The parts of the cell, such as the nucleus, the plasma membrane, the Golgi apparatus, and the endoplasmic reticulum, are usually the most prominent components in biology classes (Rubio, 2020). However, there is one element that often goes unnoticed: the lysosomes. Due to their small size or apparent simplicity, lysosomes have not received the same attention as other cellular components, even though their malfunction can cause serious health issues. For this reason, according to Rubio (2020), it is essential to give them the importance they deserve, understanding what they are, what their functions are, and what diseases can arise if they do not perform their tasks properly.
What are Lysosomes?
Lysosomes are membrane-bound structures found inside animal cells that contain enzymes involved in various metabolic processes within the cytoplasm and certain organelles, where they are responsible for breaking down substances (Rubio, 2020). Due to their digestive function, these structures can be compared to the stomachs of the cell, as the decomposition of different compounds occurs within them. In their inactive state, lysosomes appear granular, but when activated, they take on a vesicular form that varies in size, ranging from 0.1 to 1.2 μm, and they generally have a spherical shape. These structures are found in all animal cells and are formed by the Golgi apparatus, playing an essential role in the transport and utilization of the enzymes they contain. Rubio (2020) mentions that while the Golgi apparatus is also present in plant cells, lysosomes are not found in them.
What Substances Can Be Found in Lysosomes?
Lysosomes contain a wide variety of enzymes, each specialized in breaking down different types of substances (Rubio, 2020). These enzymes require an acidic environment, with a pH ranging from 4.6 to 5.0, to activate and perform their functions optimally. Among the most important enzymes are lipases, responsible for breaking down lipids or fats; glucosidases, which digest carbohydrates; proteases, which degrade proteins, and nucleases, which break down nucleic acids. In total, according to Rubio (2020), a lysosome can contain around 40 different types of hydrolytic enzymes, whose purpose is to catalyze reactions where a water molecule interacts with other substances to facilitate their breakdown.
Classification of Substances Based on Their Role in Digestion
Lysosomal enzymes are divided into two main groups: primary enzymes and secondary enzymes (Rothschuh Osorio, 2022). Primary enzymes consist only of hydrolases and have not yet started their digestive activity. These inactive enzymes are smaller and more homogeneous, ranging from 0.05 to 0.5 microns in diameter. They are located near the trans face of the Golgi apparatus, which is responsible for sending the vesicles containing these enzymes to the lysosomes. On the other hand, secondary enzymes are already active and involved in digestion processes within the cell (Rothschuh Osorio, 2022). These are a combination of primary enzymes and other vesicles, allowing them to digest damaged components of the cell in a process known as internal digestion or autophagy (Rubio, 2020). According to Rothschuh Osorio (2022), since they are active and fully functioning, they tend to be larger than primary enzymes as they are in the process of breaking down molecules.
Classification of Substances Based on the Material They Digest
Another way to classify lysosomal enzymes is based on the type of material they degrade (Rubio, 2020). In this sense, two types of vacuoles are distinguished: heterophagic vacuoles and autophagic vacuoles. Heterophagic vacuoles are responsible for digesting substances that come from outside the cell, such as bacteria and cellular debris from nearby cells. On the other hand, according to Rubio (2020), autophagic vacuoles act on internal materials of the cell itself, eliminating structures that are no longer functional through a process called apoptosis, which is a form of programmed cell death.
Structure
The structure of lysosomes is not highly complex (Rubio, 2020). They are usually spherical corpuscles, with dimensions ranging between 100 and 150 nm in diameter (Rubio, 2020). Lysosomes contain a single membrane and can represent up to 5% of the cell's volume, depending on the rate of digestion taking place in the cell (Megías et al., 2024). This is a simple membrane, whose main function is to prevent the enzymes within the lysosome from dispersing into the cytoplasm. Since these enzymes induce processes that destroy molecules, they must remain confined; otherwise, they could cause the destruction of the cell and lead to autolysis. If the enzymes are synthesized incorrectly, the consequences can be severe for the cell and, ultimately, for the organism. According to Rubio (2020), this is because the residual products of the metabolic reactions in the lysosomes would accumulate in the cell, causing damage.
Functions
Substance Degradation
The primary function of lysosomes is the digestion of substances, both internal and external to the cell (Rubio, 2020). Regarding internal substances, these may be components that the cell no longer needs but can still be degraded. Lysosomes facilitate this degradation, breaking down molecules to ease their elimination. They also participate in internal digestion when the cell is damaged. In this process, according to Rubio (2020), the affected structures, or if necessary, the entire cell, are destroyed and replaced with a new and more functional one.
