Flagella- Definition, Structure, Types, Arrangement, Functions, Examples

Flagella are hair-like or whip-like structures that protrude from the cell membrane and are composed of protein. They are used to propel cells through liquid, especially bacteria, archaea, and some eukaryotes like flagellates and sperm . Flagella can vary in number and location on the cell surface . The Latin word flagellum means "whip" to describe its lash-like swimming motion. The flagellum in archaea is called the archaellum to note its difference from the bacterial flagellum .

Flagella are important for various biological functions, such as locomotion, sensing, feeding, and reproduction. They also play a role in the colonization of tissue surfaces and the virulence of some pathogens. Flagella can respond to different stimuli, such as chemical, thermal, or mechanical signals, and change their direction or speed of movement accordingly.

Flagella have different structures, compositions, and mechanisms of propulsion across the three domains of life: Bacteria, Archaea, and Eukaryota. However, they all share the same function of providing motility. The three types of flagella are bacterial, archaeal, and eukaryotic. The flagella in eukaryotes have dynein and microtubules that move with a bending mechanism. Bacteria and archaea do not have dynein or microtubules in their flagella, and they move using a rotary mechanism. Other differences among these three types will be discussed in detail in the following sections.

In this article, we will explore the definition, structure, types, arrangement, functions, and examples of flagella in different organisms. We will also compare and contrast the similarities and differences among the flagella of different domains of life. By the end of this article, you will have a better understanding of the diversity and complexity of these fascinating cellular appendages.

A flagellum is a lash-like structure that protrudes from the cell body and acts as an organelle of locomotion in many living organisms. The structure and composition of flagella vary among different domains of life, such as bacteria, archaea, and eukaryotes. However, the basic structure of a flagellum consists of three main components: filament, hook, and basal body.

Filament

The filament is the most prominent and longest part of a flagellum that extends from the cell surface to the surrounding medium. The filament is composed of multiple subunits of a protein called flagellin, which are arranged in a helical fashion. The length and diameter of the filament depend on the type and number of flagellin subunits, which differ among different groups of organisms. For example, bacterial flagella have a diameter of about 20 nm and a length of several micrometers, while archaeal flagella have a diameter of 10-14 nm and a shorter length. Eukaryotic flagella have a different composition and structure than prokaryotic flagella. They are made up of microtubules that form a core called axoneme, which is surrounded by a membrane. The axoneme has a characteristic arrangement of nine pairs of microtubules around two central microtubules, known as the 9+2 pattern.

Hook

The hook is a short and curved tubular structure that connects the filament to the basal body. The hook acts as a flexible joint that allows the filament to rotate in different directions and orientations. The hook is also composed of protein subunits that form a polymorphic supercoil. The shape and size of the hook vary among different types of flagella. For example, bacterial flagella have a hook with a length of about 55 nm and a diameter of 25 nm, while archaeal flagella have a shorter and thinner hook.

Basal body

The basal body is the part of the flagellum that anchors it to the cell membrane and acts as a motor that drives its rotation. The basal body is located within the cell envelope and consists of a rod-like structure with a system of rings that span the cytoplasmic membrane, the peptidoglycan layer, and the outer membrane (in gram-negative bacteria). The rings are connected by spokes that transmit torque from the motor to the rod. The motor is powered by either ATP (in archaea) or proton motive force (in bacteria), which generate rotational force through interactions with proteins called stators and rotors. The basal body also contains an export apparatus that transports flagellar proteins from the cytoplasm to the hook and filament during assembly.

The mechanism of flagella formation and assembly is a complex and dynamic process that involves the transport and assembly of various protein subunits from the cytoplasm to the flagellar structure. The process differs slightly between bacterial, archaeal and eukaryotic flagella, but some common steps can be identified.

