Flagella and Cilia: Definition, Structure & Types

Have you seen some microorganisms like Euglena and Paramoecium having hair-like or thread-like appendages that help the movement? The Flagella and Cilia are microscopic, contractile and filamentous processes of the cytoplasm capable of producing a current in the fluid medium for locomotion and passage of substances. Also, they act as sensory organs and perform many mechanical functions of the cell.

Cilia and flagella are structurally identical cell organelles that differ in length and function. Flagella can be found in bacteria and sperm cells, while cilia can be seen in species like Paramecium. Cilia are more numerous and shorter than flagella. This article will learn more about Flagella and Cilia differences with examples.

What are Cytoskeletal Elements?

The cytoskeleton is a structure that helps the cells maintain their shape and internal organization. It also provides mechanical support that enables cells to carry out essential functions like division and movement.

These consist of the following types:
(i) Microtubules – These are unbranched, hollow tubules made up of tubulin protein. They contain \(13\) protofilaments and are \(25\,nm\) in diameter. They occur in centrioles, cilia/flagella, basal bodies, astral rays, spindle fibres, etc. They are non-contractile in nature.
(ii) Microfilaments – They are long, narrow, cylindrical rods made up of actin and myosin protein. They are contractile, solid structures with a diameter of about \(7\, nm.\)

Define Flagella and Cilia

Flagella

Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used in the movement of an entire cell.

Cilla

Cilia (singular = cilium) are short, hair-like structures that are used to move entire cells or substances along the outer surface of the cell.

Fig: Cilia and Flagella

Eukaryotic and prokaryotic flagella are different in chemical composition and structure.

Structure of Flagella and Cilia

Both Flagella and Cilia are structurally similar and possess similar parts, i.e., basal body, rootlets, basal plate and shaft.

Basal Body

i. It is also known as Kinetosome or basal granule, or blepharoplasty. The basal body occurs embedded in the outer part of the cytoplasm below the plasma membrane.
ii. It is like a micro cylinder that has a structure similar to a centriole with nine triplet fibrils present on the periphery without a central fibril, even though a hub of protein is present here.

Rootlets

i. They are striated fibrillar outgrowths that develop from the outer lower part of the basal body.
ii. It helps in providing support to the basal body.

Basal Plate

It is an area of high density which lies just above the basal body at the level of the plasma membrane.

Shaft

i. The shaft is the hair-like projecting structure of the flagellum or cilium. The length is \(5\) to \(20\,\mu m\) in the case of the cilium and \(100\) to \(200\,\mu m\) in the case of the flagellum.
ii. Internally, it contains a semifluid matrix having an axoneme of \(9\) peripheral doublet fibrils and 2 central singlet fibrils. Hence, this arrangement is called \(9 + 2\) or \(11 – \)stranded.
iii. Each peripheral fibril consists of two microtubules. The sub-fibre A is slightly narrower, and it bears two bent arms, the outer one having a hook. They are about \(15\, nm\) long and made up of protein dynein with ATPase activity.
iv. The peripheral doublet fibrils are interconnected by \(A-B\) linkers of a protein called nexin between the \(B\)-sub fibre of one and the inner sidearm of the \(A\)-sub fibre of the adjacent fibril.
v. Each of their \(A\) sub-fibres sends a radial proteinaceous column to the centre. It is called radial spoke; the spokes are broader internally to form heads or knobs.

Fig: Structure of Flagella and Cilia

Types of Cilia

Cilia is of two types and are as follows:

1. Motile Cilia

Motile cilia are found on the cell surface in larger numbers. These are also found in the respiratory epithelium of the human respiratory tract and help by clearing the mucus or the dust particles out of the lungs.

2. Non-motile Cilia

Non-motile Cilia were first discovered in the year \(1898.\) These are long in structure and were believed to be a vestigial organelle. Recent studies and researchers presented the biological roles of primary cilia in that they function as a sensory cellular antenna that coordinates a large number of cellular signalling pathways.