Defense Mechanism
In addition to degrading substances, lysosomes act as a fundamental defense mechanism for the cell, as they are capable of protecting it from invading bacteria (Rubio, 2020). In line with Rubio (2020), lysosomes trap these bacteria in vesicles and digest them, thereby activating the organism's immune response.
Metabolic Sensor
Lysosomes not only degrade substances but also play a role in detecting the cell's metabolic state (Rubio, 2020). Depending on their location, lysosomes may perform more degradative or sensory functions. According to Rubio (2020), it has been observed that lysosomes located near the cell nucleus are mainly involved in degradation, while those located on the periphery of the cell monitor resource availability.
Exocytosis
In recent years, it has been discovered that lysosomes also participate in exocytosis, the process by which the cell expels substances into the external environment (Rubio, 2020). According to Rubio (2020), a specific example can be found in liver cells, where lysosomes are responsible for secreting lysosomal enzymes into bile.
Vías de Degradación de Sustancias
Existen tres vías por las cuales las sustancias destinadas a ser digeridas llegan a los lisosomas (Rubio, 2020). En primer lugar, los lisosomas pueden ser vistos como la estación final de la vía endocítica, que es el proceso por el cual se introducen compuestos a la célula. La mayoría de las moléculas degradadas por esta vía deben pasar previamente por vacuolas autofágicas. En segundo lugar, se encuentran las partículas que han sido fagocitadas, como bacterias o restos celulares. Estas partículas deben quedar confinadas dentro de los lisosomas para ser digeridas, evitando así cualquier daño a la célula. El compartimento que las contiene madura hasta convertirse en un fagosoma, el cual se fusiona con el lisosoma una vez alcanza su madurez. La tercera vía es la autofagia, un proceso presente en todos los orgánulos cuando sufren deterioro. Según Rubio (2020), los lisosomas participan en distintos tipos de autofagia, ajustándose al tipo de orgánulo dañado y a las necesidades de la célula, permitiendo incluso recuperar partes que aún son funcionales.
Lysosomal Diseases
In accordance with Rubio (2020), lysosomal diseases are those that originate from the uncontrolled release of enzymes outside the cell or due to a malfunction of lysosomes, leading to the accumulation of harmful substances.
Sphingolipidoses
This term refers to a group of diseases caused by a failure in the enzymes responsible for degrading sphingolipids, common substances in the brain (Rubio, 2020). This malfunction leads to brain damage, resulting in intellectual disability and, in many cases, premature death. According to Rubio (2020), diseases belonging to this group include Krabbe disease, Tay-Sachs disease, Gaucher disease, and Niemann-Pick disease.
Wolman Disease
This is a congenital lipidosis of hereditary origin, transmitted through autosomal recessive inheritance, caused by a deficiency of the lysosomal enzyme acid lipase, whose production is encoded on the long arm of chromosome 10 (Rubio, 2020). Acid lipase is essential for the degradation of short and long-chain triglycerides, as well as cholesterol esters, converting them into their basic components. In its absence, triglycerides and esters accumulate in various organs. Initial symptoms appear during the first weeks of life and include vomiting, diarrhea, hepatomegaly, splenomegaly, abdominal distension, progressive malnutrition, and growth delay. According to Rubio (2020), the disease progresses rapidly and generally results in death within the first year of life.
Glycogen Storage Disease Type II or Pompe Disease
Pompe disease is caused by a defect in the acid maltase enzyme, leading to the accumulation of glycogen in lysosomes without proper degradation (Rubio, 2020). It is a rare and debilitating muscle disease that affects both children and adults. In children, symptoms typically manifest in the first months of life, whereas in adults, onset may be later with a slower progression. Both children and adults experience muscle weakness and respiratory problems. Children may also exhibit cardiomegaly and inability to hold up their head. This disease is pan-ethnic, occurring in all races, although its incidence varies. According to Rubio (2020), the incidence in African American children is 1 in 14,000, in Caucasian adults it is 1 in 60,000, and in Caucasian children it is 1 in 100,000.
References
Megías, M., Molist, P., & Pombal, M. Á. (2024). Lisosomas. Atlas de Histología Vegetal y Animal. https://mmegias.webs.uvigo.es/5-celulas/5-lisosomas.php
Rothschuh Osorio, U. (2022, febrero 11). Lisosomas: Definición, Características y Función. Ecología Verde. https://www.ecologiaverde.com/lisosomas-definicion-caracteristicas-y-funcion-3756.html
Rubio, N. M. (2020, marzo 10). Lisosomas: qué son, estructura y funciones en la célula. pymOrganization. https://psicologiaymente.com/salud/lisosomas
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