Flagella

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:53 PM IST

Flagella are long, whip-like structures that protrude from the cell body of many microorganisms and are vital for movement and sensation. Flagella is a topic of the chapter Cell: The Unit of Life in Biology.

What are Flagella?

These structures are likely required for the mobility of a large number of organisms, such as bacteria, protozoa, and sperm cells, so that they can move through their respective environments, search for food, and, at the same time, avoid toxic compounds. Flagella are made of structures of proteins complex in nature and vary in their structure between the prokaryotic and eukaryotic organisms. In bacteria, it contains a filament, hook, and base and operates like a motor while in eukaryotic flagella it has microtubules in arrangement “9+2” and moves in a pro-scion pattern.

This Story also Contains

What are Flagella?
Flagella Structure and Composition
Types of Flagella
Microtubule Arrangement in Eukaryotes
Function and Mechanism of Flagella
Comparison Between Prokaryotic and Eukaryotic Flagella
Flagella in Different Organisms
Diseases and Disorders Related to Flagella
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Flagella

Flagella Structure and Composition

The structure of flagella is discussed below:-

Filament

Filament is the long, twisted prolonged structure out of the flagellum that is mainly proteinaceous and is known as flagellin. This structure helps to protrude from the cell surface and is charged with the responsibility of producing the force required for movement.

Hook

The hook is thin, which is a concave section that links the filament with the basal body. The structure works as a pivot to transfer the twisting effect that is produced by the basal body to the filament, thus enabling rotation.

Basal Body

It is the basal body that secures the flagellum on the layer of the cell membrane and contains a motor that is responsible for the rotation. In bacteria, this motor is driven by proton motive force or sometimes sodium ions, on the other hand in archaea the ATP is used as fuel.

Types of Flagella

Based on the organism:

Bacterial Flagella

Structure:

Bacterial flagella consist of three main parts: these consist of the filament which is a helical structure of flagellin protein, the hook which links the filament to the motor and the basal body that are a digit that anchors the flagellum to the cell membrane and rotates.

Movement:

The bacterial flagella can rotate like a propeller; as a result, it enables the bacteria to move in any liquid substratum. The rotational direction is either clockwise or anticlockwise and this must switch so that the bacterium can perform runs and tumbles for chemotaxis.

Bacterial flagella

Eukaryotic Flagella

Structure:

Eukaryotic flagella are made up of an axoneme, a whip-like structure that has microtubules in a specific pattern of 9+2 and dynein arms which are responsible for movement through ATP breakdown.

Movement:

While bacterial flagella rotates in a corkscrew-like motion, eukaryotic flagella oscillate in a whip-like fashion which helps the cell to move forward in its habitat.

Eukaryotic flagella

Archaeal Flagella

Structure differences from bacterial flagella:

Archaella is the other type of basal body found in archaeal cells; it has a different protein makeup and assembly procedure from bacterial flagella. It does not have a core passage and therefore is not tubular like bacterial flagella.

Movement mechanisms:

It is asserted that the flagella in archaeal organisms function in the same way as that in bacterial organisms but the power used is in the form of ATP and not a proton motive force hence the difference in the way the two flagella move.

Archaeal flagella

Based on the number and the site of flagella,

Monotrichous Flagella

Definition: In a bacterial cell, one flagellum is inserted at one end of the cell. The shape of a bacterial flagellum is a long slender thread-like structure. This singular tail-like appendage propels the bacterium from spinning around but in a directed fashion.

Examples: Cholera is caused by Vibrio cholerae which is a bacterium that possesses one flagellum that assists it to swim on the villi on the surface of the intestinal lining.

Lophotrichous Flagella

Definition: These are several, situated at one and/or both poles of the cell. This organization provides higher efficiency compared to the movement with only one flagellum on the organism.

Examples: For instance, the bacterium that causes peptic ulcers, Helicobacter pylori, has a cluster of flagella by which it navigates through the thick mucus layer lining the stomach walls and through the mucus in the stomach.

