Flagella

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.

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This Story also Contains

1. What are Flagella?
2. Flagella Structure and Composition
3. Types of Flagella
4. Microtubule Arrangement in Eukaryotes
5. Function and Mechanism of Flagella
6. Comparison Between Prokaryotic and Eukaryotic Flagella
7. Flagella in Different Organisms
8. Diseases and Disorders Related to Flagella
9. Recommended video for 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.

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• MCQ Practice on Flagella
• Cell: The Unit Of Life
• Cytoplasm
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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.

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.

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.

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.

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.

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

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.

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