Cilia and Flagella

Introduction

Cilia and flagella are microscopic, hair-like tiny structures that are present extracellularly. However, they originate intracellularly from the basal body. Both cilia and flagella are found in eukaryotic cells, though flagella are also found in prokaryotic cells. 

The cilia and flagella structure is quite similar, as they possess similar body parts, including a basal body, rootlets, basal plate and shaft. 

What is Cilia?

Cilia are thread-like motile structures commonly found in paramecium organisms.They are short, slender, and can be described as a hair-like organelle extending from the surface of a living cell. Cilia have a width ranging between 0.3 and 0.5 micrometres and a length between 5 and 10 micrometres.

Cilia aid in the overall growth and development of the body and the functioning of cells. In addition, cilia also help in the locomotive and sensory functions of cells as well as their replication. They are primarily active during cell cycle growth and progression. 

Multiple cilia move in a wave-like pattern in a rhythmic motion that helps keep the internal passage free from foreign particles and mucus. They also act as an antenna that allows receiving sensory information for the cells and executes the signals from the surrounding fluids.

There are two types of cilia: motile cilia and non-motile cilia.

Motile cilia or moving cilia are commonly found in the lungs, middle ear and respiratory tract. Their role is to keep the air passage clear of dust, dirt and other foreign particles. Motile cilia also aids in the movement of sperm and the ovum from the ovaries to the uterus.

Non-motile cilia or primary cilia are present in almost all cells of mammals. They do not beat and can be mostly found in human sensory organs like the nose and the eyes. The major function of the non-motile cilia is to receive signals from other cells by acting as a receiver for the cells.

What is Flagella?

Flagella are complex, hair-like filamentous cytoplasmic structures present on the surface of cells. They have an average length of 150 micrometres and a width ranging between 0.02 and 0.025 micrometres. Flagella are composed of protein-like flagellin that is fixed in the cell envelope. 

The main function of flagella is to help in the movement of organisms. They also act as sensory organs to detect changes in the pH and temperature levels. Certain eukaryotes also use the flagellum to increase their reproduction rates. 

There are three types of flagella: archaeal flagella, bacterial flagella and eukaryotic flagella.

Bacterial flagella are found in organisms such as Salmonella Typhi and E.coli. They boast of a helical filamentous structure that rotates like screws. They are known to provide mobility to bacteria and can be one, two or many per cell.

Archaeal flagella are similar in function and structure to bacterial flagella. However, unlike bacterial flagella, they do not have a central channel.

Eukaryotic flagella are complex projections that move from one place to another, returning to their initial location in a repeated process. 

Differences between Cilia and Flagella

We will now understand the cilia and flagella difference.

• Population: Cilia are present in large numbers while flagella are fewer in number

• Structure: Although the cilia and flagella structure is quite similar, cilia are shorter while flagella are longer

• Movement: Cilia display a radical motion while flagella exhibit a coiled motion

• Location: Cilia are often present on the entire body of the cell, while flagella are located only on one end of a cell
• Where they are found: Typically, cilia are found on the outer surface of larvae like Mollusca, Annelida and Nemertea. On the other hand, flagella are found in protozoans, choanocyte cells of Metazoa and plants like algae and gamete cells

Similarities between Cilia and Flagella

Let us now take a look at the similarities between cilia and flagella.

• Structure: The cilia and flagella structures are similar. Both are hair-like outgrowths of the cell membrane. The core of both cilia and flagella is known as the axoneme. The axoneme possesses numerous microtubules that run parallel to the long axis and are present in nine doublets in a radially arranged peripheral pattern

• Function: Both the cilia and flagella help in the movement of cells

• Composition: Cilia and flagella are composed of parts like the basal body, rootlets, basal plate and shaft. The cilia and flagella emerge from a centriole-like structure known as the basal body

Conclusion

In this unit, we have learned all about cilia and flagella. We have discussed their functioning, the cilia and flagella structure, and differences and similarities.