Amphitrichous Flagella

Definition: Flagella are present at each end of the cell but only one of them is arranged. This way of a bacterium arranging allows this bacterium to move in both ways because the flagella can rotate in opposite ways.

Examples: In the case of Spirillum volutans, which is a spiral-shaped bacterium, this structure is employed to move with a corkscrew-like motion in water habitats.

Peritrichous Flagella

Definition: The flagella are arranged all over the surface of the cell. This widespread arrangement gives better and broader power for the forces and directions of propulsion that make locomotion possible.

Examples: Peritrichous flagella are used by Escherichia coli a common inhabitant of the gut to move through the thick broth of the intestines.

Types of flagella

Microtubule Arrangement in Eukaryotes

The microtubule arrangement is described below-

"9+2" structure of microtubules

Eukaryotic flagella have an axoneme core with this structure called the “9+2” structure: nine pairs of microtubules and two central microtubules. This structure helps in the flexibility of the flagellum since it requires to bend to carry out its function effectively.

Role of dynein arms in movement

Dynein arms are the motor complexes bound to the microtubules which produce force by the hydrolyzing nucleotide ATP. These proteins make the microtubules move past each other in a sliding motion giving the flagellum its whip-like behaviour.

"9+2" structure of microtubules

Function and Mechanism of Flagella

The function of flagella is listed below-

Locomotion

Flagella offer movement by rotating or beating, which is determined by the organism. Of them, in bacteria, the flagella act like a propeller to rotate and help the cell go forward. Flagella in protists and sperm cells move in a whip-like or wave motion to help the cell move through the liquid environments.

Chemotaxis

Flagella are involved in the ability of a cell to swim in its direction towards substances that would be beneficial to the cell (positive chemotaxis) or away from substances that are detrimental to the cell (negative chemotaxis). This is done by changing either the direction or the rate of the movement of the flagella or by stopping the beat of the flagella and changing it to the other direction about the chemical gradients.

Sensory Functions

Alternative to movement, the flagella can work as the receptor organelles, to determine alterations in conditions of the cell. They aid in receiving chemical, temperature, and mechanical stimuli to fit the organism appropriately to the situation.

Comparison Between Prokaryotic and Eukaryotic Flagella

The following shows the comparison between prokaryotic and eukaryotic flagella

Feature	Prokaryotic Flagella	Eukaryotic Flagella
Organisms	Bacteria, Archaea	Protists, Animal Cells (e.g., sperm cells)
Structure	Simple, rigid, helical filament	Complex, flexible axoneme
Main Component	Flagellin protein	Microtubules (tubulin)
Motor Mechanism	Rotational	Wave-like undulation
Energy Source	Proton motive force (bacteria), ATP (archaea)	ATP
Movement	Clockwise or counterclockwise rotation	Whip-like, wave motion
Basal Body	Anchored in the cell membrane, acts as a rotary motor	Anchored in the cell membrane, part of the axoneme
Microtubule Arrangement	None	"9+2" microtubule arrangement
Hook Presence	Present acts as a flexible connector	Absent
Speed	High rotational speeds (up to 100,000 RPM)	Slower, coordinated wave motion
Function	Mainly locomotion	Locomotion, sensory roles
Examples	E. coli, Salmonella	Sperm cells, Trypanosoma, Euglena

Flagella in Different Organisms

The types of flagella in different organisms are discussed below-

Bacteria

In bacteria such as E. coli and Salmonella, flagella are necessary structures for movements thus enabling these microorganisms to move towards beneficial conditions in counterpart to move away from adverse conditions. They move their flagella in that fashion; the flagella rotate like propellers, and the motor is at the base of the flagellum.

Protists

Some protists developed structures such as flagella, which they use to swim in aquatic environments for Trypanosoma and Euglena. In Trypanosoma, flagella provide the means for the parasite to move through the blood vessels of the host; amongst Euglena the organelles help it to swim towards the light to perform photosynthesis.

