Flagella are microscopic, whip-like or hair-like appendages that extend from the surface of various cells. These structures are primarily responsible for enabling movement, or locomotion, for many microscopic organisms, allowing them to navigate their environments.
General Characteristics
Flagella typically appear as long, slender, and often undulating structures when viewed under a microscope. Their shape, resembling a whip or a hair, directly relates to their primary function of propulsion. These appendages are quite thin, generally ranging from 12 to 30 nanometers in diameter. The length of flagella can vary, often measuring a few micrometers to tens of micrometers long; for instance, bacterial flagella commonly span 5 to 10 micrometers in length. This elongated and thin morphology is well-suited for generating thrust and enabling movement through liquid surroundings.
The physical form of a flagellum allows it to act like a propeller or an oar, pushing or pulling the cell through its medium. While individual flagella are too small to be seen without specialized staining under a light microscope, their collective action and characteristic movements can be observed.
Internal Architecture
Despite their relatively simple external appearance, flagella possess a complex internal structure that facilitates their precise movements. The main visible part extending from the cell is the filament, which is the long, whip-like portion. In bacteria, this filament is composed of protein subunits called flagellin, which arrange into a rigid, hollow, helical structure. In eukaryotic cells, the filament’s core, known as the axoneme, is made of the protein tubulin.
Connecting the filament to the cell body is a component that varies between life forms. In bacteria, a flexible hook acts as a universal joint, transmitting the rotational force from the motor to the helical filament. This hook is composed of specific hook proteins. Eukaryotic flagella, in contrast, feature an axoneme with a characteristic “9+2” arrangement of microtubules, meaning nine pairs of microtubules surround two central single microtubules. This arrangement allows for a wave-like bending motion, distinct from bacterial rotation.
The entire flagellar structure is anchored to the cell by a basal body, which is embedded within the cell membrane and cell wall. This basal body serves as the motor that powers the flagellum’s movement. In bacteria, the basal body acts as a rotary motor, causing the filament to spin. In eukaryotic cells, the basal body is structurally similar to a centriole and orchestrates the bending of the axoneme.
Diverse Forms in Nature
The appearance and underlying structure of flagella exhibit significant variations across the three domains of life, reflecting diverse evolutionary paths and adaptations. Bacterial flagella are characterized by their rigid, helical filaments driven by a rotary motor at their base. This rotary action propels the bacterium with a corkscrew-like motion. The arrangement of these flagella on a bacterial cell can also alter its overall appearance and swimming pattern.
Bacteria can display different flagellar arrangements:
Monotrichous bacteria have a single flagellum at one pole.
Lophotrichous bacteria possess a tuft of flagella at one or both ends.
Amphitrichous bacteria feature a single flagellum at each end.
Peritrichous bacteria have flagella distributed over their entire cell surface.
These arrangements influence a bacterium’s swimming behavior and ability to navigate various environments.
Eukaryotic flagella, found in organisms like sperm cells and some protists such as Euglena, exhibit a more complex internal structure. Their “9+2” axoneme allows for a wave-like or undulating motion, which generates propulsion. This distinct beating pattern contrasts with the rotary motion of bacterial flagella.
Archaeal flagella, often referred to as archaella, share a superficial resemblance to bacterial flagella, including a helical filament and a rotary motor. However, their evolutionary origin and protein composition are distinct; archaella are composed of proteins called archaellins, which are not related to bacterial flagellin. These flagella are typically thinner than bacterial flagella, measuring 10-15 nanometers in diameter. The way archaella are assembled also differs, with subunits added at the base of the filament rather than the tip.