Endoplasmic reticulum animal cell

The most important membranes in animal cells are the plasma membrane, the inner and outer nuclear membranes, the membranes of the endoplasmic reticulum (ER) and the Golgi apparatus, and the inner and outer mitochondrial membranes. Lysosomes, peroxisomes, and various vesicles are also separated from the cytoplasm by membranes. In plants, additional membranes are seen in the plastids and vacuoles. All membranes show polarity—e., there is a difference in the composition of the inner layer (facing toward the cytoplasm) and the outer layer (facing away from it). 

The fatty acids could be carried by proteins by a process similar to the way in which serum albumin binds fatty acid in the bloodstream of mammals. Other types of lipid might be formed into complexes analogous to low-density lipoproteins of the type found in animal tissues, where the lipid core of the lipoprotein is surrounded by a hydrophilic cortex made up of protein, phospholipid, and cholesterol (87). This allows the lipid to be moved in an aqueous environment. The protein of the lipoprotein shell could also act as possible ligands for particular receptors at the membrane of the cell at which the export occurs. The lipoproteins, if they are present, would probably be formed within the endomembrane lumen and would receive the proteins at the endoplasmic reticulum. 

Five kinds of phospholipid predominate phosphatidylcholine, phosphatidylethanolamine, phosphatidyl-serine, phosphatidylglycerols, and sphingomyelin. Usually there are also small amounts of phosphatidyli-nositol. The major phospholipid in animal cells is phosphatidylcholine, but in bacteria it is phosphatidylethanolamine. The phospholipids of E. coli consist of 80% phosphatidylethanolamine, 15% phosphati-dylglycerol, and 5% diphosphatidylglycerol (cardio-lipin). Significant amounts of cardiolipin are found only in bacteria and in the inner membrane of mitochondria. Sphingomyelin is almost absent from mitochondria, endoplasmic reticulum, or nuclear membranes. 

Hydrocarbons yield more energy upon combustion than do most other organic compounds, and it is, therefore, not surprising that one important type of food reserve, the fats, is essentially hydrocarbon in nature. In terms of energy content the component fatty acids are the most important. Most aerobic cells can oxidize fatty acids completely to C02 and water, a process that takes place within many bacteria, in the matrix space of animal mitochondria, in the peroxisomes of most eukaryotic cells, and to a lesser extent in the endoplasmic reticulum. 

Both catalyze chain cleavage and transfer reactions (Eqs. 17-14 and 17-15) that involve the same group of substrates. These enzymes use the two basic types of C-C bond cleavage, adjacent to a carbonyl group (a) and one carbon removed from a carbonyl group ((3). Both types are needed in the pentose phosphate pathways just as they are in the citric acid cycle. The enzymes of the pentose phosphate pathway are found in the cytoplasm of both animal and plant cells.n7c Mammalian cells appear to have an additional set that is active in the endoplasmic reticulum and plants have another set in the chloroplasts.117c... 

Enzyme complexes occur in the endoplasmic reticulum of animal cells that desaturate at A5 if there is a double bond at the A8 position, or at A6 if there is a double bond at the A9 position. These enzymes are different from each other and from the A9-desaturase discussed in the previous section, but the A5 and A6 desaturases do appear to utilize the same cytochrome b5 reductase and cytochrome b5 mentioned previously. Also present in the endoplasmic reticulum are enzymes that elongate saturated and unsaturated fatty acids by two carbons. As in the biosynthesis of palmitic acid, the fatty acid elongation system uses malonyl-CoA as a donor of the two-carbon unit. A combination of the desaturation and elongation enzymes allows for the biosynthesis of arachidonic acid and docosahexaenoic acid in the mammalian liver. As an example, the pathway by which linoleic acid is converted to arachidonic acid is shown in figure 18.17. Interestingly, cats are unable to synthesize arachidonic acid from linoleic acid. This may be why cats are carnivores and depend on other animals to make arachidonic acid for them. Also note that the elongation system in the endoplasmic reticulum is important for the conversion of palmitoyl-CoA to stearoyl-CoA. 

Main intracellular compartments of an animal cell. Cytosol, endoplasmic reticulum, Golgi complex, nucleus, mitochondria, endosome, lysosome, and peroxisome are distinct compartments that are isolated from the rest of the cell by at least one selective membrane. 

Complexes of PolyP and PHB were found in the membranes of the endoplasmic reticulum and mitochondria of animal cells (Reusch, 1989), which suggests their participation in the processes of transmembrane transfer. The most intriguing report was that the Ca2+-ATPase purified from human erythrocytes contains PolyP and PHB and that the plasma membrane Ca2+-ATPase may function as a polyphosphate kinase it exhibits ATP-PolyP transferase and PolyP-ADP transferase activities (Reusch et al, 1997). These findings suggest a novel supramolecular structure for the functional Ca2+-ATPase and a new mechanism of uphill Ca2+ extrusion coupled with ATP hydrolysis (Reusch et al., 1997). 

Four classes of LDL receptor mutations have been identified. Class 1 mutations are characterized by the failure of expression of the receptor protein. It is possible, however, that a modified protein is produced but it is not recognized as an LDL receptor protein. Class 2 mutations involve a nonsense mutation (premature termination of protein synthesis Chap. 17), and result in a defect in the transfer of the receptor from the endoplasmic reticulum to the cell membranes. This class of mutation is common in Afrikaners and Lebanese. The Watanabe heritable hyperlipidemic rabbit (WHHL) is an animal model which has a Class 2 defect and has been used extensively for the study of familial hypercholesterolemia. Class 3 mutations result in abnormal binding of LDL. This can be caused by alterations in the amino acid sequence of Domain 1. Class 4 mutations are those with defective internalization due to the receptor s inability to be located in coated pits. This is the result of mutations in the fifth, C-terminal domain. 

Liver. In humans, chronic Cd exposure does not typically result in hepatotoxicity. In laboratory animals, the liver accumulates the largest concentrations of Cd after acute or chronic exposures. In chronically exposed rats, liver injury occurs prior to renal dysfunction. Chronic Cd effects in the liver include increased plasma activities of alanine and aspartate aminotransferases, structural irregularities in hepatocytes, and decreased microsomal mixed function oxidase and CYP450 activities. Acute exposures in rats result in hepatic necrosis, particularly in parenchymal cells. Additionally, rough endoplasmic reticulum deteriorates, while smooth endoplasmic reticulum proliferates. Mitochondria are also degraded. As is the case with chronic exposure, microsomal mixed function oxidases and CYP450s are inhibited. 

Golgi complex Organelles in animal cells composed of a series of fattened sacs that sort, chemically modify, and package proteins produced on the rough endoplasmic reticulum. 

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