Cilia and flagella are built from the same core components: protein tubes called microtubules, motor proteins that generate movement, and a specialized membrane that wraps around everything. Despite looking different on the outside (cilia are short and numerous, flagella are long and few), their internal architecture is nearly identical across species, from single-celled algae to human cells.
The Microtubule Skeleton
The structural backbone of every cilium and flagellum is the axoneme, a cylinder of microtubules arranged in a precise geometric pattern. In motile versions, nine pairs (doublets) of microtubules form a ring around two single microtubules in the center. This is called the “9+2” arrangement, and it’s one of the most conserved structures in biology.
Each microtubule is assembled from repeating units of two proteins, alpha-tubulin and beta-tubulin, that snap together like building blocks to form a hollow tube. Within each doublet, one complete tube (the A-tubule) is fused to a partial tube (the B-tubule). The two central singlet microtubules sit inside a structure called the central pair complex, which plays a key role in coordinating the beat.
Motor Proteins That Drive Movement
Tubulin provides the tracks. Dynein provides the power. Dynein motor proteins project outward from the A-tubule of each doublet like tiny arms, and they come in two rows: outer arms and inner arms. These arms grab onto the neighboring doublet and, using energy from ATP, attempt to slide it along. A single outer dynein arm contains two or three large “heavy chain” subunits, each around 500 kilodaltons, plus several smaller chains that help regulate activity.
If the doublets could slide freely, the axoneme would simply fall apart. What converts that sliding into a controlled bend are elastic links between the doublets called nexin linkers. These connectors resist the sliding force, so when dynein pushes on one side, the whole structure curves instead of telescoping apart. The interplay between dynein’s push and nexin’s resistance is what produces the wave-like beating you see in a working cilium.
The Regulatory Machinery
A cilium doesn’t just flap randomly. Its rhythmic beat depends on radial spokes and a structure called the nexin-dynein regulatory complex (N-DRC) working together as a feedback system. Radial spokes are protein rods that extend inward from each doublet toward the central pair, like the spokes of a wheel. As the axoneme bends, the distance between the spokes and the central pair changes, and that physical shift tilts the spokes. This tilt acts as a mechanical lever that repositions nearby dynein motors, switching them on or off depending on where they are in the beat cycle.
The N-DRC sits between doublets and connects to both the inner and outer dynein arms through a network of protein linkers. It converts dynein’s sliding action into coordinated bending and relays regulatory signals from the radial spokes outward to the outer arms. Without radial spokes, cilia are either paralyzed or beat erratically, even though their dynein motors are still functional. The whole system works like a mechanical circuit: the central pair and spokes set the rhythm, and the N-DRC distributes the signal.
The Membrane and Its Lipid Composition
Surrounding the axoneme is a specialized membrane that is continuous with the cell’s outer membrane but chemically distinct. The ciliary membrane is enriched in sterols and sphingolipids, including ceramide and specific gangliosides, creating ordered lipid domains sometimes called “lipid rafts.” It also carries unusually high levels of a signaling lipid called PI(4)P compared to the rest of the cell surface, while another related lipid, PI(4,5)P2, is largely excluded from the ciliary membrane and instead concentrates at the base, creating a sharp chemical boundary.
Between the membrane and the axoneme sits the ciliary matrix, a soluble protein environment that supports signaling and transport functions. At the base, Y-shaped protein fibers connect the microtubule doublets to the membrane, forming a physical gate called the transition zone. Together with transition fibers that anchor the structure to the cell, this gate controls what enters and exits the cilium.
The Basal Body Anchor
Every cilium and flagellum grows from a basal body embedded in the cell surface. The basal body is essentially a modified centriole, the same structure that organizes cell division. Its defining feature is nine sets of triplet microtubules (three linked tubes per set) arranged in a ring. This is different from the axoneme’s doublets: when the centriole forms a cilium, only the A- and B-tubules extend upward to become the axonemal doublets. The C-tubule stays behind.
This triplet-to-doublet transition is a structural hallmark found across nearly all organisms that have cilia. The basal body provides the template that establishes the nine-fold symmetry of the entire structure above it.
How the Parts Get Delivered
Cilia and flagella can’t make their own proteins. Every component has to be manufactured in the cell body and shipped to the growing tip, and this job falls to a transport system called intraflagellar transport (IFT). IFT works like a conveyor belt: motor proteins carry large protein particles along the outer doublets from the base to the tip (anterograde transport), then a different motor brings the empty carriers back down (retrograde transport).
Notably, many axonemal components don’t travel as individual proteins. Radial spokes, outer dynein arms, and inner dynein arms are preassembled into complexes inside the cell before being loaded onto the IFT machinery. This prefabrication step likely speeds up construction and ensures that complete functional units arrive at the tip ready to be incorporated. Flagella in the single-celled alga Chlamydomonas, a widely studied model organism, grow to roughly 10 micrometers in length through this process.
Non-Motile Cilia: A Simpler Blueprint
Not all cilia are built to move. Most cells in your body carry a single primary cilium that acts as a sensory antenna rather than a propeller. These non-motile cilia have a “9+0” arrangement: the nine outer doublets are present, but the central pair, radial spokes, and dynein arms are all absent. Without motor proteins, these cilia don’t beat. Instead, their stripped-down axoneme supports a membrane packed with receptors that detect chemical signals, mechanical forces, and light.
What Happens When Components Are Missing
Because the structure depends on so many interlocking parts, defects in any one component can cause disease. Primary ciliary dyskinesia (PCD) is a genetic condition in which mutations disrupt the assembly of dynein arms, radial spokes, or other axonemal proteins. The result is cilia that beat weakly or not at all. In the airways, this means mucus isn’t cleared properly, leading to chronic respiratory infections. In the reproductive system, sperm flagella may be immotile, causing infertility. Some people with PCD also have their internal organs arranged in mirror image (a condition called situs inversus), because the embryonic cilia that normally establish left-right body asymmetry failed to function during development.