Flagella – Structure, Types and Functions

Flagella are hair-like protrusions, which are found on a variety of microorganisms as well as on some plant and animal sperm cells. The flagella function is to provide mobility to these cells. The flagella structure is composed of four parts including, the basal body, filament, hook, and motor. Flagella on bacteria are whip-like appendages that protrude from the cell surface, enabling bacterial cells to move through liquid environments.

Flagellates are a broad term for protists with flagella. Flagella and cilia sometimes interplay but flagella are longer than the cilia. Flagella are present in bacterial cells, archaea, and eukaryotic cells as well. A microbe may have a single flagellum or several. Eukaryotic flagella contain dynein and microtubules that move through bending. In this article, we will study flagella definition, meaning, types, and functions of flagella.

What is Flagella?

Flagella are hair-like protrusions from the cell body with dimensions ranging from 5 to 20 m and 10 to 30 nm. A variety of motile bacteria, including Selenomonas and Wolinella succinogenes, use it as a locomotive organelle. The Flagellum is made up of three parts: the basal body, the hook, and the filament. Bacteria can contain as little as one or two Flagella or hundreds of them.

Only with a light or electron microscope, we can directly view flagella after suitable staining. The flagellum is antigenic (for example, antigen H) and plays a role in the pathophysiology of various diseases. Flagellate bacteria include Vibrio cholerae and Campylobacter jejuni. They use a group of flagella to break through the mucus lining of the small intestine and reach the epithelium, where they produce toxins.

Flagella Diagram

The labeled diagram of flagella is shown below:

Bacterial Flagella

Bacterial flagella is a motile organelle made up of thousands of different protein subunits. The axial structure is the filamentous portion that protrudes from the cell membrane. It is made up of the filament, the hook, and the rod in addition to other smaller sections. Each of the three major components shows fairly different mechanical properties to carry out its particular job, yet they all share a basic architecture of subunit arrangement and a self-assembly mechanism.

Structure of Flagella on Bacteria

A hair-like helical structure called a flagellum (singular) arises from the cell membrane and cell wall. It controls the bacteria’s ability to move. It is thin in size, 15–20 nm in diameter. Only after dyeing with a specific stain that increases the Flagella’s diameter can a single flagellum be seen under a light microscope. Flagella are helical, not straight. It is referred to as the H antigen and is made up of the globular protein flagellin. There are three components to a flagellum: Basal body, Hook and Filament.

Basal Body

• The basal body is made up of a central rod that is inserted into a series of rings, which are connected to the cell wall and cytoplasmic membrane.
• L-ring: The outer ring that is only found in Gram-positive bacteria and is anchored in the lipopolysaccharide layer.
• P-ring: The second ring in the cell wall’s peptidoglycan layer.
• M-S ring: Cytoplasmic membrane-anchored.
• C ring: Cytoplasmic attachment.

Hook

• It is the larger area at the filament’s base.
• It joins the filament to the base’s motor protein.
• In gram +ve bacteria, the length of the hook is longer than in gram negative bacteria.

Filament

• The long, helical screw that the bacteria is propelled by when the motor rotates it through the hook is called the flagellar filament.
• The filament is made up of 11 protofilaments that are almost parallel to the filament axis in the majority of the bacteria, including the Gram-negative Escherichia coli, Salmonella typhimurium, and Vibrio alginolyticus.
• A group of protein tandem chains make up each protofilament. Campylobacter jejuni, however, possesses seven protofilaments.

Motor

At the flagellum’s anchor point on the inner cell membrane, a protein-based rotary engine called the Mot complex propels the bacterial flagellum. Proton-motive force, or the flow of protons (hydrogen ions) across the bacterial cell membrane as a result of a concentration gradient set up by the cell’s metabolism, powers the engine. Vibrio species have lateral and polar flagella, and some are driven by a sodium ion pump instead of a proton pump. Protons are transported through the membrane while being rotated by the rotor. The rotor may spin at a speed of 6,000 to 17,000 rpm on its own, but often only achieves 200 to 1000 rpm when the flagellar filament is attached.

Types of Bacterial Flagella

Variable bacterial species, called tricho (from the Greek word trichos – meaning hair) have variable flagella numbers and patterns:

• Monotrichous Bacteria: They have just one polar flagellum, like Vibrio cholerae.
• Amphitrichous Bacteria: One flagellum only functions at a time in amphitrichous bacteria (like Alcaligenes faecalis), which enables the bacterium to quickly change its direction by switching which flagellum is active.
• Lophotrichous Bacteria: Helicobacter pylori is an example of a lophotrichous bacterium, which has many flagella that are all situated at the same location on the bacterial surface and work together to propel the bacteria in a single direction. In a lot of cases, a specific area of the cell membrane known as the polar organelle surrounds the bases of numerous Flagella.
• Peritrichous Bacteria: Flagella on peritrichous bacteria, such as E. coli, project in all directions.

