Cytology · MU Pleven

Topic 10. Membranous cell organelles – endoplasmic reticulum.

Explains the endoplasmic reticulum as a network of membrane cisternae that synthesises proteins and lipids and acts as a transport system. It then separates the rough form, covering ribosome-bound protein synthesis, signal sequences, folding in the lumen and degradation of misfolded proteins with an example of an inherited disease, from the smooth form, covering lipid and steroid synthesis, detoxification, calcium regulation in muscle and tissue distribution. A picture accompanies the text.

Inside: 1 section · 1 labelled picture · 15 flashcards · 3 min

Topic 10 of 59 in the MU Pleven syllabus for Cytology (Medicine, Year 1). The free AceMed note covers it in 1 section, about 433 words (3 min of reading), with 15 flashcards.

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  1. TOPIC 10. Membranous cell organelles – endoplasmic reticulum.

TOPIC 10. Membranous cell organelles – endoplasmic reticulum.

  • Endoplasmic reticulum ER is present in the cytoplasm of cells. It is a network of intercommunicating membrane sacs called cisternae which have an inside cisternae space, separate from the cytosol. They are major sites of biosynthesis of proteins and lipids and act as transport systems.

• Rough Endoplasmic Reticulum (RER): Characterised by ribosomes (polyribosomes) attached to its cytosolic surface.

  • Function: Primarily involved in the synthesis and segregation of proteins not destined for the cytosol. This includes proteins for secretion from the cell (e.g., digestive enzymes, collagen, immunoglobulins by plasma cells and fibroblasts), membrane proteins (e.g., of the ER, Golgi, cell membrane), and enzymes to be stored in lysosomes.
  • Process: Protein synthesis begins on polyribosomes in the cytosol. Proteins destined for the ER have an N-terminal signal sequence. This sequence is bound by a signal-recognition particle (SRP), which then binds to SRP receptors on the ER membrane.
  • Translation continues, and the nascent polypeptide chain is transferred through a translocator complex into the RER lumen.
  • The signal sequence is removed by signal peptidase. o Protein Folding & Quality Control: Inside the RER lumen, molecular chaperones guide the proper folding of new proteins and prevent aggregation.
  • The RER has a highly regulated system to prevent nonfunctional proteins from being transported further. Initial glycosylation of

glycoproteins and assembly of multichain proteins also occur here.

  • ER-Associated Degradation (ERAD): Unsalvageable misfolded proteins are translocated back into the cytosol, conjugated to ubiquitin, and degraded by proteasomes. Malfunctions in this system can lead to inherited diseases, such as some forms of osteogenesis imperfecta.

• Smooth Endoplasmic Reticulum (SER): Lacks ribosomes and mainly synthesises lipids, phospholipids, and steroids (e.g., testosterone/oestrogen).

  • The SER is continuous with the RER, yet less abundant. It is not basophilic so better seen with EM. SER cisternae are more tubular, rather than flattened cisternae of RER.
  • The function of the smooth endoplasmic reticulum is to synthesize lipids and steroids in the cell, drug detoxification and transport proteins from the RER to the Golgi body to undergo PTM.
  • Major role is phospholipid synthesis, which are transferred from SER to other membranes.
  • SER is abundant in the cytoplasm of cells that secrete steroid hormones and contains the enzymes needed for steroid synthesis
  • SER is also abundant in the liver cells, where it contains enzymes for glycogen metabolism and for the detoxification of alcohol and drugs

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