• Formation of the basal body: The basal body is the motor device that anchors the flagellum to the cell membrane and drives its rotation. The formation of the basal body begins with the insertion of a ring-like protein complex called FliF into the cytoplasmic membrane. This complex forms the MS-ring, which determines the number and orientation of the flagella. The MS-ring also recruits other proteins such as FliG, FliM and FliN, which form the C-ring on the cytoplasmic face of the MS-ring. These proteins are involved in regulating the flagellar rotation and exporting other flagellar components. The next step is the formation of a rod-like structure that spans across the cell wall and connects the MS-ring to the hook. The rod is composed of several proteins that are sequentially transported through the central channel of the MS-ring and assembled at its distal end.
• Formation of the hook: The hook is a curved tubular structure that connects the rod to the filament and transmits the torque from the basal body to the filament. The hook also acts as a universal joint that allows the filament to change its direction relative to the cell axis. The formation of the hook is initiated by a protein called FlgD, which binds to the tip of the rod and induces the polymerization of hook protein subunits (FlgE) into a helical arrangement. The hook grows until it reaches a specific length (about 55 nm in bacteria), which is controlled by a molecular ruler mechanism involving another protein called FliK. Once the hook reaches its optimal length, FliK interacts with a protein complex called FlhB, which switches the export specificity from hook-type proteins to filament-type proteins.
• Filament assembly: The filament is the longest and most visible part of the flagellum that extends from the hook to the outside of the cell. The filament is composed of thousands of protein subunits called flagellins, which are arranged in a helical fashion. The filament assembly occurs at its tip, where flagellins are transported through the central channel of the flagellum and added to a cap structure that covers the growing end. The cap is composed of five proteins (HAP1-HAP5) that bind to each other and to flagellins, facilitating their polymerization and preventing their diffusion. The length of the filament can vary depending on environmental conditions and genetic factors, but it is usually several micrometers long in bacteria.

Flagella can be classified into different types based on their structure, composition, and function. The main types of flagella are bacterial, archaeal, and eukaryotic flagella.

Bacterial flagella

• Bacterial flagella are helically coiled structures that are slightly longer than the archaeal and eukaryotic flagella.
• They are thinner than eukaryotic flagella with a diameter of around 20 nanometers.
• The number of flagella in bacteria depends on different species and their arrangement can be monotrichous, lophotrichous, amphitrichous, or peritrichous.
• The main function of bacterial flagella is locomotion and they can also act as sensory structures that detect changes in the environment.
• Bacterial flagella are composed of three main parts: the basal body, the hook, and the filament. The basal body is a rod-shaped structure with a system of rings that acts as a reversible motor. The hook is a short and curved tubular structure that connects the basal body to the filament. The filament is the longest part of the flagellum that extends from the hook and is composed of flagellin subunits.
• Bacterial flagella are powered by a proton motive force that drives the rotation of the basal body and the filament. The direction of rotation determines the movement of the bacteria. Counterclockwise rotation causes forward movement or `run`, while clockwise rotation causes tumbling or `reorientation`.

Archaeal flagella

• Archaeal flagella are similar to bacterial flagella in appearance but differ in their structure, composition, and mechanism.
• Archaeal flagella are thinner than bacterial flagella with a diameter of 10-14 nanometers.
• The number and arrangement of archaeal flagella vary among different species and can be polar or lateral.
• The main function of archaeal flagella is locomotion and they can also respond to environmental stimuli such as temperature, pH, and chemicals.
• Archaeal flagella are composed of two main parts: the basal body and the filament. The basal body is a rod-shaped structure with a system of rings that acts as a reversible motor. The filament is composed of several types of flagellin subunits that form a right-handed helix.
• Archaeal flagella are powered by ATP hydrolysis that drives the rotation of the basal body and the filament. The direction of rotation determines the movement of the archaea. Clockwise rotation causes forward movement or `pushing`, while counterclockwise rotation causes backward movement or `pulling`.

Eukaryotic flagella

• Eukaryotic flagella are whip-like structures that are thicker than bacterial and archaeal flagella with a diameter of about 200 nanometers.
• The number of flagella in eukaryotes depends on different species and their arrangement can be polar or lateral.
• The main function of eukaryotic flagella is locomotion and they can also be involved in feeding, reproduction, and sensing in some organisms.
• Eukaryotic flagella are composed of two main parts: the basal body and the axoneme. The basal body is a centriole-like structure that anchors the flagellum to the cell membrane. The axoneme is a cylindrical structure that extends from the basal body and is composed of microtubules arranged in a 9+2 pattern. The microtubules are connected by dynein arms that generate sliding forces between them.
• Eukaryotic flagella are powered by ATP hydrolysis that drives the bending movement of the axoneme. The bending movement causes wave-like motion or `beating` of the flagellum. The direction and frequency of beating determine the movement of the eukaryote.

