LYSOSOMES: CHARACTERISTICS, STRUCTURE, FUNCTIONS AND TYPES - BIOLOGY - 2025

The lysosomes are organelles cell membranes that are located within animal cells. They are compartments that have an acidic pH and are rich in digestive enzymes, capable of degrading any kind of biological molecule: proteins, carbohydrates and nucleic acids.

In addition, they can degrade material from outside the cell. For this reason, lysosomes have multiple functions in cellular metabolism and thanks to their composition rich in hydrolytic enzymes, they are often called the "stomach" of the cell.

Lysosomes are formed by the fusion of vesicles that emerge from the Golgi apparatus. The cell recognizes certain sequences that function as "tags" on hydrolytic enzymes and sends them to the forming lysosomes.

These vacuoles are spherical in shape and their size varies considerably, being a fairly dynamic cell structure.

Discovery and historical perspective

Lysosomes were discovered more than 50 years ago by researcher Christian de Duve. De Duve's team was conducting experiments involving the subcellular fractionation technique, in order to investigate the location of certain enzymes.

This experimental protocol allowed the discovery of the organelles, since the researchers noticed that the release of hydrolytic enzymes increased as they added compounds that deteriorated the membranes.

Subsequently, the improvement of molecular biology techniques and the existence of better equipment - such as electron microscopes, managed to corroborate its presence. In fact, it could be concluded that lysosomes occupy 5% of the intracellular volume.

Some time after its discovery, the presence of hydrolytic enzymes was evidenced in its interior, turning the lysosome into a kind of degradation center. Furthermore, lysosomes were associated with endocytic life.

Historically, lysosomes were considered the end point of endocytosis, used only for the degradation of molecules. Today, lysosomes are known to be dynamic cell compartments, capable of fusing with a variety of additional organelles.

characteristics

Proton pumping through the lysosome membrane. Source: Alejandro Porto

Morphology of lysosomes

Lysosomes are unique compartments of animal cells that house a variety of enzymes that are capable of hydrolyzing proteins and digesting certain molecules.

They are dense, spherical vacuoles. The size of the structure is widely varied, and depends on the material that has been previously captured.

Lysosomes, along with the endoplasmic reticulum and the Golgi apparatus, are part of the endomembrane system of the cell. Although these three structures are networks of membranes, they are not continuous with each other.

Lysosomes contain multiple enzymes

The main characteristic of lysosomes is the battery of hydrolytic enzymes inside them. There are around 50 enzymes that are capable of degrading a wide range of biomolecules.

These include nucleases, proteases and phosphatases (which remove phosphate groups from phospholipid mononucleotides and other compounds). In addition, they contain other enzymes responsible for the degradation of polysaccharides and lipids.

Logically, these digestive enzymes must be spatially separated from the rest of the cellular components to avoid their uncontrolled degradation. Thus, the cell can "choose" the compounds that must be eliminated, since it can regulate the elements that enter the lysosome.

The environment of lysosomes is acidic

The interior of lysosomes is acidic (close to 4.8), and the enzymes it contains work well at this pH condition. Therefore, they are known as acid hydrolases.

The characteristic acid pH of this cell compartment is maintained thanks to the presence of a proton pump and a chloride channel in the membrane. Together, they transport hydrochloric acid (HCl) into the lysosome. The pump is located anchored in the membrane of the organelle.

The function of this acidic pH is to activate the various hydrolytic enzymes present in the lysosome and to avoid - as far as possible - their enzymatic activity at the neutral pH of the cytosol.

In this way, we already have two barriers that function as protection from uncontrolled hydrolysis: keeping the enzymes in an isolated compartment, and that these enzymes work well at the acidic pH of this compartment.

Even if the lysosome membrane were to break, the release of the enzymes would not have much effect - due to the neutral pH of the cytosol.

Features

The internal composition of a lysosome is dominated by hydrolytic enzymes, which is why they are an important region of cellular metabolism where the digestion of extracellular proteins that enter the cell by endocytosis, recycling of organelles and cytosolic proteins take place.

Below we will explore in depth the most prominent functions of lysosomes: degradation of molecules by autophagy and degradation by phagocytosis.

