Difference Between Smooth and Rough 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 forming a continuous network from the nuclear membrane, spreading throughout the cytoplasm of the cell. The inner part of the endoplasmic reticulum is called lumen (or luminaire or cisternal space).

The ER membrane has a similar structure to 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 :

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Rough endoplasmic reticulum (RER)

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

Endoplasmic Smooth Reticulum (REL)

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

Functions of the smooth endoplasmic reticulum :

Synthesis, storage, and transport of lipids.
- On the cytoplasmic side of its membrane, almost all the lipid constituents of the membranes are synthesized: cholesterol, phospholipids, glycolipids, etc.
- The fatty acids are synthesized in the cytosol and enter the membrane by the 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 by activating muscle contraction.
Detoxification. Its membrane contains detoxifying enzymes that degrade fat-soluble substances that may be harmful and transform them into soluble substances that can be excreted by the body. Invertebrates, detoxification is mainly performed in the cells of the liver, kidneys, intestine, lungs, and skin. For example, REL abounds in liver cells, where it contains enzymes that detoxify harmful products such as alcohol and other by-products of metabolism, such as ammonia .
Glucose release 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 degrades into 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 take them to the Golgi apparatus.

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