Bacterial flagella are helically coiled structures that are slightly longer than the archaeal and eukaryotic flagella. The central core or axoneme of bacterial flagellum consists of a 9+2 arrangement of microtubules. Two singlet microtubules are present in the center of the structure, that runs through the flagella.

The location, number and arrangement of flagella vary considerably in different bacterial species. Based on the arrangement of flagella on the cell surface, bacteria can be classified into four types :

• Monotrichous: A single flagellum is present at one end (pole) of the cell. Example: Vibrio cholerae, Pseudomonas aeruginosa.
• Lophotrichous: A cluster or tuft of flagella is present at one end (pole) of the cell. Example: Pseudomonas fluorescens, Spirillum.
• Amphitrichous: A single flagellum or a tuft of flagella is present at both ends (poles) of the cell. Example: Campylobacter jejuni, Rhodospirillum rubrum.
• Peritrichous: Flagella are present all over the cell surface. Example: Escherichia coli, Salmonella, Bacillus subtilis.

The arrangement of flagella affects the mechanism and direction of bacterial movement. Bacteria with monotrichous or lophotrichous flagella move by rotating their flagella in a counterclockwise direction, which results in a forward or `run` movement. When they rotate their flagella in a clockwise direction, they move backward or `tumble`, which allows them to reorient themselves. Bacteria with amphitrichous flagella move by rotating their flagella in opposite directions, which creates a push-pull effect. Bacteria with peritrichous flagella move by forming a bundle of flagella that rotates in a counterclockwise direction, which propels them forward. When they rotate their flagella in a clockwise direction, they break the bundle and `tumble`, which changes their direction.

Bacterial movement is influenced by various stimuli, such as chemicals, light, oxygen, temperature and gravity. This is called taxis. Bacteria can move towards favorable stimuli (positive taxis) or away from unfavorable stimuli (negative taxis). For example, bacteria can move towards nutrients (chemotaxis), towards light (phototaxis), towards oxygen (aerotaxis), towards higher temperature (thermotaxis) or towards the earth`s center (geotaxis).

Bacterial flagella are important for various physiological functions, such as motility, colonization, virulence, nutrient exchange and pH regulation. Flagella also play a role in the initiation of innate immune defenses by interacting with toll-like receptor 5 (TLR5) on host cells. Flagella are considered as antigens and can elicit an immune response in the host. Different types of flagellar antigens are designated by H antigens.

Some possible sentences to conclude point 5 are:

• In summary, bacterial flagella are diverse structures that enable bacteria to move and adapt to different environments and conditions.
• Therefore, bacterial flagella are essential structures for bacterial survival, growth and pathogenicity.

• Thus, bacterial flagella are remarkable structures that exhibit complex mechanisms and functions in bacteria.

  Discussion of the functions of flagella in locomotion and other physiological activities

Flagella are hair-like structures that protrude from certain cells and microorganisms to provide motility and sensory functions. Flagella can have different structures, compositions and mechanisms of movement depending on the type of organism they belong to. However, they all share some common functions that are essential for the survival and adaptation of the cells.

Locomotion

One of the main functions of flagella is to enable the movement of cells through liquid media. This is especially important for microorganisms that live in aquatic environments, such as bacteria, archaea and protists. Flagella can propel the cells towards favorable conditions, such as nutrients, light or oxygen, or away from unfavorable ones, such as toxins, predators or competitors. This process is called chemotaxis .

The mechanism of locomotion differs between prokaryotic and eukaryotic flagella. Prokaryotic flagella are helical filaments that rotate like propellers around a basal body that acts as a motor. The rotation can be clockwise or counterclockwise, depending on the direction of the proton or sodium gradient across the cell membrane. The direction of rotation determines the mode of movement: counterclockwise rotation causes the flagella to form a bundle and move the cell forward (run), while clockwise rotation causes the flagella to separate and change the direction of the cell (tumble).

Eukaryotic flagella are composed of microtubules arranged in a 9+2 pattern that slide past each other with the help of dynein motor proteins. The sliding causes the flagella to bend in a wave-like motion that pushes or pulls the cell through the fluid. The direction and frequency of the bending depend on the ATP supply and the signals from the cell.

