cells in higher organisms (with some also in mitochondria and in plant chloroplasts), RNA has a much broader cellular distribution. RNA comes in three major and distinct forms, each of which plays a cmcial role in protein biosynthesis in the ribosome, the intracellular organelle which is the site of protein biosynthesis. Ribosomal RNA (rRNA) represents two-thirds of the mass of the ribosome, messenger RNA (mRNA) encodes the information for the amino acid sequence of proteins, while transfer RNAs (tRNAs) serve as adaptor molecules, allowing the four-letter code of nucleic acids to be translated into the 20-letter code of proteins. The tRNA molecules contain a substantial number of modified bases, which are introduced by specific enzymes. 

Hydroquinone is closely related to phenol and can reduce melanin production, ft appears that it can also degrade melanosomes. It has a tyrosinase-inhibiting activity and can change the membrane structure of the intracellular organelles of melanocytes. Hydroquinone acts mostly on the first stages of melanin synthesis. Its action is therefore gradual, like any tyrosinase inhibitor. 

There is ample evidence that the sterols of plant cells are localized in the membranes of the intracellular organelles. Evidence for their function in the membranes is still only indirect. The polyene antibiotic filipin increases the cellular... 

When a free radical reacts, it usually snatches an electron from the reactant, turning it into a free radical. This in turn will steal a single electron from another nearby molecule. A chain reaction ensues until two free radicals react together, effectively neutralizing each other, or alternatively, until an unreactive free-radical product is formed. Free radicals are said to be quenched by vitamin C, because the free-radical product — the ascorbyl radical — is so unreactive. As a result, free-radical chain reactions are terminated. Lipid-soluble vitamin E (a-tocopherol) works in the same way, in membranes rather than in solution, often in cooperation with vitamin C at the interface between membranes and the cytosol (the watery ground substance of the cytoplasm that surrounds the intracellular organelles). When vitamin E reacts with a free radical, it too produces a poorly reactive (resonance-stabilized) free-radical product, called the a-tocopheryl radical. Tocopheryl radicals can be reconverted into vitamin E using electrons from vitamin C. 

Ion channels are found not only in the cell membrane but also in the membranes of the intracellular organelles such as mitochondria, lysozymes, nucleus, and secretory, endocytotic, and synaptic vesicles, as well as in the endoplasmic/sacroplasmic reticulum (ER/SR). Two major intracellular Ca channels, the inositol 1,4,5-trisphosphate (IP3) receptor and the ryanodine receptor (RyR), are located in the ER/SR membranes and contribute to changes in intracellular Ca concentration. Based on their mechanism of activation, these two channels are classified as ligand-gated channels (Section 16.4.1). 

Quinine, a cinchona alkaloid, acts primarily as a blood schizonticide. Qninine s antimalarial action is unclear. It was once believed to be dne to the intercalation of the qninoline moiety into the DNA of the parasite, thereby reducing the effectiveness of DNA to act as a template, as well as depression of the oxygen uptake and carbohydrate metabolism of the plasmodia. Recently it has been thonght that quinine s pH elevation in the intracellular organelles of the parasites plays a role in the mechanism. 

Eysosomes are the intracellular organelles of digestion enclosed by a single membrane that prevents the release of its digestive enzymes into the cytosol. They are central to a wide variety of body functions that involve elimination of unwanted material and recycling their components, including destruction of... 

The exact pH of cells is unknown, as is, of course, the exact pH of the intracellular organelles. Histo-chemical studies suggest that the intracellular pH may vary considerably and is much lower than that of body fluids (7.0). Phosphate and proteins are the most important intracellular buffers. 

Mercer, Studies on the sterols and steryl esters of the intracellular organelles of maize shoots. Biochem. J. 110 II9... 

Another finding revealed by radioautography was the accumulation of label over lipid droplets with time of perfusion (Fig. 4), which indicated that the triglyceride deposited in droplet form has a turnover rate slower than that located in the intracellular organelles. 

Of all the intracellular organelles, the mitochondrion has been the most extensively studied with respect to the compartmentation of compounds within its boimdaries. In part, this results from the ease of separation of mitochondria from mammalian tissues (most notably the liver), as well as from the key role mitochondria play in a number of metabolic processes. The mitochondrial membrane is capable of transporting metabolites on specific transporters and of segregating metabolites from the cytosol. It is important to note that some metabolites apparently move across the mitochondrial membrane in an unspecific or non-carrier-linked manner. For example, ketone bodies, water, CO2, and oxygen appear to freely diffuse into and out of mitochondria. In the following sections we will discuss specific aspects of the transport mechanisms, followed by a more general discussion of their role in regulating major metabolic pathways. We will start with the most important result of intracellular compartmentation—oxidative phosphorylation—as viewed by the chemiosmotic theory. 

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