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E.r., the golgi body andE.R., Golgi And Vesicles
E.R. And Vesicles
Thousands of structures with membranes crowd the eukaryotic cytoplasm, the most numerous being called the endoplasmic reticulum (ER), a dense network of membranes running through the cytoplasm of every cell and enclosing a network of interconnected tubes (cisternae), which form a connecting channel throughout the cytoplasm. ‘Endoplasmic’ means ‘inside the cytoplasm’, and ‘reticulum’ means ‘network’. The E.R. can make up a significant percentage of the overall volume of the cell. There are two types of ER: rough ER (RER), so-called because its cytoplasmic face is dotted with ribosomes, and smooth ER (SER), which is devoid of ribosomes. The RER is more extensive than SER and acts as a protein producer and storage vessel. Amino acid chains, freshly synthesised by the ribosomes, slide through tiny holes in its membrane into the cisternae, where they remain until they are sent elsewhere in the cytoplasm, or to the cell edge where they are secreted or incorporated into the cell membrane. The membranes of the ER usually form a tightly packed and flattened sac-like structures, that form inter-connected compartments within the cytoplasm. The internal space formed by the membrane sheets is known as the ER lumen. In most cells the ER lumen forms a single internal compartment. The membranes of other organelles are not connected to the ER and appear to form separate distinct compartments within the cytoplasm.
The ER membranes and lumen contain a large number and variety of enzymes which catalyse many different types of chemical reactions. In some cases the membranes serve as a framework for systems of enzymes that carry out sequential biochemical reactions. Other ER enzymes are located within the ER lumen.
If labelled amino acids are introduced into live cells, the radioactivity first appears in the ribosomes. Within a few minutes it is found in the membrane enclosed sacs of the rough ER. The reason for this is that proteins are produced at the ribosomes, threaded through the membrane, and are stored temporarily in the sacs of the rough ER before they are used inside the cell or are secreted to the exterior.
The SER produces steroid hormones and other lipids. The two types of endoplasmic network are not joined, so their cargoes do not get confused. RER connects directly with nuclear pores, through which mRNA molecules for protein manufacture pass. Cells that generate lots of protein, such as stomach cells that secrete digestive enzymes, possess an extensive RER. Likewise, cells in the gonads that produce steroid hormones have a large SER.
Proteins in the RER have a variety of possible destinations.
Many are shuttled to the cell's sorting and packaging factory, the Golgi apparatus, where they are prepared for secretion. As the proteins pass through each layer in the stack, moving progressively closer to the cell margins, they undergo chemical modification, usually involving the addition of sugar to form glycoproteins. Many cell secretions, such as mucus, are glycoproteins. As a...
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