Cilia and flagella are cell organelles that are structurally similar but are differentiated based on their function and/or length. Cilia are short and there are usually many (hundreds) cilia per cell. On the other hand, flagella are longer and there are fewer flagella per cell (usually one to eight). Though eukaryotic flagella and motile cilia are structurally identical, the beating pattern of the two organelles can be different. The motion of flagella is often undulating and wave-like, whereas the motile cilia often perform a more complicated 3D motion with a power and recovery stroke.
Video Explaining the Difference
This video explains the difference between cilia and flagella, as well as the function and structure of these cell organelles.
Differences in structure
Eukaryotic motile cilium and flagellum are structurally identical. Each is a bundle of nine fused pairs of microtubule doublets surrounding two central single microtubules. The movement of both cilia and flagella is caused by the interactions of these microtubules.
In non-motile or primary cilia the two central single microtubules are absent. So the central bundle consists of 9 + 0 microtubules. In prokaryotes cells the flagella are filamentous protein structures composed of flagellin. Prokaryotic flagella are much thinner than eukaryotic flagella, and they lack the typical 9 + 2 arrangement of microtubules.
Types of cilia and flagella
There two types of cilia - motile and non-motile or primary cilia.
- Non-motile or primary cilia are found in nearly every cell in all mammals and as the name suggests these do not beat. They can be found in human sensory organs such as the eye and the nose.
- Motile cilia are found on the surface of cells and they beat in a rhythmic manner. They can be found in the lining of the trachea (windpipe), where they sweep mucus and dirt out of the lungs. In female mammals, the beating of cilia in the fallopian tubes moves the ovum from the ovary to the uterus.
There are three types of flagella - bacterial, archaeal and eukaryotic.
- Bacterial flagella are helical filaments that rotate like screws. They are found in E. coli, Salmonella typhimurium. There may be one, two or many such flagella per cell. These flagella provide motility to bacteria.
- Archaeal flagella are similar to bacterial flagella but they have a unique structure which lacks a central channel.
- Eukaryotic flagella are complex cellular projections that lash back and forth. (e.g., the sperm cell, which uses its flagellum to propel itself through the female reproductive tract.
Lack of proper functioning of cilia and flagella can cause several problems in human beings. For example,
- If the cilia in the fallopian tubes are not functioning properly then the fertilized ovum will not reach the uterus and thus result in ectopic pregnancy.
- A defect of the primary cilium in the renal tube cells can lead to polycystic kidney disease (PKD).
- Flagellum dysfunction can also be responsible for male infertility because the sperm is not motile and cannot swim to the ovum.