Cilia and flagella are both hair-like structures that extend from the surface of cells, and they share a nearly identical internal architecture. The core differences come down to size, number, and how they move: cilia are short and numerous, beating in coordinated waves, while flagella are long and few, propelling cells with whip-like undulations. But the full picture is more interesting than that summary suggests, especially once you factor in the version of flagella found on bacteria, which works in a completely different way.
Size, Number, and Appearance
Cilia typically measure between 1 and 10 micrometers long. A single cell can be covered in hundreds or even thousands of them, creating a surface that looks like a dense, microscopic carpet. Flagella are significantly longer, often reaching 12 micrometers or more, and cells usually have just one or two. Sperm cells, for instance, have a single long flagellum that forms the tail.
How Each One Moves
The movement patterns are fundamentally different. Cilia beat in a two-phase cycle: a stiff “power stroke” that pushes fluid in one direction, followed by a flexible “recovery stroke” where the cilium bends and sweeps back to its starting position with minimal drag. Hundreds of cilia on a cell surface coordinate their beats in waves, much like wind rolling across a wheat field. This coordinated rhythm is what moves mucus through your airways or an egg cell along the fallopian tube.
Flagella move differently. Instead of a back-and-forth rowing motion, a eukaryotic flagellum generates smooth, sinusoidal waves that travel from the base to the tip, producing a whip-like motion that pushes the whole cell forward. If the molecular motors inside the flagellum activate sequentially around its circular core, the result can be a three-dimensional, helical wave rather than a flat, side-to-side undulation. This is the kind of propulsion that drives a sperm cell toward an egg.
The Shared Internal Skeleton
Inside both cilia and flagella sits a structural core called the axoneme, roughly 300 nanometers in diameter. In motile versions of both structures, nine pairs of protein tubes (called microtubule doublets) are arranged in a ring around two single tubes in the center. This “9+2” pattern is remarkably conserved across species, from single-celled algae to humans.
Thousands of tiny motor proteins line the microtubule doublets. These motors burn ATP (the cell’s energy currency) to walk along neighboring doublets, generating a sliding force. Because the doublets are anchored at the base, that sliding force gets converted into bending. One ATP molecule is consumed per motor step, and the coordinated activity of all those motors produces the rhythmic beat you see in a living cilium or flagellum. Research on isolated flagella has confirmed that these motors account for the vast majority of energy use in the structure.
Primary Cilia: The Non-Motile Exception
Not all cilia move. Most cells in the human body carry a single, non-motile “primary cilium” that serves as a sensory antenna rather than a paddle. These primary cilia have a simpler “9+0” arrangement, missing the central pair of microtubules and the motor proteins needed for beating. Instead, they’re packed with signaling proteins that let the cell detect chemical signals, fluid flow, and even light.
Olfactory neurons in the nose, for example, have specialized cilia at their tips loaded with receptors that detect airborne chemicals. Photoreceptor cells in the eye use a modified cilium to organize the machinery of vision. Kidney tubule cells use primary cilia to sense urine flow. The concentration of signaling molecules inside these tiny structures partly explains the remarkable sensitivity of these sensory systems.
There are also rare exceptions that break the usual rules. Nodal cilia, found in developing embryos, are motile despite having the 9+0 pattern. Their spinning motion creates a leftward fluid flow that helps establish the left-right asymmetry of your organs.
Bacterial Flagella Are a Completely Different Structure
If you’re comparing cilia and flagella across all of biology, it’s worth knowing that bacterial flagella have almost nothing in common with the eukaryotic versions beyond the name. A bacterial flagellum is a rigid, hollow tube made of a protein called flagellin, not the tubulin-based microtubules found in animal and plant cells. It doesn’t bend at all. Instead, it spins like a propeller, driven by a rotary motor embedded in the cell membrane that runs on the flow of charged particles across the membrane rather than ATP.
This is a fundamentally different engineering solution to the same problem: how to move through liquid. Eukaryotic flagella bend and wave. Bacterial flagella rotate. The two structures evolved independently.
Where You’ll Find Them in the Human Body
Your respiratory tract is lined with motile cilia, roughly 200 per cell, that sweep mucus and trapped debris up and out of the lungs. Cilia also line the brain’s ventricles, where they circulate cerebrospinal fluid, and the fallopian tubes, where they help guide egg cells toward the uterus. Meanwhile, the only human cell that uses a true flagellum is the sperm cell, which relies on its single long tail to swim.
Primary cilia appear on nearly every other cell type in the body, including kidney cells, liver bile duct cells, and connective tissue cells called fibroblasts. Their sensory roles are still being mapped, but defects in primary cilia are now linked to a growing list of diseases involving the kidneys, eyes, and brain.
What Happens When They Don’t Work
A genetic condition called primary ciliary dyskinesia illustrates how much the body depends on both cilia and flagella. People with this disorder are born with structurally abnormal versions of these organelles, often missing the motor proteins needed for proper beating. The consequences show up early: newborns frequently have breathing difficulty because their lungs can’t clear fetal fluid. Throughout childhood and into adulthood, chronic nasal congestion, persistent cough, frequent respiratory infections, and recurrent ear infections (which can cause permanent hearing loss if untreated) are hallmarks of the condition. Repeated lung infections eventually damage the airways, leading to a condition called bronchiectasis.
About 40 percent of people with primary ciliary dyskinesia have their internal organs arranged in mirror image, with the heart on the right side instead of the left. This happens because the nodal cilia that normally establish left-right body patterning during embryonic development don’t function properly, so organ placement becomes random. Another 9 to 12 percent develop more complex organ positioning problems that can affect the heart, liver, intestines, or spleen.
Fertility is also affected. Men with the condition typically have reduced fertility because their sperm flagella can’t generate the vigorous swimming motion needed to reach an egg. Some women experience decreased fertility as well, since cilia in the fallopian tubes can’t effectively transport the egg cell.