Flagella: Structure, Functions, Types

Flagella: A flagellum is a long, whip-like appendage on the surface of a cell that is utilised for motility or movement. It can be present in single-celled creatures such as protozoa and multicellular species such as sperm cells.

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Occurrence of Flagella

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Structure in Bacteria

• Basal Body: The basal body is  the flagellum's base and is embedded in the cell membrane and cytoplasm. It comprises a succession of ring-like structures known as the rod, hook, and motor.
• Filament : A filament is a long, thin structure comprising the protein flagellin that extends from the basal body. This portion of the flagellum extends beyond the cell and controls cell movement.
• Hook: A flexible junction that links the filament to the basal body and allows the filament to move and change direction.
• Motor: The motor is a protein complex found at the base of the flagellum that is in charge of rotating the filament. The motor is propelled by the passage of ions across the cell membrane, which produces a proton motive force.

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Structure in Eukaryotes

• Basal body: This is the base of the flagellum and is composed of microtubules and associated proteins. It is anchored in the cell membrane and cytoplasm and is the structural support for the rest of the flagellum.
• Axoneme: The axoneme is the core of the flagellum and is composed of microtubules, which provide the structural framework for the movement of the flagellum. The microtubules are arranged in a specific pattern, with two central microtubules surrounded by nine peripheral microtubules.
• Dynein arms are molecular motors attached to the axoneme's microtubules. They move along the microtubules and cause the bending of the flagellum, which generates the movement.
• Nexin links: These proteins connect the axoneme's adjacent microtubules and help maintain proper alignment.

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Differences between bacterial and eukaryotic flagella:

Function of Flagella

• Movement
• Locomotion
• Sensory Input
• Cell-to-Cell Communication
• Propulsion
• Fertilization
• Attachment and adhesion
• Signaling and communication
• Maintenance of Cell shape
• Regulation of Cell division

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Types of Flagella

• Monotrichous flagellum: This type of flagellum is found at one end of the cell and is a single, trailing flagellum.

• Amphitrichous flagellum: This type of flagellum is found at both ends of the cell and comprises two trailing flagella.

• Lophotrichous flagellum: This type of flagellum is found at one end of the cell and is composed of multiple trailing flagella.

• Peritrichous flagellum: This type of flagellum is found all over the cell's surface and is composed of multiple flagella in various arrangements.

• Cilia : This type of flagellum is found on the surface of eukaryotic cells and plays a key role in the movement of fluid and particles across the cell's surface and is found in various tissues, including the respiratory tract and the fallopian tubes.
• Cephalotrichous: This type of flagellum is characterised by a single flagellum at the cell's front or "head" end. Example: Caulobacter crescentus .

Evolutionary Significance of Flagella

VI. A to Z of Flagella:

• A - Appendage: A flagellum is a long, whip-like appendage that extends from a cell or organism.
• B - Bacteria: One of the organisms that contain flagella, used for movement and motility.
• C - Complex structure: A flagellum in eukaryotes is more complex, involving a microtubule structure and motor proteins.
• D - Development: The development of flagella has played an important role in the survival and success of many organisms.
• E - Evolutionarily conserved: The flagellum is a highly conserved structure with a similar structure and function in evolutionarily distant organisms.
• F - Filament: The flagellum in bacteria comprises a flagellin protein filament.
• G - Growth: Flagella play a role in the growth and survival of many organisms.
• H - Helical structure: The filament of the bacterial flagellum has a helical structure that rotates like a propeller to move.
• I - Importance: Flagella are important for the survival and success of many organisms, playing a role in movement, motility, and fertilisation.
• J - Jointed structure: In eukaryotes, the flagellum structure involves a jointed structure made of microtubules and motor proteins.
• K - Kinetics: The movement of flagella is driven by the kinetics of the motor proteins that move.
• L - Long and slender: A flagellum is a long, slender structure extending from a cell or organism.
• M - Motile structure : A flagellum is a common motile structure in many unicellular and some multicellular organisms.
• N - Nucleation: Forming flagella in eukaryotes involves the nucleation of microtubules and the assembly of motor proteins.
• O - Organic: A flagellum is an organic structure of proteins and other biological molecules.
• P - Protein: The filament of the bacterial flagellum is made of a flagellin protein.
• Q - Quick movement: Flagella enable quick movement in organisms, allowing them to swim or move towards a target.
• R - Role : The role of flagella varies depending on the organism they are found in, but they typically play a role in the movement, motility, and fertilisation.
• S - Sperm cells: Sperm cells contain flagella, which provide the movement required to swim towards the egg cell for fertilisation.
• T - Tube-like: The structure of the flagellum in eukaryotes is typically tube-like, involving a microtubule structure and motor proteins.
• U - Unique: Each type of organism that contains flagella has a unique flagellar structure and function.
• V - Variation: The flagella structure can vary depending on the organism they are found in, reflecting the diversity of life on Earth.
• W - Whip-like: A flagellum is a long, whip-like structure that extends from a cell or organism.
• X - Xenopus: Xenopus, a genus of African frogs, contains sperm cells with flagella, which play a role in fertilisation.
• Y - Yeast: Some yeast species contain flagella, which affects their motility and movement.
• Z - Zone of polarity: The flagellum is often organised with a "zone of polarity" that helps to coordinate its movement. This zone determines the direction of movement and helps maintain the flagellum's orientation during movement.

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