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

An overview of the components, functions and characteristics of lysosomes.

All of us who have studied biology in high school have been given the parts of the cell. What if cell nucleus, what if plasma membrane, what if Golgi and 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 have not been given much prominence in biology classes, despite the fact that, if they do not perform their functions correctly, the associated medical problems are fatal.

Let's give them some prominence and see what they are, what functions they perform and what diseases they can cause if they do not work properly.

What are lysosomes?

The lysosomes are membranous structures found in the interior of cells.. Inside these are enzymes, which are used to carry out various metabolic processes in the cell cytoplasm and in some organelles, degrading substances. So to speak, these bubbles with enzymes inside are like the stomachs of the cell.

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

What substances can be found in lysosomes?

Inside the lysosomes we can find different types of enzymes, which will be specialized in digesting different types of substances.. In order for the enzymes to be activated, it is necessary for the lysosomes to have a medium with an acid pH of between 4.6 and 5.0. Among the main enzymes we have three:

• Lipases: digest lipids or fats.
• Glycosidases: break down and digest carbohydrates.
• Proteases: digest proteins.
• Nucleases: deal with nucleic acids.

Each lysosome can contain about 40 hydrolytic enzymes, i.e. enzymes that catalyze the breakdown of carbohydrates.i.e. enzymes that catalyze a chemical reaction between a molecule of water (H2O) and one or more molecules of other substances.

Classification of substances according to their role in digestion

Likewise, the aforementioned enzymes can be classified according to their role during the whole process of digestion of substances. Thus, we speak of primary enzymes and secondary enzymes:

1. Primary enzymes

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

2. Secondary enzymes

They are the combination of primary enzymes with other vesicles.. These enzymes would be in charge of digesting the cell, cleaning the structures that are damaged, this process being known as 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 what type of material they are in charge of digesting, having heterophagic and autophagic vacuoles:

1. heterophagic vacuoles.

These are the enzymes that are responsible for attacking and digesting material external to the cell.. Among particles external to the cell we have bacteria and debris from neighboring cells.

2. Autophagic vacuoles

The substances to be digested by 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 take the form of spherical corpuscles, with variable dimensions that can range from 100 to 150 nanometers (nm) in diameter. Although small, these corpuscles can represent up to 5% of the total volume of the cell.This percentage can be modified depending on the rate of digestion that the cell is carrying, i.e. the amount of substances that it is "disassembling".

The most important part, after the enzymes that we have already seen above, is the lysosomal membrane.. It is a simple membrane, which has the purpose of preventing the enzymes inside the lysosome from being dispersed in the cytoplasm. Since enzymes are substances that induce processes in which molecules are "destroyed", it is advisable to keep them in a safe place, since otherwise the cell is destroyed by inducing cell autolysis.

If enzymes have been improperly synthesized, this can have serious consequences for the cell and, consequently, for the whole organism. This is because, in this case, the residual products arising from the metabolic reactions occurring within the lysosomes would be stored in the cell and could damage it.

An example of a disease due to lysosomal problems is glycogenosis type II, where the enzyme β-glucosidase is absent, causing large amounts of glycogen to accumulate in the organs, which is fatal for the organism.

Functions

Although small, lysosomes have very important functions for the organism.

1. Degradation of substances

The main function of lysosomes is to digest substances, both external and internal to the cell. Internal substances may be components that are no longer needed by the cell, but can still be degraded.but which can be further degraded. Lysosomes are responsible for reducing the complexity of these substances so that they can be more easily eliminated.

They also carry out internal digestion, which occurs if the cell is damaged. In this way, the damaged structures or, if necessary, the entire cell is digested and replaced by a new, more functional one.

2. Defense mechanism

Lysosomes, in addition to digesting substances, are a very important defense mechanism for the cell, since they are capable of defending it from attack by bacteria. they are capable of defending the cell from attack by invading bacteria..

They are in charge of defending the organism from bacterial attack, trapping them in vesicles and digesting them, thus activating the immune response.

3. Metabolic sensor

Lysosomes, in addition to degrading substances, are involved in sensing the metabolic state of the cell. In fact, depending on the location of the lysosome populations, they have a more degradative or a more sensory function..

It has been shown that the perinuclear lysosome population, i.e. near the cell nucleus, is more involved in degradation, while another, more peripheral, population is responsible for knowing the state of resource availability.

4. Exocytosis

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

A particular case is found in liver cells. The lysosomes of liver cells are responsible for the secretion of lysosomal enzymes into the bile.

Pathways of substance degradation

There are three pathways by which substances to be digested in the lysosomes arrive:

In the first one, the lysosomes could be considered as the final station of the endocytic pathwayIn the first, the lysosomes could be considered as the final station of the endocytic pathway, that is the pathway by which compounds of various kinds are introduced into the cell. Most of the molecules that are degraded by this pathway must first pass through the autophagic vacuoles.

The second refers to unprofitable particles that have been phagocytosed, such as bacteria or residual particles from other cells.. These particles must be contained within the lysosomes so that they can be digested, thus ensuring that they are eliminated without damaging the cell by their passage. The compartment in which they are trapped will mature into what is called the phagosome, which will fuse with the lysosome once the former matures.

The third degradation pathway is autophagy. This is a process that occurs in all organelles when they are impaired. Lysosomes are involved in different types of autophagy, adapting to the damaged organelle in question and the needs of the cell, or how salvageable it is.

Lysosomal diseases

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

Sphingolipidosis

This is a medical condition that induces a set of diseases. It is caused by a malfunction in one of the enzymes in charge of degrading sphingolipids, substances very common in the brain.sphingolipids, a very common substance in the brain.

Because of this, the condition induces brain damage, causing intellectual disability and premature death. Among the diseases due to sphingolipidosis we can find Krabbe disease, Tay-Sachs disease, Gaucher disease and Niemann-Pick disease.

Wolman disease

This is a congenital lipidosis. It is hereditary, transmitted by autosomal recessive inheritance, and is caused by the deficiency of a lysosomal enzyme, acid lipase, whose production is encoded on the long arm of chromosome 10..

Gastric lipase has the function of degrading short and long chain triglycerides and cholesterol esters to their basic units. When this enzyme is absent, these triglycerides and esters accumulate in various organs.

The first symptoms are manifested during the first weeks of life, being vomiting, diarrhea, enlargement of the liver and spleen, abdominal distension, progressive malnutrition and arrest of the weight curve. and arrest of the weight curve. It evolves very rapidly to worse symptoms and ends with the death of the infant after one year.

Glycogenosis type II or Pompe disease

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

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

In both age groups there is muscle weakening and the appearance of respiratory problems.. In children the Heart appears enlarged, besides not being able to support the head.

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

Bibliographic references:

• Cooper, G. M., Hausman, R. E (2011). The Cell. Madrid: Marban.
• Kuehnel, W. (2003). Color Atlas of Cytology, Histology, & Microscopic Anatomy (4th edition). Thieme. p. 34. ISBN 1-58890-175-0.