One of the main differences between eukaryotic and prokaryotic cells is their high degree of compartmentalization (Sabatino et al., n.d.). Within this organization, a clearly defined nucleus stands out, surrounded by a nuclear envelope that encloses the genetic material inside. This is just one example of the spatial separation of functions that characterizes eukaryotic cells. On the other hand, the cytoplasm is traversed in all directions by a complex system of sacs and tubules, whose membranes act as boundaries between the cytoplasmic matrix and the lumen or cavity of this system. According to Sabatino et al. (n.d.), this collection of membranous structures, which also includes the nuclear envelope, is referred to as the endomembrane system (ES) or cytoplasmic vacuolar system (CVS).
What is the Endomembrane System?
The endomembrane system is a set of membranes and organelles in eukaryotic cells that, in a coordinated manner, perform key functions in the modification, packaging, and transport of lipids and proteins (Jung, 2020). This system includes various organelles, most notably the nuclear envelope, lysosomes, endoplasmic reticulum, and Golgi apparatus. Although not strictly part of the cell, the plasma membrane is considered part of this system due to its constant interaction with other endomembrane organelles and its crucial role in the export of secretory proteins, such as pancreatic enzymes. According to Jung (2020), it is essential to note that the endomembrane system does not include mitochondria, chloroplasts, or peroxisomes.
Endoplasmic Reticulum
The endoplasmic reticulum is a membranous structure that presents a series of channels and systems at its base, composed of folds of a lamellar structure similar to a protein that spans the cytoplasm (María, 2022). This organelle plays a fundamental role in protein modification and lipid synthesis (Jung, 2020). Its structure consists of a network of membranous tubules and flattened sacs. According to Jung (2020), the disks and tubules of the endoplasmic reticulum are hollow, and the internal space is called the lumen.
Rough Endoplasmic Reticulum
The rough endoplasmic reticulum gets its name from the presence of ribosomes attached to its cytoplasmic surface (Jung, 2020). As ribosomes synthesize proteins, the newly formed polypeptide chains enter the lumen. Some of these chains are fully introduced into the endoplasmic reticulum and float inside, while others are anchored to the membrane. Inside the endoplasmic reticulum, proteins fold and undergo modifications, such as the addition of carbohydrate side chains. These modified proteins are incorporated into the membranes of the cell, either of the endoplasmic reticulum or other organelles, or are secreted by the cell. In line with Jung (2020), if these proteins are not intended to remain in the endoplasmic reticulum, they are packaged into vesicles, small membranous spheres that facilitate transport, and subsequently sent to the Golgi apparatus or complex.
Furthermore, the rough endoplasmic reticulum produces phospholipids for other cell membranes, which are transported in the vesicles formed (Jung, 2020). Consequently, according to Jung (2020), since the rough endoplasmic reticulum is crucial in modifying proteins that will be secreted, cells that need to secrete large amounts of enzymes or other proteins, such as liver cells, have a high amount of rough endoplasmic reticulum.
Smooth Endoplasmic Reticulum
The smooth endoplasmic reticulum is a continuation of the rough endoplasmic reticulum, but has few or no ribosomes on its cytoplasmic surface (Jung, 2020). The functions of the smooth endoplasmic reticulum include the synthesis of carbohydrates, lipids, and steroid hormones, as well as the detoxification of drugs and poisons, and the storage of calcium ions. In muscle cells, a special type of smooth endoplasmic reticulum known as the sarcoplasmic reticulum is responsible for storing the calcium ions necessary to trigger the coordinated contraction of muscle fibers. Likewise, according to Jung (2020), within the rough endoplasmic reticulum, there are small sections of smooth endoplasmic reticulum that serve as exit sites for vesicles that bud off from the rough endoplasmic reticulum, known as transitional endoplasmic reticulum.
