The endoplasmic reticulum (ER) is present in every eukaryotic cell except mature erythrocytes. It is a cell organelle with multiple functions. Without ER, the cell and thus the organism would not be viable.
What is the endoplasmic reticulum?
The endoplasmic reticulum (ER) is a very structurally rich cell organelle with a channel system of cavities. The cavities are surrounded by membranes. In the eukaryotic cell, the ER performs a variety of tasks. It has storage, detoxification, control or synthesis functions. It is located in the cytoplasm of the cell and is in close contact with the nuclear envelope of the nucleus. Furthermore, a distinction is made between rough and smooth ER. Both forms belong to the endoplasmic reticulum, but have different functions. The rough ER has ribosomes on the surface of the membrane. In contrast, the smooth ER, as the name implies, is smooth. It does not have ribosomes. The term endoplasmic reticulum is composed of three parts. Endo is the Greek term for inside. Plasmatic indicates the cell plasma and reticulum is the Latin term for reticulum. Thus, translated, the endoplasmic reticulum is a reticular organelle, which is located in the cytoplasm of a cell.
Anatomy and structure
The endoplasmic reticulum is a maze of channels, vesicles, and sac-like structures (cisternae), all surrounded by a membrane. Due to this fact, half of all membranes in a cell are located in the ER. The membranes seal off the lumen (the inside of the ER) from the cytoplasm. The ER membrane is directly connected to the nuclear envelope of the nucleus and forms a unit with it. Thus, the ER lumen is in direct communication with the membrane space between the nuclear envelope, which is called the perinuclear space. The ER is partly smooth and partly rough. The rough ER has ribosomes on its surface, whereas the smooth ER is ribosome-free. Both forms of the ER differ in their functions. The endoplasmic reticulum is not static, but is subject to constant changes. Thus, there are constant extensions, splittings and fusions of membrane structures. These activities are significantly influenced by the cytoskeleton of the cell. In plant and animal cells, different proteins play a role in the changes of the endoplasmic reticulum. Whereas in plant cells and yeast, F-actin is the main determinant, in animal and human cells the ER is mainly influenced by microtubules. Microtubules are tubular filaments of proteins that form the basis of the cytoskeleton. During cell division, these proteins ensure that the endoplasmic reticulum is passed on to daughter cells.
Function and tasks
Both at and in the endoplasmic reticulum, many processes take place for protein synthesis, control, modification, or transport. Furthermore, it forms new nuclear membranes after cell division and laces them off for transmission. The ER is also an important calcium store in the cell and therefore plays a key role in signal transduction. Specifically in muscle cells, it is responsible for muscle contraction due to its function as a calcium mediator. There it is also referred to as the sarcoplasmic reticulum. As mentioned earlier, smooth and rough ER exert different functions. The smooth ER does not have ribosomes. It is responsible for calcium storage, for carbohydrate metabolism, for the formation of certain lipids such as phospholipids, fatty acids or steroids and for detoxification reactions inside and outside the liver. Testes and ovaries contain particularly large amounts of smooth ER because the steroid hormones testosterone and estrogen are produced there. The parenchymal cells of the liver are also rich in smooth ER. Excess glucose is stored there in the form of the polymer glucogen. Here, smooth ER is responsible for the renewed cleavage of glucogen (glycogenolysis). The smooth ER contains enzymes in its membrane that can also be released outside the liver and initiate detoxification reactions. These so-called CYPs oxidize exogenous substrates and make them water-soluble. This allows the breakdown products of the toxins to leave the body via the kidneys. The rough ER performs two important functions. It controls both protein biosynthesis and membrane production via the ribosomes.Protein chains are bound to the membrane-bound ribosomes and are immediately shuttled into the lumen of the ER. Proteins formed in the cytosol also first enter the interior of the ER. There, the protein chains are folded, acquiring their three-dimensional structure. For membrane production, the membrane of the ER first grows, is split off and transported to other structures of the inner membrane system. The ribosomes of the endoplasmic reticulum form the corresponding membrane proteins.
Diseases
Without the endoplasmic reticulum, the organism would not be viable. Many functions of the ER are absolutely essential for life. Malfunctions of the ER would also lead to the death of the organism. For example, detoxification reactions could no longer take place in the body. Testes and ovaries would be non-functional because sex hormones could no longer be produced. Muscles and nerves would no longer function without ER because the calcium storage function would be lost. Thus, stimulus transmission would also no longer take place. Cell division would also no longer be possible. This fact leads to the fact that the ER must be fully functional in order to keep the organism viable. Any dysfunction is fatal. Therefore, no diseases have yet been described that are directly caused by dysfunction of the endoplasmic reticulum.
