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Cilia vs. Flagella

OK, let's start by getting a few thing straight. Cilia (pronounced "silly-ah", from the Latin for "eyelash") (and not spelt "cillia") is the plural of cilium. A cilium is a microscopic hairlike process extending from the surface of a cell. Capable of rhythmic motion, it acts in unison with other cilia to cause the movement of the cell or the surrounding medium.
Flagella (pronounced "fla-gel-ah") (not spelt "flagellae") is the plural of flagellum - long, threadlike appendages on certain cells which function as organs of locomotion.

So what's the difference?

Cilia beat in organized, rythmic waves - rigid in the power stroke, flexible in recovery. Under the microscope, cilia are usually revealed by a flickering appearance. There are usually fewer flagella on the surface of a cell and they show a longer beat pattern with a variety of waveforms. Under a good microscope, it is usually possible to pick out individual flagella.

Cilia and and flagella are covered by membrane continuous with cell membrane. They consist of an array of microtubules running longitudinally through entire organelle, with a characteristic arrangement of nine peripheral doublets of microtubules and two central single microtubules known as the axoneme. There is a high degree of evolutionary conservation among all eukaryotes, from the simplest single-celled organisms to humans.

However, in prokaryotes, flagella are filamentous protein structures composed of flagellin, attached to the cell surface. Prokaryotic flagella are much thinner than eukaryotic flagella, and they lack the typical 9 + 2 arrangement of microtubules. The diameter of a procaryotic flagellum is about 20 nanometers, below the resolving power of the light microscope. Learn more about bacterial motility.


Ciliated Epithelial Cells:

Many epithelial surfaces in the body are ciliated - covered in cells which contain tiny hair-like structures beating in synchrony to move secretions or objects around. Examples include ciliated epithelia in the vesciles of the brain which circulate the cerebrospinal fluid, and in the oviduct which move the ova from the ovary to the uterus. In the respiratory tract, ciliated epithelial cells sweep clean dust and germs trapped in mucus secreted by "goblet cells" in the epithelium.

How important are these cells? Well, they are the primary target of infection for "common cold viruses" such as coronaviruses, influenza virus and rhinoviruses. These viruses may kill the ciliated cells or simply stop the cilia beating. In either case, mucus builds up and forms a good site for secondary bacterial infections, resulting in the gooey green nasties we are all familar with when we have a cold. This show how important healthy cilia are!


In some bacteria, there is only a single flagellum - such cells are called monotrichous. In these circumstances, the flagellum is usually located at one end of the cell (polar). Some bacteria have a single flagellum at both ends - amphitrichous. However, many bacteria have numerous flagella; if these are located as a tuft at one end of the cell, this is described as lophotrichous (e.g. Chromatium), if they are distributed all over the cell, as peritrichous. The following digital video shows motile Chromatium cells. Watch for the tumbles as the cells change direction:


Naegleria gruberi is an amoeba-flagellate, so called because the organism can exist as an amoeba and also as swimming flagellates. The differentiation of Naegleria provides a valuable system for examining a number of interesting problems in eukaryote cell and developmental biology. Some of the advantages of the system arise from the ease and speed with which cells can be grown and induced to differentiate. Another important advantage is that the differentiation of amoebae into flagellates is relatively synchronous. This means that following the population of amoebae as they differentiate into flagellates is similar to following the changes in a single cell:


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