Sperm Cells

It is concerned with the locomotive system in human sperm where the flagellum is used to guide the sperm through the female reproductive tract to the egg to be fertilized. This is a key characteristic that is indispensable for successful reproduction owing to the length of space that sperm has to travel to get to the egg.

Diseases and Disorders Related to Flagella

The disorders are listed below-

Flagellar Defects

They also can cause various medical conditions as defects in cellular flagella affecting the ability of a cell to move. For example, primary ciliary dyskinesia (PCD) is a genetic disorder associated with structural or functional abnormality in cilia and flagella. This condition results in chronic respiratory tract infections, low fertility rates, and other related complications since the cilia and flagella cannot move.

Pathogenic Bacteria

In pathogenic bacteria for example Helicobacter pylori flagella have great importance in infection and disease progression. These bacteria can move within the mucus of the stomach that is thick with the help of flagella and thus they can infect the gastric epithelium, which in turn causes certain diseases like gastritis and peptic ulcers. Flagella helps the bacteria find host tissues as motility is so critical to the bacterial infection process.

Also read-

Semi autonomous Organelles	Golgi Apparatus
Endomembrane System	Lysosomes
Endoplasmic Reticulum	Vacuoles

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Frequently Asked Questions (FAQs)

1. What is the function of flagella?

Flagella are long, whip-like structures on the cell surface of many microorganisms that are mainly involved in movement and feeling. They facilitate movements within the fluid media, directions towards desirable states, and interactions with the surrounding stimuli, for instance, chemical concentrations.

2. How does bacterial flagella differ from eukaryotic flagella?

Comparing bacterial flagella and eukaryotic flagella it can be stated that the latter are more similar in structure and function to animal cilia. These structures are the filament, hook, and basal body; they rotate or propeller-like to move the bacteria. Nonetheless, the flagella of eukaryotes have the ‘9+2’ microtubule structure and the mode of movement is typically whiplash. Also, bacterial flagella operate through the proton motive force, and eukaryotic flagella operate through ATP.

3. What proteins make up flagella?

Flagella consists of structures that are specific to the cytoplasm of cells and flagellin is the main protein in bacterial flagella. Eukaryotic flagella are made of tubulin proteins that assemble the microtubule axoneme, and other motor proteins such as dynein arms. Other proteins like pericentrin are also involved in the development of flagellar structure and its functionality.

4. How do flagella contribute to cell movement?

Flagella are involved in the process of locomotion using whirling or waging movements. Bacterial flagella spin like a propeller while those of eukaryotic cells describe a pattern of coordinated to and fro motion called the whip-like motion. This movement phasing drives the cell to move and enables it to make its move within its operational space and act in response to stimuli.

5. What are some examples of organisms with flagella?

Flagella exist in different forms in different organisms belonging to the various domains of life. Some examples are bacterium Escherichia coli and Salmonella, protists Trypanosoma and Euglena and sperms of man. All these organisms utilize flagella to afford them the ability to swim, to move, and to perform their activities in their places of habitat.

6. How do prokaryotic and eukaryotic flagella differ?

Prokaryotic flagella are simpler, composed of the protein flagellin and rotate like a propeller. Eukaryotic flagella are more complex, with the "9+2" microtubule arrangement, and move with a whip-like motion.

7. What is the function of the hook in bacterial flagella?

The hook in bacterial flagella acts as a universal joint, connecting the rigid filament to the basal body. It allows the filament to rotate freely while transmitting torque from the motor, enabling efficient propulsion of the bacterium.

8. How do intraflagellar transport (IFT) proteins contribute to flagellar function?

Intraflagellar transport (IFT) proteins are crucial for the assembly and maintenance of flagella. They move materials up and down the flagellum, transporting building blocks to the tip for assembly and removing turnover products.

9. How do bacterial flagella rotate, and what drives this rotation?

Bacterial flagella rotate like a propeller, driven by a molecular motor at their base. This motor is powered by the flow of ions (usually protons or sodium ions) across the cell membrane, converting electrochemical energy into mechanical energy.

