Cilia and Flagella Definition
- Complex filamentous cytoplasmic structures like cilia and flagella are projecting through the cell wall.
- They are little, especially the differentiated cell appendices.
- Long, hair-like projections called flagella (plural: flagellum) protrude from the plasma membrane and are used to transport whole cells.
- Short, hair-like structures called cilia (plural: cilium) are used to transport materials or complete cells, like paramecia, along the surface of cells (such as the cilia of cells lining the Fallopian tubes, which transport the ovum into the uterus, or the cilia of cells lining the respiratory system, which capture particulate matter and transport it toward the nose).
- Arbitrarily used terminology include cilium, which means an eyelash, and flagellum, which means a whip.
- Generally speaking, cilia are shorter than flagella (10 vs. >40 m).
- The number of cilia on the cell surface is significantly greater (ciliated cells typically contain hundreds of cilia, whereas flagellated cells possess a single flagellum).
- The true distinction, though, may be seen in how they move. The cilia act as oars. The action is biphasic, with the effective stroke causing the cilium to remain stiff and bend just at the base, and the recovery stroke causing the bend to extend to the tip.
- The flagella move like eels. They produce waves that propagate down their length, often at constant amplitude from base to tip.
- A cilium transports water perpendicular to its axis and, as a result, perpendicular to the cell surface, whereas a flagellum moves water parallel to its axis.
Structure of cilia and flagella
Although they beat in distinct ways, cilia and flagella are fundamentally identical.
All cilia and flagella are constructed using the same basic framework:
- The axoneme is a bundle of microtubules that is surrounded by a membrane that is a component of the plasma membrane and is 1 to 2 nm in length and 0.2 m in diameter.
- The basal body, an intracellular granule that starts from the centrioles and is located in the cell cortex, is related to the axoneme.
- There are nine outside pairs of microtubules, known as doublets, and two centre singlet microtubules, each with 13 protofilaments, embedded in the ciliary matrix that fills each axoneme. The 9 + 2 array is the name given to this recurrent theme.
- One full microtubule, known as the A sub-fibre, with all 13 protofilaments, may be found in each doublet. Each A sub-fibre is connected to a B sub-fibre that has 10 protofilaments.
- Two clockwise-oriented dynein arms are present in subfibre A.The nexin connections that connect doublets
- An ATPase called dynein transfers the energy generated during ATP hydrolysis into the mechanical labour required to beat cilia and flagella.
- Additionally, radial spokes that terminate in fork-like structures known as spoke knobs or heads connect each subfibre A to the central microtubules.
Centrioles also exhibit this nine-way pattern of microtubules and related proteins in a regular configuration. However, unlike centrioles, cilia and flagella feature a centre pair of microtubules, leading to the term “9 + 2 axoneme” for the total structure.
Prokaryotic and eukaryotic flagella differ in composition. Eukaryotic flagella have far more proteins than motile cilia and share some of their motion and control mechanisms.
Working of Cilia and Flagella
The dynein arms press on the neighbouring outer doublets, causing a sliding movement to happen between adjacent outer doublets, using ATP generated by mitochondria near the base of the cilium or flagellum as fuel. Sliding is changed into bending because the arms are activated in a certain order both inside and outside the axoneme and because the radial spokes and inter-doublet linkages limit how much sliding is possible.
The mechanism used by bacterial flagella is substantially different. The bacterial flagellum is totally propelled by the rotary motor at its base, much like a boat’s propeller. One protein (flagellin), which bears no resemblance to tubulin or dynein, makes up the bacterial flagellum, a specialised component of the extracellular cell wall. Cytosol fills cilia and flagella all the way to their tips, and the ATP in that cytosol is used to produce force along the length of both structures.
Functions of Cilia
- In solitary cells, such as certain protozoans, cilia are utilised as a kind of propulsion (e.g., Paramecium).
- Motile cilia remove materials such as dirt, dust, microorganisms, and mucus using their regular undulation to ward off illness.
- Cilia play a part in both animal development, such as the formation of the heart, and the cell cycle.
- Selected proteins can operate normally thanks to cilia.
- Additionally, cilia are involved in molecular transport and cellular communication.
- Cells use non-motile cilia as their sensory organs to pick up messages. They are essential for sensory neurons. The kidneys use non-motile cilia to detect urine flow, and the photoreceptors in the retina of the eyes also have non-motile cilia.
- They also offer symbiotic animal microbiomes homes or locations for recruitment.
- Additionally, it has been found that cilia have an involvement in the vesicular secretion of ectosomes.
Functions of Flagella
- Flagella are often employed by cells to move around, including the spermatozoon and the euglena (protozoan).
- Flagella actively contributes to eukaryotic reproduction and cell nutrition.
- Flagella operate as propulsion motors in prokaryotes like bacteria and are the main means by which bacteria move through liquids.
- Additionally, it gives harmful germs a means of helping to colonise hosts and spread illness.
- Additionally, flagella serve as scaffolding or bridges for attachment to host tissue.
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- Stephen R. Bolsover, Elizabeth A. Shephard, Hugh A. White, Jeremy S. Hyams (2011). Cell Biology: A short Course (3 ed.).Hoboken,NJ: John Wiley and Sons.
- Alberts, B. (2004). Essential cell biology. New York, NY: Garland Science Pub.