Introduction

Flagella refers to hair-like structures that are available on a cell’s body and play an essential role in controlling and maintaining its different physiological functions. The term ‘flagellum’ has a Latin origin meaning a whip. This indicates a long slender whip-like structure. The flagellum is a characteristic of the members of Mastigophora, a type of protozoan group. However, these structures have also been found in bacteria, fungi, algae, and animals. Flagellum serves the role of being an organelle of locomotion in different organisms. These structures also help these organisms in gathering food and in circulation. Let us study some more details about the basic structure, functions, and types of the flagellum.

Structure of Flagella

The size, structure, and number of the flagellum are different between eukaryotes and prokaryotes. In prokaryotes, a difference exists between the archaeal type and bacterial type. Similarly, a lot of diversity exists in the composition and mechanism of flagella formation. However, some flagellum structures are most common in every domain of life. The basic structure involves three essential parts: the filament, hook or anchoring structures and the basal body or the motor device.

The filament

The most prominent part of a flagellum is the filament. In fact, about 98% of all the flagellum’s structures are represented by the filament. The filament extension is from the hook-like structure found within the cell’s cytoplasm. Variations occur in the filament’s length in different living beings groups. These structures are self-assembling in nature. They are made up of hook proteins and flagellins. Moreover, there may be a variation in the number of flagellin subunits and hook protein subunits in different cells.

Hook or anchoring structures

The flagellar hook is a tubular structure that is short and curved. It acts as a connection between the basal body to the long filament. The hook’s primary function is the transmission of the motor torque to the helical filament. This facilitates movement in the helical filament in a different orientation for different functions. The hook is also essential in the assembly of the flagellum.

Basal body or motor device

The basal body is the only flagellum structure within the cell membrane. Its connection is with the hook, which connects to the long filament. The structure of the basal body is rod-shaped. There is a system of microtubule rings (alpha- and beta-tubulin subunits)  inside it. The component of the basal body differs from one cell to another.

Functions of Flagella

Below are the various flagella functions:

• They serve as the primary locomotion structures in various types of bacteria. As such, they allow the movement of bacteria towards the most beneficial environment.
• The flagellum is crucial in colonising tissue surfaces for the invasion of the host tissue.
• In some bacteria, there is an involvement of the flagellum in the nutrient and waste exchange. This happens by distributing the waste-rich shell that is available within the bacteria.
• They play an essential role in motility and ultimately fertilisation in eukaryotic cells like sperm.
• The flagellum is significant for colonising surfaces like soil, plant, or animal surfaces in a non-pathogenic manner.
• In alkaliphilic bacteria, flagellum acts in a sodium-driven manner. It facilitates the re-entry of sodium into the cytoplasm to maintain the neutral cytoplasmic pH.

Types of Flagella

There are three types of flagellum: bacterial, archaeal, and eukaryotic.

Bacterial

• They are coiled structures that are slightly longer than both the other types- archaeal and eukaryotic
• Bacterial flagella are made up of a type of low molecular weight protein
• They are thinner in comparison to the eukaryotic flagella
• Their diameter is measured to be around 20 nanometres

Note- Depending on the number of flagella bacteria are of following types- Atrichous- No flagella present 

Monotrichous- One flagellum present.

Amphitrichous- Two flagella present opposite to each other on the bacterium.

Lophotrichous- Many flagella present at any one point on the bacterium.

Peritrichous- Flagella distributed all over the surface of the bacterium.

Cephalotrichous- Two groups of flagella at two opposites ends on the bacteria.

Archaeal

• This type is a unique motility apparatus. This is because it is different in composition; it is similar when it comes to assembly of bacterial types
• The occurrence of the archaeal type is in almost all the main groupings of various domains. Such domains are thermophiles, methanogens, and halophiles
• There is a difference between the archaeal type and the bacterial type in diameter. This is because archaeal filaments are thinner
• The arrangement of the proteins in the archaeal flagellum is in a right-handed helix, which results in a clockwise rotation of the archaeal type

Eukaryotic

• The eukaryotic flagellum is present in various types of algae and some animal cells like sperms
• There is an association of eukaryotic flagellum with reproduction, cell feeding and cell motility in eukaryotic animals. It also acts as sensory antennae in some types of algae
• There is a difference between the eukaryotic type and the bacterial type in assembly, mechanism, composition and architecture. The eukaryotic type is composed of many different types of proteins. In contrast, there are only 30 protein types in the bacterial type

Conclusion

Flagella are long threadlike cell organelle found on the surface of living cells. The controlling and maintaining of physiological functions of the cell is made possible by the flagellum. There is a difference in the size, structure and number of flagellum between the eukaryotes and prokaryotes. A flagellum’s basic structure comprises the filament, hook and basal body. There are various functions of the flagellum in organisms. Three types of flagellum exist in nature- bacterial, archaeal, and eukaryotic.