FLAGELLA: EUKARYOTIC, PROKARYOTIC (STRUCTURE AND FUNCTIONS) - BIOLOGY - 2024

A flagellum is a whip-shaped cellular projection that participates in the locomotion of unicellular organisms and in the movement of various substances in more complex organisms.

We find flagella in both the eukaryotic and prokaryotic lineage. Prokaryotic flagella are simple elements, formed by a single microtubule composed of flagellin subunits configured in a helical manner, forming a hollow nucleus.

Source: LadyofHats. Spanish version of Alejandro Porto

In eukaryotes the configuration is nine pairs of tubulin microtubules and two pairs located in the central region. One of the typical examples of flagella is the sperm extensions, which give them mobility and allow the ovum to fertilize.

Cilia, another type of cell prolongation, have a similar structure and function to flagella, but should not be confused with flagella. They are much shorter and move differently.

Flagella in prokaryotes

In bacteria, flagella are helical filaments whose dimensions are in the range of 3 to 12 micrometers in length and 12 to 30 nanometers in diameter. They are simpler than the same elements in eukaryotes.

Structure

Structurally, the flagella of bacteria are composed of a protein molecule called flagellin. Flagellins are immunogenic and represent a group of antigens called "H antigens" that are specific to each species or strain. This is configured in a cylindrical way, with a hollow center.

In these flagella, we can distinguish three main parts: a long external filament, a hook that is located at the end of the filament and a basal body that is anchored to the hook.

The basal body shares characteristics with the secretory apparatus for virulence factors. This similarity could indicate that both systems have been inherited from a common ancestor.

Classification

Depending on the location of the flagellum, bacteria are classified into different categories. If the flagellum is located at the poles of the cell as a single polar structure at one end it is monoteric and if it does so at both ends it is amphibian.

The flagellum can also be found as a “plume” on one or both sides of the cell. In this case, the assigned term is lophotric. The last case occurs when the cell has multiple flagella homogeneously distributed over the entire surface, and is called peritrichous.

Each of these types of flagellation also exhibits variations in the type of movements that the flagella makes.

Bacteria also display other types of projections on the cell surface. One of them is the pili, these are more rigid than a flagellum and there are of two types: the short and abundant, and the long ones involved in sexual intercourse.

Movement

The thrust or rotation of the bacterial flagellum is the product of the energy coming from the proton-motive force and not directly from ATP.

Bacterial flagella are characterized by not rotating at a constant speed. This parameter will depend on the amount of energy that the cell is producing at any given time. The bacterium is capable not only of modulating speed, it can also change flagellar direction and movement.

When the bacteria are directed to a particular area, they are likely to be attracted to a stimulus. This movement is known as taxis and the flagellum allows the organism to move to the desired location.

Flagella in eukaryotes

Like prokaryotic organisms, eukaryotes exhibit a series of processes on the surface of the membrane. Eukaryotic flagella are made up of microtubules and are long projections involved in movement and locomotion.

Furthermore, in eukaryotic cells there may be a series of additional processes that should not be confused with flagella. Microvilli are extensions of the plasma membrane involved in the absorption, secretion and adhesion of substances. It is also related to motility.

Structure

The structure of eukaryotic flagella is called an axoneme: a configuration made up of microtubules and another class of proteins. The microtubules are configured in a pattern called "9 + 2", which indicates that there is a central microtubule pair surrounded by 9 outer pairs.

Although this definition is very popular in the literature, it can be misleading, since only one pair is located in the center - and not two.

Structure of microtubules

Microtubules are protein elements made up of tubulin. Of this molecule, there are two forms: alpha and beta tubulin. These group together forming a dimer, which will form the unit of microtubules. The units polymerize and aggregate laterally.

There are differences between the number of protofilaments that microtubules have that are located around the central pair. One is known as tubule A or complete because it has 13 protofilaments, in contrast to tubule B, which has only 10 to 11 filaments.

Dynein and nexin

Each one of the microtubules is attached by its negative end to a structure known as the basal body or kinetosome, which is similar in structure to the centriole of the centrosomes with nine triplets of microtubules.

The protein dynein, of great importance in eukaryotic flagellar movement (an ATPase), is associated by two arms to each A tubule.

Nexin is another important protein in the composition of the flagellum. This is responsible for joining the nine pairs of outer microtubules.

Movement

The movement of eukaryotic flagella is directed by the activity of the protein dynein. This protein, together with kinesin, are the most important motor elements that accompany microtubules. These "walk" on the microtubule.

Movement occurs when the outer microtubule pairs shift or slip. Dynein is linked to both type A and type B tubules. Specifically, the base is associated with A and the head with B. Nexin also plays a role in movement.

There are few studies that have been in charge of elucidating the specific role of dynein in flagellar movement.

Differences between prokaryotic and eukaryotic flagella

Dimensions

The flagella in prokaryotic lineages are smaller, reaching 12 um long and the average diameter is 20. Eukaryotic flagella can exceed 200 um in length and the diameter is close to 0.5 um.

Structural configuration

One of the most outstanding characteristics of eukaryotic flagella is their 9 + 0 microtubule organization and 9 + 2 fiber configuration. Prokaryotic organisms lack this organization.

Prokaryotic flagella are not enveloped in the plasma membrane, as is the case with eukaryotes.

The composition of prokaryotic flagella is simple and only includes flagellin protein molecules. The composition of eukaryotic flagella is more complex, consisting of tubulin, dynein, nexin, and an additional set of proteins - as well as other large biomolecules such as carbohydrates, lipids, and nucleotides.

Energy

The energy source of prokaryotic flagella is not given by an ATPase protein anchored in the membrane, but by the proton motive force. The eukaryotic flagellum does possess an ATPase protein: dynein.

Similarities and differences with cilia

Similarities

Role in locomotion

Confusion between cilia and flagella is common. Both are cytoplasmic processes that resemble hair and are located on the surface of cells. Functionally, both cilia and flagella are projections that facilitate cellular locomotion.

Structure

Both arise from the basal bodies and have a fairly similar ultra-structure. Likewise, the chemical composition of both projections is very similar.

Differences

Length

The crucial difference between the two structures is related to length: while the cilia are short projections (between 5 and 20 um in length), the flagella are considerably longer and can reach lengths greater than 200 um, almost 10 times as long. than cilia.

Quantity

When the cell has cilia, it usually does so in significant numbers. In contrast to cells that have flagella, which generally have one or two.

Movement

In addition, each structure has a peculiar movement. The cilia move in powerful strokes and the flagella in an undulating, whip-like fashion. The movement of each cilium in the cell is independent, while that of the flagella is coordinated. Cilia are anchored to an undulating membrane and flagella are not.

Complexity

There is a peculiar difference between the complexity of cilia and flagella throughout each structure. Cilia are complex projections along their entire length, while the complexity of the flagellum is restricted only to the base, where the motor that is responsible for rotation is located.

Function

Regarding their function, cilia are involved in the movement of substances in some specific direction and flagella are only related to locomotion.

In animals, the main function of the cilia is the mobilization of fluids, mucus or other substances on the surface.

References

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