Sensory functions

Another function of flagella is to act as sensory organelles that can detect changes in temperature, pH, osmolarity, light, chemicals and other environmental factors . Flagella can transmit these signals to the cell and trigger appropriate responses, such as altering gene expression, metabolism, motility or cell cycle .

Flagella can sense environmental changes by using different mechanisms. Some flagella have specialized receptors or ion channels on their surface that bind to specific molecules or ions and modulate their activity . For example, bacterial chemoreceptors can sense changes in nutrient concentrations and regulate the direction and speed of flagellar rotation. Other flagella have mechanosensitive proteins that respond to changes in fluid flow or pressure and alter their bending pattern . For example, eukaryotic cilia can sense fluid flow in the kidney tubules and regulate fluid reabsorption.

Other physiological activities

Flagella can also perform other physiological activities that are specific to certain types of cells or organisms. Some examples are:

• Flagella can help in colonization of tissue surfaces by acting as virulence factors that enable bacteria to invade host cells and evade immune responses. For example, Helicobacter pylori uses its flagella to penetrate the mucus layer and reach the stomach epithelium, where it causes gastric ulcers.
• Flagella can help in nutrient and waste exchange by creating water currents that bring fresh nutrients and remove waste products from the cell surface. For example, sponges and coelenterates use their flagellated cells to filter feed and respire.
• Flagella can help in reproduction by facilitating fertilization or increasing reproduction rates. For example, sperm cells use their flagella to swim towards the egg cell and penetrate it. Some algae use their flagella to increase their chances of sexual reproduction by forming gamete aggregations.

• Flagella can help in secretion by transporting proteins or other molecules from the cell to the outside environment. For example, some algae use their flagella as secretory organelles that release toxins or signaling molecules.

  Examples of flagella in Helicobacter pylori and human sperm cell

Flagella are present in different types of cells, ranging from bacteria to eukaryotes. Here are two examples of flagella in Helicobacter pylori and human sperm cell.

• Flagella in Helicobacter pylori

Helicobacter pylori is a Gram-negative bacterium that has a helical or spiral shape and has 6-8 flagella at one end. The flagella are essential for the colonization of the human gastric mucosa by this pathogen, as they provide motility and chemotaxis. The flagella also influence the inflammation, immune evasion, and virulence of H. pylori.

The flagella of H. pylori consist of three structural elements: a basal body, a hook, and a filament. The basal body is embedded in the cell wall and contains the proteins required for rotation and chemotaxis. The hook serves as a joint between the basal body and the filament, which is an external helical structure that works as a propeller when rotated at its base. The filament is composed of flagellin subunits and has a bulb-like structure at its distal end that represents a dilation of the flagellar sheath. The sheath is an extension of the outer membrane and protects the flagellar structure from the acidic environment.

The formation and assembly of the flagella in H. pylori involve more than 50 proteins that are involved in expression, secretion, and assembly of the flagellar apparatus. The process begins with the formation of the FliF ring complex in the basal body, followed by the formation of the rod, hook, and filament. The proteins are exported from the cytoplasm through a flagellum-specific type III protein export system that binds and moves them into the central channel of the flagellum.

• Flagellum in human sperm cell

The flagellum in human sperm cell is essential for motility and fertilization in humans. The failure to move or propel the flagellum can result in infertility or reduced fertility.

The flagellum in human sperm cell is composed of a core structure called axoneme, which consists of microtubules arranged in a 9+2 pattern. The axoneme extends from the centriole, which is considered the basal body of the flagellum. The axoneme is surrounded by an outer dense fiber and a fibrous sheath that provide structural support. The movement of the flagellum is driven by dynein motors that slide along the microtubules and generate bending waves.

The formation and assembly of the flagellum in human sperm cell involve hundreds of proteins that are transported from the cytoplasm to the axoneme by intraflagellar transport (IFT) complexes. The process begins with the formation of the centriole at the base of the sperm head, followed by the elongation of the axoneme by adding tubulin subunits at its tip. The outer dense fiber and fibrous sheath are also assembled along the axoneme by specific proteins.

We are Compiling this Section. Thanks for your understanding.