Endoplasmic Reticulum – The Lipid and Protein Processing Machinery of the Cell

By | July 23, 2021
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The endoplasmic reticulum (ER) is divided into – smooth endoplasmic reticulum (SER) and rough endoplasmic reticulum (RER). ER consists of a system of membranes that enclose a space or lumen, that separates it from the cytosol. The composition of the luminal or cisternal space inside the ER membranes is quite different from the cytosolic space. The research study of fluorescently labeled lipids and proteins shows that they diffuse from one type of ER to another, implying that their membranes are continuous. Both types of ER do involve common activities such as the synthesis of certain lipids and cholesterol. On the other hand, certain types of proteins are found only in one or the other type of ER. Hence, there are structural and functional differences between SER and RER.

RER is defined by the presence of ribosomes bound to its cytosolic surface and these associated ribosomes are absent in SER. The RER is composed of flattened sacs, called cisternae and its outer membrane is continuous with a nuclear envelope which also bears ribosomes on its cytosolic surface. Whereas, membranous elements of SER are highly curved and tubular, forming an interconnected pipeline system that is continuous with the cell cytoplasm. This structural difference can be visualized in laboratory experiments when a cell is homogenized, the SER fragments into smooth-surfaced vesicles, and the RER fragments into rough-surfaced vesicles.

The abundance of the two types of ER depends on cell metabolism and state of cell activities. For example, the cells of the pancreas and salivary glands that secrete large amounts of proteins, have extensive regions of RER.

Functions of SER: The SER is found in several cell types, including skeletal muscle, kidney tubules, and steroid-producing endocrine glands.

  • Steroid hormones are synthesized in the endocrine cells of the adrenal cortex and gonad.
  • A variety of organic compounds, such as barbiturates and ethanol, are subjected to detoxification in the liver through proliferation of the SER. Such detoxification is carried out by a system of oxygen transferring enzymes (oxygenases), including cytochrome P450 family. These enzymes are not substrate-specific, oxidize thousands of hydrophobic compounds and convert them into hydrophilic compunds that are excreted. The effects are not always positive. For example, benzopyrene formed when meat is charred on a grill is converted into a potent carcinogen by the “detoxifying” enzymes of SER.
  • Sequestration of calcium ions in the cell is another major function of the SER. For example, sarcoplasmic reticulum of the cardiac muscles realease calcium ions in a regulated manner to trigger contraction.

Functions of RER: The RER is found in the cell types that secrete large quantities of proteins, such as the acinar cells of the pancreas or the mucus-secreting cells in the lining of the digestive tract. The nucleus and the extensive array of cisternae of the RER are located on the basal surface of the cell, facing the blood supply.

  • The RER serves as the site for biosynthetic pathway initiation, site for synthesis of proteins, carbohydrate chains, and phospholipids that travels through the membranous compartments of the cell.
  • The RER is also involved in the synthesis of polypeptide hormones by endocrine cells, antibodies by plasma cells, and blood serum proteins by the liver cells.
  • Certain polypeptides are produced by the ribosomes attached to the cytosolic surface of RER, such as integral membrane proteins, secreted proteins, soluble proteins of the endomembrane system, including the ER, Golgi complex, lysosome, endosomes, and vesicles.
  • There are other types of polypeptides that are secreted by the free-ribosomes, that are not attached to the ER, and are subsequently released into the cytosol. This includes the enzymes of glycolysis pathway, proteins transported into the nucleus, proteins incorporated into other organelles (mitochondria and peroxisomes), and proteins of the cytoskeleton.

Image credit: openstax.org