10. What is the role of flagella in the life cycle of Chlamydomonas?

In Chlamydomonas, a type of green algae, flagella are crucial for motility and sexual reproduction. During their life cycle, these organisms can shed and regrow their flagella, and use them for swimming towards light (phototaxis) and potential mates.

11. How do flagella contribute to the virulence of some pathogenic bacteria?

In some pathogenic bacteria, flagella contribute to virulence by enabling the bacteria to move towards host tissues, evade immune responses, and form biofilms. The flagella can also act as adhesins, helping bacteria attach to host cells.

12. What is flagellar shedding, and why does it occur?

Flagellar shedding is the process where some organisms deliberately detach their flagella in response to stress or as part of their life cycle. This can occur to conserve energy, avoid detection by predators, or as part of cellular differentiation.

13. How do environmental factors affect flagellar function?

Environmental factors like temperature, pH, and viscosity can affect flagellar function. For example, increased viscosity can slow flagellar movement, while extreme pH or temperature can denature the proteins involved in flagellar structure and function.

14. What is the role of flagella in biofilm formation?

Flagella play a role in the early stages of biofilm formation by enabling bacteria to reach surfaces and initiate attachment. Once attached, many bacteria lose their flagella as they transition to a sessile lifestyle within the biofilm.

15. What is meant by "flagellar beat frequency"?

Flagellar beat frequency refers to the number of complete back-and-forth movements a flagellum makes per second. This frequency can vary depending on the organism and environmental conditions, affecting the speed of cellular movement.

16. What is the basal body, and what is its function in flagella?

The basal body is a structure at the base of the flagellum that anchors it to the cell. It's composed of nine triplet microtubules and serves as the organizing center for the growth and assembly of the flagellum.

17. What is the function of the transition zone in eukaryotic flagella?

The transition zone is a region at the base of the flagellum that acts as a selective filter, controlling what enters and exits the flagellum. It plays a crucial role in maintaining the unique protein composition of the flagellum.

18. What is the glycocalyx, and how does it relate to flagella?

The glycocalyx is a carbohydrate-rich layer surrounding many cells. In some flagellated organisms, it can extend to cover the flagella, potentially affecting their interaction with the environment and their function in cellular adhesion.

19. What is the function of the paraflagellar rod in some protists?

The paraflagellar rod is a lattice-like structure found alongside the axoneme in some protist flagella, particularly in trypanosomes. It provides additional structural support and may enhance the power and efficiency of flagellar beating.

20. How do flagella contribute to bacterial chemotaxis?

In bacteria, flagella allow for directed movement towards or away from chemical stimuli (chemotaxis). By rotating their flagella, bacteria can swim towards favorable conditions or away from harmful ones.

21. How do flagella differ in plant and animal cells?

Animal cells may have flagella for locomotion (e.g., sperm cells), while most plant cells lack flagella. However, some plant reproductive cells (like male gametes in certain plants) may have flagella for movement in liquid environments.

22. What is the evolutionary significance of flagella?

Flagella are evolutionarily significant as they represent one of the earliest forms of cellular motility. Their presence across diverse groups of organisms suggests they evolved early in life's history and have been adapted for various functions.

23. How does the structure of a flagellum differ from that of a cilium?

While both are motile cellular appendages, flagella are typically longer and fewer in number compared to cilia. Flagella are usually found singly or in pairs, whereas cilia are shorter and often present in large numbers on a cell's surface.

24. How does the length of a flagellum affect its function?

The length of a flagellum affects its propulsive force and efficiency. Longer flagella generally provide more propulsive force but require more energy to move. The optimal length depends on the specific needs of the organism and its environment.

25. What is a flagellum and what is its primary function?

A flagellum is a long, whip-like structure found on some cells that enables movement. Its primary function is locomotion, allowing cells to swim through liquid environments by creating a propelling force.

