Lysosomes: what they are, their structure and their functions in the cell

All of us who studied biology in high school donated parts of the cell. That if the cell nucleus, that if the plasma membrane, that if Golgi and the endoplasmic reticulum … but there is a part that almost always goes unnoticed.

Either because of their size, or because in appearance they are very simple, lysosomes did not play a big role in biology classes, although, if they do not perform their duties properly, the associated medical problems are fatal.

Let’s give them some importance and see what they are, what functions they perform and what diseases they can cause if they don’t function properly.

    What are Lysosomes

    Lysosomes these are membranous structures that are found inside cells. Within these are enzymes, which are used to carry out various metabolic processes in the cell cytoplasm and in some organelles, degrading substances. To put it bluntly, these bubbles containing enzymes are like the stomachs of the cell.

    In the inactive state, their appearance is grainy, while when activated, they have a vesicular shape, of variable size. This size can be between 0.1 and 1.2 microns and is in a spherical shape. These structures can be found in all animal cells, being a characteristic part of this type of cell and being formed by the Golgi apparatus, having as its main function the transport and use of enzymes. Although the Golgi apparatus is also found in the plant cell, there are no lysosomes in it.

    What substances can be found in lysosomes?

    In the lysosomes we can find different types of enzymes, which will be specialized in the digestion of different types of substances.. For enzymes to be activated, lysosomes must have an acidic medium pH between 4.6 and 5.0. Among the main ones we have three:

    • Lipases: digest lipids or fats.
    • Glucosidases: break down and digest carbohydrates.
    • Proteases: digest proteins.
    • Nucleases: are responsible for nucleic acids.

    Each lysosome can contain around 40 hydrolytic enzymesThat is, enzymes that catalyze a chemical reaction between a water molecule (H2O) and one or more molecules of other substances.

    Classification of substances according to their role in digestion

    Likewise, the enzymes mentioned above can be classified according to their role throughout the process of digestion of substances. So, let’s talk about primary enzymes and secondary enzymes:

    1. Primary enzymes

    They are composed only of hydrolase enzymes and do not contain other vesicles. These are enzymes that have not yet been involved in the digestion of substances.

    2. Secondary enzymes

    They are the combination of primary enzymes with other vesicles. These enzymes would be responsible for the digestion of the cell, for cleaning damaged structures, knowing this process as an internal digestion of the cell.

    Classification of substances according to the material they digest

    In addition to this classification, we have the one that refers to the type of material responsible for digestion, having heterophagous and autophagous vacuoles:

    1. Heterophagous vacuoles

    These are the enzymes that they are responsible for attacking and digesting material outside the cell. Between the particles external to the cell we have bacteria and the remains of neighboring cells.

    2. Autophagous vacuoles

    The substances that will be digested with these lysosomes come from the internal environment of the cell.

      Structure of this part of the cell

      The structure of lysosomes is not very complex. They are in the form of spherical corpuscles, of variable dimensions which can be between 100 and 150 nanometers (nm) in diameter. Still small, these corpuscles they can represent 5% of the total volume of the cell, Modifiable percentage according to the speed of digestion carried by the cell, that is to say of the quantity of substances which it “dismantles”.

      The most notable part, after the enzymes we have already seen above, is the lysosomal membrane. It is a simple membrane that aims to prevent enzymes present in the lysosome from dispersing through the cytoplasm. Since enzymes are substances that induce processes in which molecules are “destroyed”, it is advisable to keep them in a safe place, otherwise the cell is destroyed by inducing their autolysis.

      If the enzymes have been poorly synthesized, this can have serious consequences for the cell and, consequently, for the whole organism. This is due to the fact that, in this case, the residual products from the metabolic reactions that took place in the lysosomes would be stored in the cell, which could damage it.

      An example of a disease due to lysosome problems is type II glycogenosis, where the enzyme β-glucosidase is absent, which results in the accumulation of large amounts of glycogen in the organs, which is fatal for the body.

      the functions

      Always small, lysosomes perform very important functions for the body.

      1. Degradation of substances

      The main function of lysosomes is to digest substances, both external and internal to the cell. Internal substances can be components that the cell no longer needs, But they can be further degraded. Lysosomes are responsible for reducing the complexity of these substances to make them easier to remove.

