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6.9.1.1. The endoplasmic reticulum

Endoplasmic reticulum

The endoplasmic reticulum is the largest organelle of many eukaryotic cells. It is formed by tubules (tubes) and sacs (cisterns) that communicate in a continuous network from the nuclear membrane, extending throughout the cytoplasm of the cell. The inner part of the endoplasmic reticulum is called the lumen (or luminal or cisternal space).

The ER membrane has a similar structure to that of the plasma, although it is somewhat thinner and has fewer lipids and more proteins than the plasma membrane.

According to its functions and its composition, two types of endoplasmic reticulum are distinguished:

Rough endoplasmic reticulum (RER)

  • It has ribosomes attached to its membranes, on the cytosol side. Ribosomes are linked by their largest subunit by the help of proteins from the group of riboforins, which are not found in the REL.
  • It communicates with the nuclear membrane and the smooth endoplasmic reticulum.
  • It is made up of sacks and large flattened cisterns.
  • Its main function is to synthesize proteins with ribosomes attached to its membrane. The synthesized proteins pass from the RER to the REL, then to the Golgi apparatus and from there, to the lysosomes, to the plasma membrane or to the exterior. On the way through these organelles, the molecule will undergo additional processing.
  • The proteins synthesized and stored in the RER, before being transported to other cytoplasmic organelles (Golgi apparatuslysosomes), to the plasma membrane or to the outside of the cell, must be glycosylated to become glycoproteinsThe glycosylation of proteins occurs in the lumen of the reticle, while proteins of the cytosol usually not glycosylated.

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Smooth endoplasmic reticulum (REL)

  • It lacks ribosomes, so its surface is smooth.
  • Formed mainly by a network of tubules, which join the RER, which extends throughout the cytoplasm.
  • Most cells have a sparse smooth endoplasmic reticulum, but it is especially abundant in:
    • Striated muscle cells.
    • Leydig's ovarian interstitial cells, testis, and cells of the adrenal cortex that secrete steroid hormones.
    • Hepatocytes, where it participates in the synthesis of lipoprotein molecules.

Smooth endoplasmic reticulum functions:

  • Synthesis, storage and transport of lipids.
    • Almost all the constituent lipids of the membranes are synthesized on the cytoplasmic side of its membrane : cholesterol , phospholipids , glycolipids , etc.
    • The fatty acids are synthesized in the cytosol and enter the membrane by action of a flippase.
    • In some cells, the smooth endoplasmic reticulum produces large amounts of lipids, such as steroid hormones.
  • Muscle contraction. In striated muscle tissue cells, REL releases calcium, activating muscle contraction.
  • Detoxification. Its membrane contains detoxifying enzymes that break down fat-soluble substances that can be harmful and transform them into soluble substances that can be excreted by the body. In vertebrates, detoxification takes place mainly in the cells of the liver, kidneys, intestine, lungs and skin. For example, REL is abundant in liver cells, where it contains enzymes that detoxify harmful products such as alcohol and other by-products of metabolism, such as ammonia.
  • Release of glucose from glycogen (in hepatocytes). The glycogen stored in the liver is in the form of small granules attached to the membranes of the REL. When energy is needed, glycogen is degraded, obtaining glucose-6-phosphate in the cytoplasm. The REL removes the phosphate group and the glucose molecules enter the REL and are sent to the blood where they are required.
  • It produces transport vesicles with newly synthesized proteins and lipids to transport them to the Golgi apparatus.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Castilla y León, September 2017, option B, question 2.

In relation to the endoplasmic reticulum:

a) Indicate the types that there are, make a drawing of each one of them and indicate the functions of each one of them. (1.0)

b) What relationships do they have with other organelles in the cell? (0.5)

c) Cite a cell type where one of them predominates and explain why. (0.25)

Madrid, September 2017, option A, question 2.

In relation to various structures that we can find in eukaryotic cells:

b) Cite the differences in function between rough endoplasmic reticulum and smooth endoplasmic reticulum (0.5 points).

Valencian Community, June 2019, option B, block 2, question 4

It indicates the structural differences between the rough and the smooth endoplasmic reticulum and indicates their functions.

Navarra, July 2019, option A, question 5

a) Explain the function of the endoplasmic reticulum (ER) indicating its location in the cell.

b) Cite the main function of RER

c) Cite the main function of the REL

Fundamental ideas about the endoplasmic reticulum

The endoplasmic reticulum is an organelle made up of tubules (tubes) and sacs (cisterns) communicating with each other forming a continuous network from the  nuclear membrane, extending throughout the cytoplasm of the cell.

There are two types of endoplasmic reticulum: