Flagella are found across all three domains of life: bacteria, archaea, and eukaryotes (which includes protists, some algae, certain fungi, some plants, and even animal cells like sperm). Despite sharing a name, the flagella in each group are built from entirely different proteins and work through different mechanisms. Understanding which organisms have them, and how they differ, reveals just how universal the need for cellular movement really is.
Bacteria: The Most Familiar Flagellates
Bacteria are the organisms most closely associated with flagella. A bacterial flagellum is a relatively simple structure made of a single protein called flagellin. Tens of thousands of flagellin subunits stack together to form a long, hollow filament that can extend several times the length of the cell body. This filament connects to a flexible hook that acts as a universal joint, which in turn attaches to a rotary motor embedded in the cell membrane. The motor converts the flow of charged particles (usually protons) across the membrane into mechanical spinning, much like a tiny turbine. These motors can reach staggering speeds of up to 100,000 revolutions per minute.
Bacteria arrange their flagella in several distinct patterns. Monotrichous bacteria have a single flagellum. Amphitrichous bacteria carry one flagellum at each end of the cell. Lophotrichous bacteria grow a tuft of flagella clustered together. Peritrichous bacteria, like E. coli and Salmonella, have flagella distributed across the entire cell surface. In peritrichous species, the flagella bundle together during forward swimming (“runs”) and fly apart to cause random reorientation (“tumbles”), letting the bacterium navigate toward food or away from threats.
Archaea: A Distinct Spinning Filament
Archaea also have rotating filaments used for swimming, but these structures are so different from bacterial flagella that scientists gave them their own name: archaella. While a bacterial flagellum can involve dozens of different proteins, the simplest known archaellum, from the heat-loving organism Sulfolobus acidocaldarius, is composed of only seven proteins. Archaella belong to the same protein family as structures used in secretion systems and certain types of pili, meaning they share an evolutionary ancestry with molecular machinery that moves substances across membranes rather than with bacterial flagella.
The energy source is also different. Bacterial flagellar motors run on the flow of ions across the membrane. Archaella, by contrast, appear to be powered by ATP, the cell’s universal energy currency. A single enzyme in the system handles ATP breakdown, and researchers believe it drives both the assembly of the filament and its rotation. Like bacterial flagella, archaella spin to propel the cell forward, but they are built from the base up by adding new subunits at the bottom of the growing filament rather than at the tip.
Protists: The Most Diverse Flagellate Group
Among eukaryotes, protists represent the widest variety of flagellated organisms. These single-celled creatures use one or more whip-like flagella to swim, feed, or sense their environment. Unlike the simple protein filaments of bacteria, eukaryotic flagella are far more complex. They contain a core structure called the axoneme: a central pair of microtubules surrounded by nine doublet microtubules, forming the characteristic “9+2” arrangement. Molecular motors called dynein arms slide these microtubules against each other, producing a bending, wave-like motion rather than rotation. The whole structure is also enclosed within the cell’s outer membrane, making it technically an extension of the cell itself.
Free-living flagellated protists include euglenids, the familiar green single-celled organisms often studied in biology classes, and dinoflagellates, which are enormously abundant in ocean ecosystems. Parasitic flagellates include trypanosomes, which have a single flagellum running the length of their body and cause diseases like sleeping sickness, and Giardia lamblia, a multi-flagellated parasite that colonizes the small intestine and causes the diarrheal illness giardiasis. Some flagellated protists blur the line between plant and animal, using their flagella to swim toward light while also photosynthesizing.
Algae: Flagella in Photosynthetic Organisms
Many algae use flagella at some point in their life cycle. Green algae like Chlamydomonas are classic examples of single-celled photosynthetic organisms that swim using two flagella. Colonial green algae such as Volvox coordinate the beating of flagella across hundreds or thousands of cells to roll through the water. Many brown algae and other marine algae produce flagellated reproductive cells even if the mature organism is stationary and anchored to a surface.
Not all algae have flagella, though. Red algae completely lack them at every stage of their life cycle. This absence is one of the defining features that sets red algae apart from other photosynthetic protists.
Plants: Flagellated Sperm in Mosses and Ferns
Flowering plants and conifers do not produce flagellated cells, but earlier-diverging plant groups still rely on them for reproduction. Mosses, liverworts, and ferns all produce biflagellated sperm cells, tiny swimming cells equipped with two flagella that must travel through a film of water to reach a stationary egg. When a mature sperm-producing structure in a moss is submerged in water, it bursts open and releases a mass of these flagellated sperm. This water-dependent fertilization is one reason mosses and ferns are most successful in moist environments.
Cycads and ginkgo trees are the only seed plants that still produce flagellated sperm, a feature that reflects their ancient lineage. In all other seed plants, pollen tubes deliver non-swimming sperm directly to the egg, eliminating the need for water as a transport medium.
Fungi and Animals
Most fungi do not have flagella. The familiar mushrooms, molds, and yeasts all lack them entirely. The major exception is a group called chytrids, aquatic fungi that produce flagellated spores (zoospores) for dispersal through water. Chytrids are among the most ancient lineages of fungi, and their retention of flagella reflects a time before fungi became primarily land-based organisms.
In animals, the most well-known flagellated cell is the sperm cell. Sperm across a huge range of animal species, from sea urchins to humans, use a single flagellum (often called a tail) to swim toward an egg. The internal structure is the same 9+2 microtubule arrangement found in protist flagella, with dynein motors generating the characteristic whipping motion. Some animal cells also carry non-motile versions of flagella, called primary cilia, that serve as sensory antennae rather than propulsion devices.
Three Flagella, Three Origins
One of the most important things to understand about flagella is that the bacterial, archaeal, and eukaryotic versions evolved independently. They are not variations on a single ancestral structure. Bacterial flagella are extracellular protein filaments powered by ion flow. Archaella are built using a completely different assembly system and powered by ATP. Eukaryotic flagella are membrane-enclosed, microtubule-based structures that bend rather than rotate. The fact that all three domains of life independently arrived at whip-like appendages for swimming speaks to how fundamental motility is for survival, but the molecular machinery behind each one is entirely distinct.