Structure of Prokaryotic Flagella

The flagella structure and chemical composition are different in eukaryotes and prokaryotes. The bacterial flagella have the following features:

1. The bacterial flagellum is made up of the flagellin protein.
2. Each flagellum has three parts, hook, shaft, and the basal body.
3. This has a helical structure and very sharp bending outside the outermost membrane. This is called the hook.
4. The hook is made up of different types of proteins.
5. A long shaft runs between the hook and the basal body.
6. The basal body has protein rings (called \(L,P,S\) and \(M\)) in the wall and cell membrane. These act as bearings.
7. Gram-positive organisms have two rings, and Gram-negative bacteria have all four rings. The \(L\) ring is associated with lipopolysaccharides, \(P\) with a peptidoglycan layer, \(S\) attached to the plasma membrane and \(M\) embedded in the plasma membrane.
8. The filament of the flagellum is the longest and prominent part which is made up of flagellin protein. The shape is like a \(20-\)nanometer-thick hollow tube. This could be \(1\) to \(70\,nm\) long. It consists of eight vertical rows of flagellin.

Fig: Structure of Prokaryotic Flagellum

Types of Flagella Arrangement in Bacteria

Based on the number and arrangement of flagella, there are the following types of bacteria:

1. Monotrichous – Vibrio cholerae has a single flagellum only at one end of the cell.
2. Amphitrichous – Nitrosomonas has a single flagellum at each pole.
3. Lophotrichous – Spirillum volutans has a cluster of flagella at one or both ends.
4. Peritrichous – Flagella are spread fairly evenly over the whole surface. Like E. coli.
5. Cephalotrichous – Pseudomonas fluorescens has a cluster of flagella only at one end.
6. Atrichous –Lactobacillus, Pasteurella have no flagella.

Fig: Arrangements of Flagella

Difference Between Eukaryotic and Bacterial Flagella

What is the Main Function of Flagella?

Some of the functions of flagella and cilia are as follows:
i. These help in locomotion in flagellated and ciliated organisms.
ii. The flagella or cilia also help capture food in many protozoans and some animals.
iii. They create water currents in certain aquatic animals for obtaining food.
iv. The flagella circulate food in the gastrovascular cavity of coelenterates.
v. The cilium of the respiratory tract helps to remove dust particles from it.
vi. The cilia in the alimentary canal of tunicates and lanceolate help in food movements and egestion.
vii. Motile cilia use their rhythmic undulation to sweep away substances, as in clearing dust, dirt, microorganisms and mucus, to prevent any diseases.
viii. Cilia plays an important role in the cell cycle and animal development, such as in the heart.
ix. Non-motile cilia serve as sensory apparatus for cells by detecting signals, and also they play crucial roles in sensory neurons.
x. Non-motile cilia can also be found in the kidneys to sense urine flow and the eyes of the photoreceptors of the retina.
xi. Flagella have an active role in aiding cell feeding and eukaryotic reproduction.
xii. In prokaryotes such as bacteria, flagella serve as propulsion mechanisms; they are the chief way bacteria swim through fluids.
xiii. Flagella also act as bridges or scaffolds for adhesion to host tissue.

What is the Difference Between Cilia and Flagella?

The differences between cilia and flagella are:

Summary

Through this article, we understood the structure and functions of flagella and cilia and the difference between them. These are the microscopic contractile and filamentous structure of the cytoplasm. It creates food currents, acts as sensory organs and performs many mechanical functions of the cell. Cilia and Flagella are identical structures, but both can be distinguished by their number and function.

FAQs

Q.1. Write one function of cilia.
Ans: The cilia also help in capturing food in many protozoans and some animals. They also help in feeding, locomotion, aeration, circulation, etc.

Q.2. Do all bacteria have flagella?
Ans: Flagella occur on both Gram-positive and Gram-negative bacteria, and their presence can be useful in identification. For example, they are found on many species of bacilli but rarely on cocci. E. coli has flagella all over the body.

Q.3. What are flagella made of?
Ans: Flagella are composed of subunits of a low-molecular-weight protein called flagellin that is arranged in a helical manner.

Q.4. What are the types of flagella?
Ans: There are two main types of flagella in eukaryotes:
1. Whiplash flagellum is one that does not have hairy flimmers on the surface.
2. The tinsel flagellum is one that has lateral hair-like projections or flimmers, or mastigonemes on the surface.

Q.5. Where are flagella found in the human body?
Ans: In humans, the flagellum is only seen in gametes, i.e., in sperms that help to swim towards the ovum.

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