Flagella is a thin hair-like filamentous appendage that is one of the locomotory cellular organelles. Flagella is derived from the word “flagellum,” which means whip-like. It is present external to the cell’s surface but is developed intracellularly from the basal body of the cytoplasm.
Flagella is present in some prokaryotes (bacteria, archaea) and eukaryotes (protozoa, sperm cells). The main function of the flagella is locomotion. In the bacteria, flagella are arranged as monotrichous, lophotrichous, amphitrichous and peritrichous.
Types of flagella
Different types of flagella are present in bacteria, archaea, and eukaryotes.
Some bacteria are motile and can move from one place to another, while others are non-motile. Bacterial flagella are made up of the protein called flagellin. This protein wraps around in a helical fashion to form a rigid, hollow cylinder. The protein rings act as proton pumps that allow the movement of hydrogen ions across the cell membrane down their electrochemical gradient. This movement of ions rotates the ring, which rotates the flagellum. If the flagellum rotates counterclockwise, the cell will move in one direction. To change direction, it can rotate clockwise. Flagella are present external to the cell wall. It is attached to one of the ends of the cell while the next end is free. After staining it with the flagella stains, it can be observed in the light microscope.
The flagella present in the archaea is called the archaellum. Flagella are found in methanogens, halophiles, thermophiles, and hyperthermophiles. They possess the same rotatory movement as bacterial flagella. Flagella is present in some of the species of archaea and differs from the bacterial flagellum. Archaeal flagella are about half the diameter of the bacteria, and it measures about 10-13 nm in width.
- The diameter and speed of the archaeal are less than that of the bacterial flagella.
- The source of energy for the archaeal flagella is ATP, whereas proton motive force in the bacteria.
- The bacterial filament comprises a single type of protein, but the archeal filament comprises different types of protein.
Eukaryotic flagella are made up of a protein called tubulin. Microtubules are arranged in the 9+2 arrangements in the eukaryotic cell. Flagella is present in some algae and the sperm of animals. It’s different from prokaryotic flagella in the following ways.
Difference between eukaryotic flagella and prokaryotic flagella
|Eukaryotic flagella||Prokaryotic flagella|
|Eukaryotic flagella are made up of tubulin protein.||Prokaryotic flagella are made up of flagellin protein.|
|Microtubules are present, showing 9+2 arrangements.||Microtubules are absent.|
|Eukaryotic flagella are covered by the membranous sheath.||Prokaryotic flagella are not covered by the membranous sheath.|
|The size of the eukaryotic flagella is large and thick.||The size of the prokaryotic flagella is small and narrow.|
|Eukaryotic flagella have undulatory movement, which means it moves back and forth.||Prokaryotic flagella have rotatory movement, which means it moves in a clockwise and anticlockwise direction.|
Structure of bacterial flagella
Flagella consists of three major parts: basal body, hook, and filament.
Filament: Filament is a long, hollow, cylindrical structure made of the flagellin protein. The length of the filament in the bacteria is 20 nm and in archaea is 10-14 nm. The motor controls its movement.
Hook: Hook is the short curved structure connecting the basal body with the filament. It is wider than the filament.
Basal body: Basal body is attached to the hook of the flagellum. It is also called the motor device and is present within the cell membrane. It is a rod-shaped structure and consists of different rings. The different rings present in the Gram-negative bacteria are
- L ring: L ring is anchored in the lipopolysaccharide layer.
- P ring: P ring is present in the periplasmic space. It is located in the peptidoglycan layer of the cell wall.
- MS ring: MS ring is found in the cytoplasmic membrane. One is a smaller stator ring (S-ring), and another is a motor ring (M-ring). These two rings are attached and form the MS ring.
- C ring: C -ring anchors all the parts to the cell.
In Gram-positive bacteria, flagella contain only the two basal body rings. One ring is embedded in the peptidoglycan layer, and the next is in the cell membrane.
Arrangement of the bacterial flagella
Atrichous flagella: Flagella is absent. Example: Lactobacillus
Monotrichous flagella: Single flagella are present at one end of the cell. Example: Vibrio cholerae
Lophotrichous flagella: Tufts of flagella are present at one of the poles of flagella. Example: Spirillum
Amphitrichous flagella: Either single flagella or tufts of flagella is present on both ends of the cell. Examples: Alcaligenes faecalis, Rhodospirillum rubrum, Magnetospirillum.
Peritrichous flagella: Flagella are present over the entire surface of the cell. Example: Salmonella Typhi, Escherichia coli
Structure of Eukaryotic Flagella
In eukaryotes, the flagellum is composed of protein filaments called microtubules. One of the centrioles of the cell develops into the basal body, which then forms the flagellum. In the periphery, nine, and in the center, two fibrils show the 9+2 arrangement. This peripheral fibril is called doublet due to two sub-fibrils in each of them. Out of the two sub-fibrils, one gives out two projections in a clockwise direction. Unlike the peripheral fibrils, each central fibril is made of a single tubule. In the distal end of the flagellum, additional fibrils are present between peripheral and central fibrils, known as secondary fibrils. Secondary fibrils are smaller in diameter than peripheral and central fibrils. There is the common sheath that encloses the two central fibrils. The dynein motor proteins utilize ATP to force the doublet microtubules to slide against one another, creating a bending motion and propelling the cell forward.
Functions of flagella
- Flagella helps in the movement of organisms from one place to another.
- Flagella plays an important role in the pathogenesis of some bacteria. Flagella of Escherichia coli and Proteus spp. helps in the movement of bacteria from the urethra into the bladder, which causes urinary tract infections.
- Flagella also helps in the identification of the organisms. Example:
- Salmonella can be identified by using antibodies against the flagellar proteins.
- Vibrio cholerae can be identified by its darting motility.
- Flagella also act as the sensory organ. It detects the different variations in the temperature in the environment and the change in the chemical composition.
- The flagella of the sperm helps to move and find the oocyte for fertilization.
- In Euglena, flagella help to find out the direction in which the cell moves.
- Madigan, M. T., Martinko, J. M., Stahl, D. A., & Clark, D. P. (2011). BROCK Biology of Microorganisms (13th edition). Benjamin Cumming.
- Pelczar Jr., M., Chan, E., & Krieg, N. (2007). Microbiology (5th edition). Tata McGraw-Hill.