Flagella are commonly found in bacteria, but can also be found in archaea and eukaryotic organisms as well. A flagellum is a lash-like structure that protrudes from the cell body. It is very akin to a whip, which is what its name translates to.
“For the first half of geological time our ancestors were bacteria. Most creatures still are bacteria, and each one of our trillions of cells is a colony of bacteria.” — Richard Dawkins
The main function of a flagellum is to serve a means of locomotion and assist the cell in finding its way. Among the three groups that flagella are found in, there is a wide variation in the structure of the flagellum. However, a flagellum is defined by its function rather than its structure.
Beyond being a means of movement, a flagellum can also be a sensory organelle. In some organisms, the flagellum is used to detect temperature and/or chemical changes outside of the cell.
In bacteria, the flagellum is composed of proteins called flagellin. These proteins form a hollow tube and create a helical tail structure with a sharp bend at the base of the exterior cell wall. This bend is followed by the flagellar filament, which makes up most of the whip-like structure, that serves to propel the bacteria.
The exterior part of the flagellum is connected to a rotary motor system via a shaft. When activated, this motor generates the movement that is seen in the flagellar filament and guides the bacteria to where it needs to go. It is powered by the flow of protons across the cell membrane that follows a concentration gradient, moving from one concentration to another.
The flagellum is capable of reaching 200 to 1000 rpm and also capable of quickly changing the rotation direction. There is no off-switch on the motor, so it is controlled via a protein.
“What’s great about bacteria is you have a surprise every day waiting for you because they’re so fast, they grow overnight.” — Bonnie Bassler
When the flagellum is active and moving, the movement is done via a biased random walk composed of “running” motions and “tumbling” motions as the rotation on the flagellum changes. This is biased because the bacteria, like E. coli, is still moving towards a target or away from something with an intention, but it is done in a random-like way.
Bacteria do not only have a single flagellum. There are some that have 2, 3, or even 30.
Based on the types of protein within the flagellum, there is evidence to suggests that the bacterial flagellum evolved from a secretory and transport system. Known as a Type III system, it is an appendage that moves toxins from the cell to the exterior. So, the going theory is that the flagellum evolved from this Type III system, but instead of exporting toxins, it exported flagellin to form itself.
Archaeal flagella are structurally similar to bacterial flagella on a superficial level. The differences between the structure have allowed archaeal flagella to become known as archaellum because they were distinct enough from other flagella types. Despite this, it can still be considered a flagellum because its function is still locomotion.
It still serves the same function, being a means of locomotion, and can be considered a flagellum on that basis alone. However, there are many differences that allow the archaellum to be its own distinct appendage.
The archaellum is made of different proteins compared to the bacterial flagellum. These proteins form a structure that is thought to have evolved from another appendage called Type IV pilus. These are thinner filaments, sometimes used to connect bacteria to bacteria.
Archaellum is powered by ATP, a common component in power generation across many organisms including humans. While bacterial flagella can be numerous and act independently, the filaments of an archaellum form a bundle that rotates together. These filaments are thinner than the bacterial counterparts.
The formation of the archaellum is also different from bacterial flagellum. In bacterial flagellum, filament subunits are added to the tip of the growing flagellum. In archaea, components are added to the base of the flagellum instead of the tip. This difference is due to the thickness of the filaments as the bacterial filaments are hollow, allowing subunits to move through them to the tip. Whereas, archaellum is too thin to be hollow.
Studies on archaellum are still new and more information will continue to gather to give us an improving look at what it means to be an archaellum. It was only recently that archaellum was adopted as the name used to refer to these flagella.
In Eukaryotes, like sperm cells, the flagella is closely similar to the cilia, which is a hair-like strand responsible for sensory functions. They are similar structurally. A eukaryotic flagellum is composed of a bundle of 9 fused pairs of microtubules that surrounds 2 single microtubules. This is considered a “9+2” structure.
The eukaryotic flagellum is composed of the cell membrane, which covers the axoneme (the “9+2” structure). This tail-like structure leads into the cell connected to a basal body.
“The fossil showing two swarm cells in fused position and shedding of their flagella is evidence that the two cells had sex.” — Ranjit K. Kar
Force is generated through the use of ATP and occurs along the axoneme. Proteins, receptors, and other components are constantly moving throughout the flagellum to ensure it remains functioning and stable.
There are two competing views on how the eukaryotic flagella evolved. The first is that it formed as remnants from pre-existing components of the cell as it developed. The second is that it formed through a symbiotic relationship between a eukaryotic cell and either a primitive eukaryote or archaea.
Eukaryotic flagella are similar to cilia because they both have similar structure and functions involved in motility. However, the flagellum exhibits propellor like motions while the cilia exhibit a whip-like motion. There are variations on these movements as these are generalizations.
There is a wide diversity of flagella structure throughout the eukaryotic world. Some can be hair, non-hair, have spines or scales, and have multiple of them. This wide variation is useful as it ensures that a particular organism can move in its particular environment.