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

All of us who have studied biology in high school have given the parts of the cell. What if cell nucleus, what if plasmatic 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 had much prominence in biology classes, despite the fact that if they do not carry out their functions correctly, the associated medical problems are fatal.

We are going to give them a little prominence and see what they are, what functions they fulfill and what illnesses they can cause if they do not work correctly.

• Related article: "The Most Important Cell Parts and Organelles: A Summary"

What are lysosomes?

lysosomes are membranous structures 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. So to speak, these bubbles with enzymes inside them are like the stomachs of the cell.

In the inactive state, their appearance is granular, while 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. 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 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?

Within 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 acidic pH inside them, 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 about 40 hydrolytic enzymes., that is, enzymes that catalyze a chemical reaction between a molecule of water (H2O) and another or several 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 entire process of digestion of substances. So, we talk about primary enzymes and secondary enzymes:

1. primary enzymes

They consist 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 the ones that are 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 one that refers to what type of material they are responsible for digesting, having heterophagic and autophagic vacuoles:

1. heterophagic vacuoles

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

2. autophagic vacuoles

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

• You may be interested in: "Main cell types of the human body"

Structure of this part of the cell

The structure of lysosomes is not very complex. They appear in the form of spherical corpuscles, with variable dimensions that can be around between 100 and 150 nanometers (nm) in diameter. Although small, these corpuscles They can represent 5% of the total volume of the cell., a percentage which is modifiable depending on the digestion rate that the cell is carrying, this is the amount of substances that it is "disassembling".

The most remarkable part, after the enzymes that we have already seen, is the lysosomal membrane.. It is a simple membrane, which has the purpose of preventing the enzymes found within the lysosome from dispersing through the cytoplasm. Since enzymes are substances that induce processes in which molecules are "destroyed", it is convenient to Keep them safe, since, otherwise, the cell is destroyed, inducing autolysis of the cell. same.

If the enzymes have been improperly synthesized, it can have serious consequences for the cell and, consequently, for the whole organism. This is because, in this case, the residual products that arise from the metabolic reactions that occur within the lysosomes would remain stored in the cell, potentially damaging it.

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

functions

Although small, lysosomes perform very important functions in the body.

1. substance degradation

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 that can be further degraded. Lysosomes are responsible for reducing the complexity of these substances so that their elimination is easier.

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

2. Defense mechanism

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

They are in charge of defending the organism from the attack of bacteria, trapping them in vesicles and digesting them, 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, they have a more degrading function or a more sensory one.

It has been seen that the perinuclear population of lysosomes, that is, close to the cell nucleus, is more involved in the degradation, while another, more peripheral, would be in charge of knowing the state of the availability of resources.

4. exocytosis

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

We have a particular case in liver cells. The lysosomes of the liver cells are responsible for these cells secreting lysosomal enzymes into the bile.

Substance degradation pathways

There are three routes by which the substances that are going to be digested in the lysosomes arrive:

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

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

The third pathway of degradation is autophagy. It is a process that occurs in all organelles when they are damaged. Lysosomes participate 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 also due to a malfunction of the lysosomes, which lead to the accumulation of harmful substances.

sphingolipidosis

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

Due to 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.

Wolfman's disease

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

Gastric lipase has the function of breaking down short and long chain triglycerides, as well as cholesterol esters to their basic units. When you don't have this enzyme, these triglycerides and esters are accumulated in various organs.

The first symptoms appear during the first weeks of life, being vomiting, diarrhoea, enlarged liver and spleen, abdominal distension, progressive malnutrition and stopping the weight curve. It evolves very quickly to worse symptoms and ends with the death of the baby after a year.

Glycogenosis type II or Pompe disease

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

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

In both age groups there is muscle weakness and the appearance of respiratory problems. In children the heart appears enlarged, in addition to 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 this is 1 in 60,000 and in children it is 1 in 100,000.

Bibliographic references:

• Cooper, G. M., Hausmann, 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.