The endoplasmic reticulum (meaning "within the cytoplasm" and "net") consists of flattened sheets, sacs, and tubes of membrane that cover the entire expanse of a eukaryotic cell's cytoplasm. This internal system of membrane is continuous with the double membrane that surrounds the cell's nucleus. Therefore, the encoded instructions that the nucleus sends out for the synthesis of proteins flow directly into the endoplasmic reticulum. Within the cell, the endoplasmic reticulum synthesizes lipids and proteins. The proteins that the endoplasmic reticulum synthesizes, such as enzymes, are exported from the cell to perform various functions in the body. Proteins that are made in the cell for use by the cell-for instance, as channels in the plasma membrane-are made by the free ribosomes that dot the cytoplasm.
Two types of endoplasmic reticulum are found in the eukaryotic cell. Rough endoplasmic reticulum is studded with ribosomes on its outer face. These ribosomes are the sites of protein synthesis. Once a protein is synthesized on a ribosome, it is enclosed within a vesicle, a small, membrane-bound "bubble." The vesicle travels to another organelle, the Golgi body. Within the Golgi body, the proteins within the vesicle are further modified before they are exported from the cell. Cells that specialize in protein secretion contain large amounts of rough endoplasmic reticulum. For instance, cells of the pancreas that produce the protein insulin, have abundant rough endoplasmic reticulum. Plasma cells, white blood cells that secrete immune proteins called antibodies, are so crowded with rough endoplasmic reticulum it is difficult to distinguish other organelles within the cytoplasm.
The other type of endoplasmic reticulum is smooth endoplasmic reticulum. Smooth endoplasmic reticulum does not have ribosomes and is the site of lipid metabolism. Here, macromolecules containing lipids are broken down into their constituent parts. In addition, smooth endoplasmic reticulum functions in the synthesis of lipid-containing macromolecules. Smooth endoplasmic reticulum is not as common in cells as rough endoplasmic reticulum. Large amounts of smooth endoplasmic reticulum are found in cells that specialize in lipid metabolism. For instance, liver cells remove alcohol and drugs from the bloodstream. Liver cells have an impressive network of smooth endoplasmic reticulum. Similarly, cells of the ovaries and testes, which produce the lipid-containing hormones estrogen and testosterone, contain large amounts of smooth endoplasmic reticulum.
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