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Q. Which Gine the paltarn of separation
in increasi oder in these organisms
a) Ribosome
6 Membrane vesicle
O ER
od Cellular deberies
e Mitachondria
8 Nucleave fraction
harge proteins
Answers
Answer:
All eucaryotic cells have an endoplasmic reticulum (ER). Its membrane typically constitutes more than half of the total membrane of an average animal cell (see Table 12-2). The ER is organized into a netlike labyrinth of branching tubules and flattened sacs extending throughout the cytosol (Figure 12-35). The tubules and sacs are all thought to interconnect, so that the ER membrane forms a continuous sheet enclosing a single internal space. This highly convoluted space is called the ER lumen or the ER cisternal space, and it often occupies more than 10% of the total cell volume (see Table 12-1). The ER membrane separates the ER lumen from the cytosol, and it mediates the selective transfer of molecules between these two compartments.
Figure 12-35. Fluorescent micrographs of the endoplasmic reticulum.
Figure 12-35
Fluorescent micrographs of the endoplasmic reticulum. (A) Part of the ER network in a cultured mammalian cell, stained with an antibody that binds to a protein retained in the ER. The ER extends as a network throughout the entire cytosol, so that all (more...)
Image ch12fu5.jpg
The ER has a central role in lipid and protein biosynthesis. Its membrane is the site of production of all the transmembrane proteins and lipids for most of the cell's organelles, including the ER itself, the Golgi apparatus, lysosomes, endosomes, secretory vesicles, and the plasma membrane. The ER membrane makes a major contribution to mitochondrial and peroxisomal membranes by producing most of their lipids. In addition, almost all of the proteins that will be secreted to the cell exterior—plus those destined for the lumen of the ER, Golgi apparatus, or lysosomes—are initially delivered to the ER lumen.
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Membrane-bound Ribosomes Define the Rough ER
The ER captures selected proteins from the cytosol as they are being synthesized. These proteins are of two types: transmembrane proteins, which are only partly translocated across the ER membrane and become embedded in it, and water-soluble proteins, which are fully translocated across the ER membrane and are released into the ER lumen. Some of the transmembrane proteins function in the ER, but many are destined to reside in the plasma membrane or the membrane of another organelle. The water-soluble proteins are destined either for the lumen of an organelle or for secretion. All of these proteins, regardless of their subsequent fate, are directed to the ER membrane by the same kind of signal sequence and are translocated across it by similar mechanisms.
In mammalian cells, the import of proteins into the ER begins before the polypeptide chain is completely synthesized—that is, import is a co-translational process. This distinguishes the process from the import of proteins into mitochondria, chloroplasts, nuclei, and peroxisomes, which are posttranslational processes. Since one end of the protein is usually translocated into the ER as the rest of the polypeptide chain is being made, the protein is never released into the cytosol and therefore is never in danger of folding up before reaching the translocator in the ER membrane. Thus, in contrast to the posttranslational import of proteins into mitochondria and chloroplasts, chaperone proteins are not required to keep the protein unfolded. The ribosome that is synthesizing the protein is directly attached to the ER membrane. These membrane-bound ribosomes coat the surface of the ER, creating regions termed rough endoplasmic reticulum, or rough ER (Figure 12-36A).