Types of Flagella

Flagella exist in three different types: bacterial, archaeal, and eukaryotic. Dynein and microtubules in eukaryotic flagella move through bending. Bacterial and archaeal flagella lack dynein and microtubules, and they spin to move.

• Bacterial Flagella: Each of the helical filaments that make up bacterial flagella has a rotary motor at its base that may rotate either clockwise or counterclockwise. They offer two of the several bacterial motility types.
• Eukaryotic Flagella: Animal, plant, and protist cell eukaryotic flagella are intricate cellular appendages that lash back and forth. Although the construction of eukaryotic Flagella and motile cilia is the same, their lengths, waveforms, and uses vary. The 9+0 axoneme, which is structurally distinct from the 9+2 axoneme found in both Flagella and motile cilia, is present in primary cilia, which are immotile.
• Archaeal Flagella: Although archaeal flagella (archaella) are thought to be non-homologous, they are superficially similar to bacterial Flagella in that they both include a rotary motor.

Comparison Between Bacterial, Archaeal, and Eukaryotic Flagella

The below table shows the comparison between Bacterial, Archaeal, and Eukaryotic Flagella including their structure, movement, growth, presence etc.

Feature	Bacterial Flagella	Archaeal Flagella (Archaella)	Eukaryotic Flagella
Structure	Composed of flagellin protein. Thin and rigid structure.	Composed of archaellins. Thinner than bacterial flagella.	Composed of microtubules in a 9+2 arrangement. Thicker and more complex.
Movement	Rotates like a propeller.	Rotates like a propeller but is structurally and genetically distinct from bacterial flagella.	Waves in a whip-like motion.
Energy Source	Proton motive force (PMF).	ATP or possibly a different form of ion motive force.	ATP.
Growth	Grows from the base.	Grows from the base.	Grows from the tip.
Function	Primarily for motility.	Primarily for motility.	Motility, but also plays roles in cell signaling and sensory functions.
Presence	Found in many bacteria.	Found in many archaea.	Found in eukaryotic cells, including animals, plants, and fungi (in sperm cells and some unicellular organisms).

Functions of Flagella

Flagella conducts the following activities:

• They facilitate mobility in an organism.
• Few eukaryotes have a flagellum to speed up reproduction.
• They serve as sensory organs, detecting changes in pH and temperature.
• Recent studies have demonstrated that flagella can function as a secretory organelle as well. Example: Chlamydomonas.

Cilia versus Flagella

Motion is produced at the cellular level by the regular beat patterns of eukaryotic cilia and Flagella. Examples range from the movement of fluid along a stationary layer of cells, such as in the respiratory tract, to the propulsion of single cells, like the swimming of spermatozoa.

Even though eukaryotic cilia and Flagella are fundamentally the same, they are occasionally categorized based on how they move, a practice that dates back to when the structures of these structures were known. Unlike motile cilia, which frequently execute a more complex three-dimensional motion with a power and recovery stroke, flagella frequently move in a planar, wave-like fashion. Another common method of differentiation is based on the cell’s assembly of 9+2 organelles.

Conclusion : Flagella – Structure, Types and Functions

In conclusion, flagella are essential cell organelles found on various microorganisms and some sperm cells. The flagella function to providing mobility. Flagella structure consists of a basal body, hook, filament, and motor, they enable bacteria to move through liquid environments. Flagella play crucial roles in detecting environmental changes, facilitating reproduction, and even functioning as sensory organs. Moreover, they differ from cilia in their movement patterns and assembly, contributing significantly to cellular motion and function.

FAQs on Flagella – Structure, Types and Functions

Where are Flagella Located?

Flagella can be found single at one cell pole (monotrichous flagella), in great numbers along the length of the cell (peritrichous flagella), or as a tuft at a polar end (lophotrichous flagella). They can also be found at both poles (amphitrichous flagella).

Are Flagella found in Sperm?

The flagellum, a single motile cilium that generates movement to propel the cell through the female reproductive system and transfer paternal genetic material into an egg, is a representation of the tail of mammalian sperm cells.

What is the Largest part of Flagella?

The largest part of Flagella is the filament. The Flagella of the motile bacteria are tiny filamentous protrusions from their cell walls. The number and placement of flagella vary among bacteria.

Are Flagella found in Humans?

Flagella are only found in sperm cells in the human body. Near the cell’s extreme end, Flagella are frequently found. Normally, flagella do not fuse. Flagella, which are typically unicellular, contribute in cell movement.

What is Flagella Biology Definition?

In biology, flagella are whip-like appendages found in some cells, facilitating cell movement through their waving motion.

Describe the Structure of Flagella and Cilia?

Flagella and cilia are hair-like structures on cell surfaces, made of microtubules arranged in a 9+2 pattern, enabling movement. Flagella are longer and less numerous, while cilia are shorter and more abundant.

What is Flagella Function?

Flagella function primarily to provide mobility to cells, enabling them to move through liquid environments. They also play roles in detecting changes in pH and temperature and can even function as sensory organs.