      They also perform internal digestion, which occurs in the event that the cell is damaged. In this way, damaged structures or, if necessary, the entire cell are digested to be replaced with a new and more functional one.

      2. Defense mechanism

      Lysosomes, in addition to the digestion of substances, are a very important defense mechanism for the cell, because they are able to defend it against the attack of invasive bacteria.

      They are responsible for defending the body against bacterial attack, trapping in vesicles and digesting, thus activating the immune response.

      3. Metabolic sensor

      Lysosomes, in addition to degrading substances, participate in the perception of the metabolic state of the cell. In fact, depending on the location of the lysosome populations, these have a more degrading or more sensory function.

      We have seen that the population of perinuclear lysosomes, that is to say near the cell nucleus, is more involved in the degradation, while another, more peripheral, would be responsible for knowing the state of availability of resources. .

      4. Exocytosis

      In recent years, we have seen that lysosomes have the capacity to participate in exocytosis, that is to say in the elimination of substances from the internal environment of the cell.

      We have a special case in liver cells. Lysosomes in liver cells are what cause these cells to secrete lysosomal enzymes into the bile.

      Pathways of substance degradation

      The substances to be digested in the lysosomes arrive in three ways:

      As a first step, the lysosomes could be considered as the final station of the endocytic pathwayIt is the route by which compounds of different types are introduced into the cell. Most molecules degraded by this route must first pass through autophagic vacuoles.

      The second refers to nonprofit particles that have been phagocytosed, such as bacteria or residual particles from other cells. These particles must be contained inside the lysosomes to be digested, and thus ensure that they are eventually eliminated without damaging the cell as it passes. The compartment in which they are trapped matures and will become what is called a phagosome, which will fuse with the lysosome once the first one ripens.

      The third route of degradation is autophagy. It is a process that occurs in all organelles when damaged. Lysosomes participate in different types of autophagy, adapting to the damaged organelle in question and the needs of the cell, or its degree of recovery.

      lysosomal diseases

      Lysosomal diseases are those caused by the uncontrolled release of enzymes outside the cell, Or also due to a dysfunction of the lysosomes, which induce the accumulation of harmful substances.

      Sphingolipidosis

      It is a medical condition that induces a set of diseases. It is caused by dysfunction of one of the enzymes responsible for breaking down sphingolipids, Very common substances in the brain.

      Because of this, the disease induces brain damage, leading to intellectual disability and premature death. Among the diseases caused by sphingolipidosis are Krabbe disease, Tay-Sachs disease, Gaucher disease and Niemann-Pick disease.

      Wolman disease

      It is a congenital lipidosis. It is inherited, transmitted by autosomal recessive inheritance, and is caused by deficiency of a lysosomal enzyme, acid lipase, encoded its production in the long arm of chromosome 10.

      Gastric lipase is responsible for breaking down short and long chain triglycerides, as well as cholesterol esters into their basic units. When this enzyme is not present, these triglycerides and esters accumulate in various organs.

      The first symptoms appear during the first weeks of life, being vomiting, diarrhea, growth of liver and spleen, abdominal distension, progressive malnutrition and stopping the weight curve. It progresses very quickly to more serious symptoms and ends with the death of the child after a year.

      Glycogenosis type II or Pompe disease

      It is a defect of acid maltase, a defect which causes glycogen stored in lysosomes to appear without being properly broken down.

      It is a muscular disease, very rare and debilitating, affecting both children and adults. In childhood, it manifests itself already in the first few months, but in more adult stages it can appear suddenly, with slower progression.

      In both age groups there is muscle weakness and the onset of breathing problems. In children, the heart seems enlarged, in addition to not being able to hold its head.

      This disease is considered panethnic, that is, it appears in all races, but the percentages vary from race to race. The incidence in the population of African American children is very high, 1 in 14,000, while in Caucasian adults it is 1 in 60,000 and in children 1 in 100,000.

      Bibliographical references:

      • Cooper, GM, Hausman, R. E (2011). The cell. Madrid: Marban.
      • Kuehnel, W. (2003). Color Atlas of Cytology, Histology and Microscopic Anatomy (4th Edition). Thieme. p. 34. ISBN 1-58890-175-0.

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