Cilia, Flagella, Centrosomes and Centrioles

Cilia and flagella move cells or surrounding fluids

Many eukaryotic cells have slender membrane-covered projections that generate movement. Cilia are commonly shorter and more numerous, while flagella are usually longer and fewer, although these are general patterns rather than universal definitions. Ciliary beating may propel a single-celled organism or move fluid over a stationary tissue; for example, motile cilia in respiratory passages help move mucus. A eukaryotic flagellum can propel a sperm cell by bending. The force comes from interactions within an organised internal scaffold.

A bacterial flagellum is different despite sharing the name. Many bacterial flagella are filaments rotated by a membrane motor, often built from flagellin, whereas eukaryotic cilia and flagella are membrane-covered projections with an internal microtubule array whose bending is generated by motor proteins. A single picture of a long hair-like appendage is therefore insufficient to determine whether its structural mechanism is prokaryotic or eukaryotic.

A typical motile eukaryotic axoneme follows a 9+2 plan

The internal core of a typical motile cilium or flagellum is called the axoneme. In cross-section it has nine peripheral pairs of microtubules arranged as doublets around a central pair, called the 9+2 organisation. The peripheral doublets are linked and interact through motor proteins, while radial spokes connect their vicinity towards the central apparatus. This architecture maintains ordered bending rather than allowing all microtubules to slide freely past one another.

Dynein motor proteins use ATP to generate relative sliding forces between adjacent doublets. Because the axoneme contains connections that constrain free sliding, those forces produce bending. Repeating this action in a coordinated pattern creates a beat or propagating wave. The description 'cilia work like oars' is a useful visual analogy, but the molecular basis is regulated microtubule bending rather than the movement of a single rigid rod.

Basal bodies anchor motile projections

Cilia and flagella extend from basal bodies located near the cell surface. Basal bodies are closely related in architecture to centrioles and help organise the microtubules that extend into the projection. The basal-body core typically contains nine microtubule triplets rather than the axoneme's nine doublets plus central pair. The transition from the basal region into the shaft involves changes in microtubule arrangement, so one cross-section should not be used to represent both structures indiscriminately.

Centrosomes and paired centrioles

In many animal cells, a centrosome lies near the nucleus and contains a pair of centrioles within surrounding pericentriolar material. The two centrioles are often approximately perpendicular. Each centriole is a cylindrical structure with nine peripheral groups of microtubule triplets arranged around a central hub and associated radial connections, producing a cartwheel-like appearance in an appropriate cross-section. Unlike a typical motile axoneme, a centriole does not have the same central pair of microtubules.

Centrosomes help organise cellular microtubules and establish spindle poles during division in many animal cells. The spindle itself is assembled from microtubules and associated proteins; a centriole is not a complete spindle fibre. In addition, centrioles can become basal bodies for motile cilia or flagella. Many land plant cells organise microtubules without centrioles, showing that the basic processes of division and microtubule organisation do not require an animal-style centrosome in all eukaryotes.

Observe the correct section before interpreting a diagram

A transverse micrograph through a ciliary shaft should show nine peripheral doublets and, in a typical motile example, a central pair. A section through a centriole instead should reveal nine peripheral triplets and a cartwheel arrangement. If you see nine dots around the edge but the image resolution does not separate doublets from triplets, you cannot confidently classify the structure from that feature alone; examine labels or neighbouring longitudinal information.

Although the 9+2 arrangement is the characteristic example here, not all eukaryotic cilia have a central pair. Primary sensory cilia are frequently organised differently, often described as 9+0, and many are not motile in the same way. This variation reinforces the principle that textbook structural models are representative rather than universal.

Quick Quiz

Model Answers

1. Show nine peripheral microtubule doublets, two central microtubules, radial spokes, connecting proteins and a surrounding plasma membrane.

2. Centriole: nine triplet microtubule groups in a cartwheel without central pair. Axoneme: nine peripheral doublets plus a central pair inside a membrane-covered motile shaft.

3. Dynein uses ATP to generate sliding between doublets; connecting elements constrain this sliding so the shaft bends, producing coordinated strokes or waves.

4. They act as microtubule-organising centres, helping establish spindle poles and organise spindle microtubules involved in chromosome segregation.

5. Some sensory or specialised cilia possess different arrangements, such as 9+0, with distinct motility or signalling functions.