26. What is the role of dynein in flagellar movement?

Dynein is a motor protein that generates the force for flagellar movement. It causes adjacent microtubule doublets to slide past each other, resulting in the bending motion of the flagellum.

27. How does ATP contribute to flagellar motion?

ATP (adenosine triphosphate) provides the energy needed for flagellar motion. It powers the dynein motor proteins, which use this energy to create the sliding motion between microtubules, resulting in the flagellum's movement.

28. What is meant by the term "undulatory motion" in relation to flagella?

Undulatory motion refers to the wave-like movement of eukaryotic flagella. This motion is created by the sequential activation of dynein arms along the length of the flagellum, resulting in a propagating bend.

29. What is the function of the central pair of microtubules in eukaryotic flagella?

The central pair of microtubules in eukaryotic flagella plays a crucial role in regulating the bending pattern and waveform of the flagellum. They help coordinate the activity of the dynein arms on the outer doublets.

30. What is the "9+2" arrangement in flagella?

The "9+2" arrangement refers to the internal structure of flagella, visible in cross-section. It consists of nine outer doublet microtubules arranged in a ring around two central singlet microtubules. This structure is crucial for flagellar movement.

31. How do flagella in sperm cells differ from those in other cell types?

Sperm cell flagella are specialized for propelling the sperm through the female reproductive tract. They are typically longer and have additional structures like the outer dense fibers and fibrous sheath, which provide extra support and energy for prolonged motility.

32. What is the role of calcium ions in flagellar movement?

Calcium ions play a crucial role in regulating flagellar movement. They can affect the beat pattern and frequency by interacting with proteins in the axoneme. Changes in intracellular calcium concentration can trigger changes in flagellar motion.

33. What is the flagellar pocket in trypanosomes, and what is its function?

The flagellar pocket in trypanosomes is a specialized invagination of the cell membrane where the flagellum emerges. It serves as the sole site for endocytosis and exocytosis in these organisms, playing a crucial role in nutrient uptake and waste removal.

34. How do flagella contribute to the mating process in some organisms?

In some organisms, like certain algae and protists, flagella play a crucial role in mating. They enable gametes to swim towards each other and can also be involved in the recognition and fusion processes between mating cells.

35. What is the difference between monotrichous and peritrichous flagellar arrangements?

Monotrichous arrangement refers to a single flagellum at one end of the cell, while peritrichous arrangement involves multiple flagella distributed around the cell surface. These different arrangements affect how the cell moves through its environment.

36. What is the difference between a flagellum and a pilus?

While both are cellular appendages, flagella are primarily used for motility and are typically longer and fewer in number. Pili are generally shorter, more numerous, and are used for adherence to surfaces or other cells, and in some cases, for DNA transfer.

37. How do flagella contribute to the formation of bacterial swarms?

Flagella play a crucial role in bacterial swarming, a form of collective motion over surfaces. They provide the propulsive force needed for movement and help coordinate the behavior of individual cells within the swarm through physical interactions.

38. How do flagella contribute to the symbiotic relationship between some bacteria and their hosts?

In some symbiotic relationships, bacterial flagella can help in colonizing host tissues. For example, in the squid-Vibrio symbiosis, bacterial flagella are crucial for the initial colonization of the squid's light organ by Vibrio fischeri.

39. How do flagella contribute to magnetotaxis in certain bacteria?

In magnetotactic bacteria, flagella work in conjunction with magnetosomes (intracellular magnetic particles) to orient and propel the cell along magnetic field lines. This allows these bacteria to navigate towards optimal growth conditions in aquatic environments.

40. How do flagella contribute to feeding in some single-celled organisms?

In some single-celled organisms, like certain protists, flagella create water currents that draw food particles towards the cell. This process, known as filter feeding, allows these organisms to capture nutrients from their aquatic environment.

41. How do flagella contribute to the pathogenicity of Helicobacter pylori?

Helicobacter pylori uses its flagella to move through the mucus layer of the stomach and colonize the epithelial lining. The motility provided by flagella is crucial for the bacterium's ability to establish infection and cause gastric diseases.

