Understanding Flagella: Structure, Function, and Diversity
Last Updated : 11 Jun, 2024
Flagella are hair-like protrusions, intricate whip-like appendages crucial for cellular movement in various organisms. These microstructures propel cells through liquids, helping in navigation, nutrients, and defense mechanisms. Flagella are important features in bacteria, archaea, and eukaryotic cells, showcasing a variety of diversity in structure and function. Explore more of the mechanisms of flagellar movement, their roles in microbial biology, and their significance in understanding cellular motility and adaptation in diverse environments.
In this article, we study types, structures, functions, and facts about flagella in detail.
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.
Also Read: Microscopy
Flagella Location
Flagella can be found in various locations on a cell.
- In bacteria, they often protrude from the cell surface, extending outward to propel the cell through its environment.
- Some bacteria have flagella at one or both ends of the cell, allowing for efficient movement in different directions.
- In eukaryotic organisms like sperm cells, flagella are typically found as a single long tail-like structure extending from the cell body, facilitating locomotion through fluid environments.
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.
Difference 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.
Flagella Example
Examples of flagella include:
- Bacterial Flagella: Found in bacteria like Escherichia coli, these whip-like appendages protrude from the cell surface and enable bacterial movement through liquid environments.
- Eukaryotic Flagella: Present in organisms like sperm cells, eukaryotic flagella are long, tail-like structures that aid in cell locomotion, allowing sperm to swim towards the egg for fertilization.
- Archaeal Flagella: Archaea, a group of single-celled microorganisms, also possess flagella that assist in their movement and navigation.
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
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.
Also Read:
- Cilia And Flagella - Definition, Structure, Functions and FAQs
- Euglenoid
- Bacterial Chemotaxis
- Cell Organelles – Structure, Types and their Functions
- What is Zoospore?
- Cilia – An Overview
- Difference between Cilia, Stereocilia and Microvilli
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