Cilia And Flagella – Definition, Structure, Functions and FAQs

Cilia and Flagella are cell organelles that enable cells to move or protect themselves from their surroundings. The human eye is unable to see these. The prokaryotic flagella are structurally distinct from the eukaryotic flagella. Cilia and Flagella are extensions that help to move substances around the cells. Cilia are usually short and multiple in number whereas Flagella are long and very few in number.

What is Cilia?

Cilia are thin thread-like structures that are responsible for the locomotion of different types of cells in various organisms. E.g In Paramecium, the Cilia help it to move around in liquid environments. The beating of the Cilia helps in forward propulsion. Cilia are made up of the protein Tubulin, which is responsible for the formation of microtubules, the main functional unit of Cilia.

Characteristics of Cilia

The following points illustrate the key characteristics of Cilia:

• It is present on certain microorganisms like ciliates and also on the surface of membrane-bound organelles.
• The singular form of cilia is known as cilium.
• It is absent in prokaryotes.
• The shape of the cilia is slender like long threadlike projections and it extends from the surface of the cell.
• The number of cilia present in an organism can vary from one to many.

Structure and Functions of Cilia

Cilia are of two types- motile and nonmotile. The cilium class is determined by the structure of the cilium core known as the Axoneme. 9+2 axoneme means having a central pair of single microtubules surrounded by nine pairs of double microtubules. This type of arrangement is usually seen in motile cilia whereas in non-motile cilia the arrangement is 9+0 which means it doesn’t have a central pair of microtubules. Due to the absence of the central pair of microtubules, it also lacks the mobility of the inner and outer arms along with radial spokes. Primary cilia and sensory cilia are the usual names of non-motile cilia.

In vertebrates, the cilia are usually single, non-motile in nature and act as a cellular antenna. Many non-motile cilia are possessed by olfactory neurons. The respiratory epithelial cells contain a large number of motile cilia i.e., approx. around 200 cilia per cell. The function of cilia on the respiratory epithelial cells is mucociliary clearance and mechanosensory and chemosensory functions. Cerebrospinal fluid is moved through the ventricular system of the brain by the motile cilia present in ependymal cells. Eggs in females are transported from the ovary to the uterus by the cilia present in the Fallopian tubes present there.

What is Flagella?

Flagella, similar to Cilia is a cell organelle related to the motility of the cell. It has long and slender appendages that protrude from its surface. Flagella’s principal purpose is to promote movement. Flagella are necessary for swimming and navigating through watery environments in single-celled organisms. Flagella are important in the propulsion of sperm for fertilization in multicellular organisms like humans.

Characteristics of Flagella

The following are the key characteristics of Flagella:

• Its singular form is called a Flagellum.
• It is present in both prokaryotes and some eukaryotes.
• The number of Flagella is usually one or two in an organism.
• The Protozoan group Mastigophora has the characteristic feature of having flagella.
• It is also found in the gametes of mosses, fungi, slime molds, algae, and animals.

Structure and Functions of Flagella

Prokaryotic flagella and eukaryotic flagella differ in their structure and the pattern of movement. The characteristic pattern in eukaryotes is a whip-like manner. Flagella can be found in eukaryotes in varying numbers, ranging from one to many. The base of the flagella is joined to the cell via its basal body. Bacterial flagella are helical in shape and contain the protein Flagellin.

Flagella move in a manner akin to that of a propeller, that is, either clockwise or anticlockwise. Eukaryotes require ATP to move their flagella, whereas prokaryotes use the proton-motive force to do so. Protists that have flagella are known as Flagellates. In sponges and coelenterates, the water current that is necessary for respiration and circulation is generated by flagella. Locomotion in bacteria mostly occurs through flagella. Below is a diagram showing the structure of Flagella:

Differences Between Cilia And Flagella

Below is a table that lists the differences between Cilia and Flagella:

Characteristics	Cilia	Flagella
Structure	Multiple hair-like protrusions	Few long whip-like appendages
Location	Found in eukaryotic cells	Found in both prokaryotic and eukaryotic cells
Length	Short	Long
Thickness	0.3-0.5 nanometre	0.02-0.025 nanometres
Density	Higher as they are more in number	Lesser, as there are only a couple of them
Type of movement	Fast and rotational movement	Rotational movement (prokaryotes), Bending movement (Eukaryotes)
Coordination	Highly coordinated as they beat one after the other	None, as they beat independently of each other
Function	Locomotion: In single-celled creatures, it is used for motion and also by multicellular organisms for the movement of things along surfaces. Sensory: Some species use their sensory systems to detect clues from their surroundings.	It is used for movement in cells to perform actions like swimming.
Example	Paramecium	Salmonella, Sperm cells

FAQs on Cilia and Flagella

1. What are the Cilia and Flagella made up of?

Answer:

Cilia and Flagella are made up of a central core unit known as axoneme. Microtubules arranged in a 9+2 configuration and coupled by dynein, which uses ATP to produce a wave-like bending action, make up the cilia and flagella of eukaryotic cells.

2. Which Type of Organism has both Cilia and Flagella?

Answer: 

Eukaryotic organisms have both cilia and Flagella. But prokaryotic organisms lack Cilia.

3. Which of the following is Longer between the two- Cilia or Flagella?

Answer: 

Flagella are longer as they are few in number. On the other hand, cilia are short and are in great numbers.

4. Write a few Functions of Cilia.

Answer:

A few of the functions of Cilia are:

• Cilia help in the movement of cells within a system. It helps in the movement of single celled organisms through their environments.
• In respiratory system, cilia help in moving mucus and other dirt particles towards the mouth to be expelled.

5. List a few functions of Flagella.

Answer:

Few of the functions of Flagella are:

• It helps in locomotion and also in the transport of egg from the ovary to the uterus.
• Change in pH and temperature is detected by flagella.