42. What is the role of flagella in the infection process of plant pathogens?

In many plant pathogens, flagella are important for reaching and colonizing plant surfaces. They enable bacteria to swim towards plant wounds or natural openings, facilitating the initial stages of infection.

43. How do eukaryotic cells regulate the length of their flagella?

Eukaryotic cells regulate flagellar length through a balance of assembly and disassembly at the flagellar tip. This process involves intraflagellar transport (IFT) and is influenced by various cellular signals and environmental factors.

44. How do flagella contribute to quorum sensing in bacteria?

While flagella themselves are not directly involved in quorum sensing, the genes controlling flagellar synthesis and function are often regulated by quorum sensing systems. This allows bacteria to coordinate their motility based on population density.

45. What is the evolutionary relationship between bacterial flagella and the type III secretion system?

There is evidence suggesting that bacterial flagella and the type III secretion system (used by some pathogenic bacteria to inject proteins into host cells) share a common evolutionary origin. Both systems have similar protein components and assembly mechanisms.

46. How do flagella contribute to the formation of bacterial rosettes?

In some bacteria, like Caulobacter crescentus, flagella play a role in forming rosette-like structures. These structures, where multiple cells attach to a surface or each other via their flagellar ends, can aid in surface colonization and biofilm formation.

47. What is the role of flagella in bacterial conjugation?

While not directly involved in the transfer of genetic material, flagella can play a role in bacterial conjugation by bringing cells into close proximity. In some species, the expression of flagella and conjugation genes are co-regulated.

48. How do flagella contribute to the motility of spirochetes?

Spirochetes have a unique form of motility using internal flagella called endoflagella or periplasmic flagella. These flagella, located between the outer membrane and cell wall, rotate to produce a corkscrew-like motion of the entire cell.

49. How do flagella contribute to the bioluminescence of some marine bacteria?

While flagella don't directly produce light, they play a crucial role in some bioluminescent bacteria by allowing them to form symbiotic relationships with marine animals. For example, Vibrio fischeri uses its flagella to colonize the light organ of the Hawaiian bobtail squid.

50. What is the role of flagella in bacterial chemotaxis?

Flagella are essential for bacterial chemotaxis, allowing bacteria to move towards attractants or away from repellents. The direction of flagellar rotation (clockwise or counterclockwise) determines whether the bacterium tumbles or swims straight, enabling it to navigate chemical gradients.

51. How do flagella contribute to the virulence of Pseudomonas aeruginosa?

In Pseudomonas aeruginosa, a common opportunistic pathogen, flagella contribute to virulence in several ways. They enable motility for initial colonization, act as adhesins for attachment to host cells, and can trigger inflammatory responses in the host.

52. What is the role of flagella in the life cycle of Chlamydomonas reinhardtii?

In Chlamydomonas reinhardtii, a model organism for studying flagella, these structures are crucial for motility, mating, and sensing environmental cues. During the sexual phase of its life cycle, flagella enable gametes to find and fuse with their mating partners.

53. How do flagella contribute to the formation of bacterial streamers in flowing systems?

In flowing systems, bacterial flagella can contribute to the formation of streamers - filamentous biofilm structures that form in the direction of flow. Flagella-mediated motility helps bacteria to initially attach to surfaces and aggregate, initiating streamer formation.

54. What is the relationship between flagella and the cell cycle in Caulobacter crescentus?

In Caulobacter crescentus, flagellar assembly and function are tightly linked to the cell cycle. The motile swarmer cell has a single polar flagellum, which is shed when the cell differentiates into a stalked cell. This coordination ensures that only specific cell types are motile.

55. How do flagella contribute to the survival of extremophiles in harsh environments?

For extremophiles living in harsh environments, flagella can be crucial for survival. They allow these organisms to move towards more favorable microenvironments, escape unfavorable conditions, and in some cases, form biofilms for protection against extreme conditions.