Autophagy

What is autophagy?

One mechanism that captures cellular proteins is called "self-eating" autophagy. This event helps to maintain cell homeostasis, degrading cell structures that are no longer needed and contributes to the recycling of organelles.

Through this phenomenon, the formation of vesicles called autophagosomes occurs. These are small regions of the cytoplasm or other cellular compartments, coming from the endoplasmic reticulum, that fuse with the lysosomes.

Both organelles have the ability to fuse, since they are delimited by a plasma membrane of a lipid nature. It is analogous to trying to match two soap bubbles together - you are making a bigger one.

After the fusion, the enzymatic content of the lysosome is responsible for degrading the components that were inside the other vesicle formed. The capture of these molecules seems to be a process that lacks selectivity, causing the degradation of proteins located in the long-lived cytosol.

Autophagy and periods of fasting

In the cell, the autophagy event appears to be regulated by the amount of available nutrients.

When the body experiences a deficiency of nutrients or experiences prolonged periods of fasting, the degradation pathways are activated. In this way, the cell manages to degrade proteins that are not essential and achieves the reuse of certain organelles.

Knowing that lysosomes play an important role during periods of fasting has increased the interest of researchers in this organelle.

Autophagy and the development of organisms

In addition to their active participation in periods of low nutritional content, lysosomes play an important role during the development of certain lineages of organic beings.

In some cases, development involves the total remodeling of the body, which implies that certain organs or structures must be eliminated during the process. In the metamorphosis of insects, for example, the hydrolytic content of lysosomes contributes to the remodeling of tissues.

Endocytosis and phagocytosis

Endocytosis and phagocytosis have a role in the uptake of elements external to the cells and their subsequent degradation.

During phagocytosis, certain cells - such as macrophages - are responsible for ingesting or degrading large particles, such as bacteria or cell debris.

These molecules are ingested by a phagocytic vacuole, called a phagosome, which, as in the previous case, will fuse with the lysosomes. Fusion results in the release of digestive enzymes within the phagosome and the particles are degraded.

Types of lysosomes

Some authors distinguish this compartment into two main types: type I and type II. Those of type I or primary lysosomes are involved in the storage of hydrolytic enzymes, while secondary lysosomes are related to catalysis processes.

Formation of lysosomes

The formation of lysosomes begins with the uptake of molecules from the outside through endocytic vesicles. The latter fuse with other structures called early endosomes.

Later, the early endosomes undergo a maturation process, giving rise to late endosomes.

A third component appears in the formation process: the transport vesicles. These contain the acid hydrolases from the trans network of the Golgi apparatus. Both structures - transport vesicles and late endosomes - fuse and become a lysosome, after acquiring the set of lysosomal enzymes.

During the process, recycling of membrane receptors occurs by means of recycling endosomes.

Acid hydrolases are separated from the mannose-6 phosphate receptor during the fusion process of the organelles that give rise to lysosomes. These receptors enter the Golgi trans network again.

Differences between endosomes and lysosomes

Confusion between the terms endosomes and lysosome is common. The former are membrane-enclosed cell compartments - like lysosomes. However, the crucial distinction between the two organelles is that the lysosomes lack mannose-6-phosphate receptors.

In addition to these two biological entities, there are other types of vesicles. One of them are vacuoles, whose content is mainly water.

Transport vesicles, as their name implies, participate in the movement of substances to other locations in the cell. Secretory vesicles, for their part, remove waste material or chemicals (such as those involved in the synapse of neurons.)

Associated diseases

In humans, mutations in the genes that code for lysosome enzymes are associated with more than 30 congenital diseases. These pathologies are encompassed by the term "lysosomal storage diseases."

Surprisingly, many of these conditions arise from damage to a single lysosomal enzyme.

In affected individuals, the consequence of having a non-functional enzyme inside the lysosomes is the accumulation of waste products.

The most common lysosomal deposition alteration is known as Gaucher disease, and it is associated with a mutation in the gene that codes for the enzyme responsible for glycolipids. As a curious fact, the disease shows a fairly high frequency among the Jewish population, affecting 1 in every 2,500 individuals.

References

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