Flagella: Structure, Types, Functions and Sample Questions

Flagella is a microscopic hair-like outgrowth of the cell membrane, which aids in the locomotion of the organism. Flagella means “whip” due to its whip-like appearance that helps to propel a cell through fluids. They also have some sensory and chemotaxis which can sense pH and temperature changes in the surroundings. Some special flagella are applied in few organisms as sensory organs that help sense changes in pH and temperature.

Key terms: Flagella, Basal Body, Axoneme, hook and filament, eukaryotes. 

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What is Flagella?

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A flagellum that has a 15-20nm diameter is visible under a microscope. The whip-like shape of flagella helps motion of single-celled organisms. These are filamentous systems made from microtubules. They are found in eukaryotes and bacteria.

1. Archaeal flagella are generally nonhomologous.
2. Bacterial flagella looks like a coiled, thread-like structure, sharp bent, consisting of a rotary motor at its base and composed of the protein flagellin. In between a hook, a shaft exists between a hook and a basal body passing through the protein rings in the cell membrane.
3. Eukaryotic flagella are referred to as complicated cellular projections that pummel backwards and forward and are found in protist cells, gametes of plants, and animals. It is made up of a protein called tubulin.

Also read: Differences between cell membrane and plasma membrane

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Structure of a Flagella

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Flagella is a helical structure, which is divided into three parts, which are enlisted below-

Basal Body

It is attached to the cell membrane and cytoplasmic membrane. It consists of rings surrounded by a pair of proteins. The core of a flagella is called an axoneme, which possesses a number of microtubules running parallel to the long axis. The axoneme has nine doublets of radially arranged peripheral microtubules, and a pair of centrally located microtubules. This arrangement of axonemal microtubules is referred to as the 9+2 array.

The rings of the basal body includes: -

L-ring - Outer ring anchored in the lipopolysaccharide (LPS) layer and found in gram +ve bacteria.

P-ring - Anchored in the peptidoglycan layer.

C-ring - Anchored in the cytoplasm

M-S ring: Anchored in the cytoplasmic membrane

Hook

It is a broader area present at the base of the filament, and it connects filament to the motor protein in the base.

Filament

It is a rigid, helical, hair-like structure arising from the hook, which extends from the cell surface.

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Types of Flagella

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• Atrichous: Organism doesn’t have a flagellum. Example: Lactobacillus
• Monotrichous: Single polar flagellum can rotate both clockwise and anti-clockwise resulting in forward movement and backward movement respectively. They are also known as Polar Flagella. Example: Vibrio cholerae
• Amphitrichous: One flagellum is present on each end, movement is similar to monotrichous flagella. Example: Alkaligens faecalis
• Lophotrichous: Several flagella or the other tufts of flagella are present at one or both end of the organism. Propagates clockwise and anticlockwise. Example: Spirillum
• Peritrichous: Many flagella are present all over the bacterial body. These flagella rota anti-clockwise and form a bundle that moves the organism in one direction. If some of the flagella break and start rotating clockwise, the organism does not move in any direction and begins tumbling. Example: Salmonella Typhi
• Cephalotrichous: Several flagella are present at both ends. Movements are similar to monotrichous flagella. Example: Pseudomonas

Also read: Difference between Chromatin and Chromosomes

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Functions of Flagella

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• They help an organism in locomotion and movement.
• Acts as a sensory organ
• Some flagella act as a secretory organelle.
• Help in reproduction.

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Things to remember

• Flagella act as sensory organs in order to detect temperature and pH changes.
• Few eukaryotes use flagellum so that they can increase reproduction rates.
• Recent researches have found that flagella are also used as a secretory organelle. For eg., in Chlamydomonas.
• Flagella is a helical structure, which is divided into three parts, basal body, hook and filament.