Golgi Apparatus
The vesicles that bud off from the endoplasmic reticulum have a specific destination, but before reaching their final target, it is essential to classify, package, and label the lipids and proteins they contain (Jung, 2020). These activities occur in the Golgi apparatus, an organelle composed of flattened membranous discs. The receiving side of the Golgi apparatus is called the cis face, while the opposite side is known as the trans face. Transport vesicles coming from the endoplasmic reticulum move towards the cis face, where they fuse and empty their contents into the lumen of the Golgi apparatus. During their transit through the Golgi apparatus, proteins and lipids can undergo further modifications. For instance, according to Jung (2020), short sugar chains may be added or removed, or phosphate groups may be incorporated as tags.
Finally, the modified proteins are classified according to markers such as amino acid sequences and chemical tags, and are packaged into vesicles that bud from the trans side of the Golgi apparatus (Jung, 2020). Some of these vesicles deliver their contents to various parts of the cell, such as lysosomes or vacuoles. Others fuse with the plasma membrane, releasing membrane-bound proteins that perform specific functions or secreting proteins outside the cell. Cells that secrete proteins, such as salivary gland cells, which produce digestive enzymes, or immune system cells, which secrete antibodies, contain multiple Golgi apparatuses. In plants, according to Jung (2020), the Golgi apparatus is also responsible for the production of polysaccharides, some of which are incorporated into the cell wall.
Lysosomes
The lysosome is an organelle that houses digestive enzymes and acts as the recycling facility for organelles in an animal cell (Jung, 2020). Its function is to break down old and unnecessary structures, allowing for the reuse of their molecules. Lysosomes are part of the endomembrane system, and some vesicles that emerge from the Golgi apparatus are destined for this organelle. Most proteins present in the lysosomal membrane have an unusually high amount of carbohydrates attached to them. According to Jung (2020), these sugars protect membrane proteins by preventing the digestive enzymes located inside the lysosome from degrading them.
Additionally, lysosomes have the ability to digest foreign particles that enter the cell from the outside (Jung, 2020). An example is the white blood cell known as a macrophage, which is part of the human immune system. In a process called phagocytosis, a section of the macrophage's plasma membrane invaginates, that is, it folds inward to engulf a pathogen. The invaginated section, containing the pathogen, separates from the plasma membrane and forms a structure known as a phagosome. Subsequently, in accordance with Jung (2020), the phagosome fuses with a lysosome, creating a combined compartment in which digestive enzymes act to destroy the pathogen.
Vacuoles
Plant cells are distinguished by having a lysosome-like organelle known as a vacuole (Jung, 2020). The large central vacuole performs various functions, such as storing water and waste, isolating hazardous materials, and containing enzymes capable of breaking down macromolecules and cellular components, similar to what occurs in lysosomes. Furthermore, according to Jung (2020), vacuoles in plants play an important role in osmotic balance and can store compounds such as toxins and pigments (particles that confer color).
Functions of the Endomembrane System
The endomembrane system houses enzymes involved in the synthesis of different macromolecules: proteins and glycoproteins in the rough endoplasmic reticulum, lipids in the smooth endoplasmic reticulum, and complex carbohydrates in the Golgi apparatus (Sabatino et al., n.d.). It also provides an intracellular pathway for the circulation of these products and acts as a "packaging" system for the export of some of them. Additionally, in line with Sabatino et al. (n.d.), it manages a signaling system that allows assigning each molecule its final destination, whether within the cell or in the extracellular medium, functioning as a postal code system that directs molecules to their correct location.
References
Jung, S. (2020). Khan Academy. https://es.khanacademy.org/science/ap-biology/cell-structure-and-function/cell-compartmentalization-and-its-origins/a/the-endomembrane-system
María, F. (2022, junio 3). ¿Sabes lo que es un Sistema Endomembranoso? Ok Diario. https://okdiario.com/ciencia/sabes-lo-que-sistema-endomembranoso-9176997
Sabatino, V., Lassalle, A., & Márquez, S. (s. f.). Sistema de Endomembranas. GenomaSur. Recuperado 19 de septiembre de 2024, de http://www.genomasur.com/lecturas/Guia05.htm
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