Cilia and Flagella

Structurally cilia and flagella are similar but have different functions and lengths. Paramecium and bacteria have cilia, while sperm cells and bacterial flagella have flagella. Cilia are many and smaller than flagella.

Cilia and flagella are two of the most common organelles used for locomotion in unicellular organisms. With cilia, organisms can move more quickly and more effectively. Recently, we discovered that white blood cells move similarly. When we have an injury,  cilia aids WBCs in passing through the blood vessels. The flagella of most prokaryotic and eukaryotic microorganisms are similar in structure but differ in function.

Cilia and flagella cause the cells’ movement. Many microscopic unicellular and multicellular organisms rely on them for activity. For example, bacteria and protists use these structures to move toward or away from a stimulus (toxin). Higher organisms use cilia to direct substances. Some cilia, however, function in sensing rather than movement. Some organs and vessels have primary cilia that may feel environmental changes. Inside blood vessels, cell linings show this characteristic. Primary cilia in endothelial cells monitor blood flow.

Cilia Overview

Cilia, meaning hair-like appendages on the surface of eukaryotic cells, enable animals and protozoans to move around.

Types of cilia

Primary cilia

• Except for blood cells, non-motile primary cilia are present on nearly every type of animal cell. 
• The olfactory sensory neurons, which contain the odorant receptors, have around ten cilia each, whereas motile cilia cells have one. 
• The primary cilia of retinal photoreceptor cells are highly specialised.

The discovery of the primary cilium was in 1898 but ignored for a century as a vestige organelle with no function. Newly discovered physiological roles in chemosensation, signal transduction, and cell growth control have shown their importance in cell function. Its function in diseases such as congenital heart disease, mitral valve prolapses, polycystic kidney disease, and retinal degeneration has highlighted its importance in human biology. Ciliopathies are these diseases. The primary cilium is now known to play a role in numerous human organ functions. During the G1 phase, cilia are assembled and then disassembled before mitosis.

The Aurora A kinase is necessary for the disassembly of cilia. 

• Current scientific understanding of primary cilia is sensory cellular antennae that coordinate various cellular signalling pathways, sometimes coupling signalling to ciliary motion or cell division and differentiation.   
• The cilium consists of subdomains surrounded by a continuous plasma membrane with the cell’s plasma membrane. 
• The ciliary pocket is a membrane invagination that surrounds the basal body of many cilia. 
• Distal appendages connect the cilium membrane to the basal body microtubules. 
• At the distal appendages, vesicles carry molecules for the cilia dock. 
• The transition zone regulates molecule entry and exit to and from the cilia distal to the transition fibres.
• Some cilia signalling have ligand binding, such as Hedgehog signalling.   
• The somatostatin receptor 3 in neuronal cells is another form of G protein-coupled receptor signalling.

Motile cilia

Larger eukaryotes, including mammals, have motile cilia—a cell surface covered with many cilia, which beat in a coordinated wave.

• For example, the respiratory epithelium lining the respiratory tract contains motile cilia, which aid in removing mucus and other debris from the lungs through mucociliary clearance. 
• In the respiratory epithelium, each cell has 200 moving cilia.
• Female mammals’ fallopian tubes move the ovum from ovary to uterus. 
• The optimal level of periciliary fluid bathing the cilia is critical to the proper operation of motile cilia. 
• Epithelial sodium channels (ENaC) are sensors that regulate the liquid level surrounding cilia.
• Ciliates are microscopic organisms with only motile cilia that transport liquid over their surface.

Nodal cilia

Nodal cilia are motile cilia with a 9+0 microtubule arrangement in the axoneme present throughout the embryo’s early development.

Although primary cilia are structurally similar to nodal cilia, dynein arms are necessary for movement and spinning. Extraembryonic fluid flows through the nodal surface due to the action of these cilia in the clockwise direction.

• The cilia are present on the gastrula stage embryo’s node or embryonic organiser cells. 
• Nodal cilia regulate the left-right orientation and the liquid matrix’s movement around the embryo. 
• These cilia receive sensory responses around the embryo surrounded by primary cilia.

Flagella Overview

The complex filamentous cytoplasmic structure protruding through the cell wall is flagella. Intricately embedded in the cell envelope, these unbranched, long thread-like structures are mostly flagellin. They have a diameter of 12-30 nm and a length of 5-16 nm. For bacterial motility, they are responsible. Motility is essential for bacteria’s survival and their ability to cause disease.

Types of flagella

Bacterial flagella

Salmonella typhi and E. coli contain these, and they are one or two.

Eukaryotic flagella

They beat back and forth to create movement—for example, a sperm cell.

Archaeal flagella

Although it is similar to bacterial flagella, it lacks a central channel.

Where to Find Cilia and Flagella

Numerous cell types contain cilia and flagella. Flagella is present in the sperm of multiple animals, algae, and ferns. Some prokaryotic organisms, however, only have a single flagellum. Montrichous, amphitrichous, lophotrichous flagella are distributed throughout the cell (peritrichous). Cilia are also present in the respiratory tract and female reproductive tract. Cilia in the respiratory tract aids in removing mucus that contains dust, germs, pollen, and other contaminants from the lungs. Cilia help sweep sperm into the uterus in the female reproductive tract.

Conclusion

Cilia and flagella are cellular appendages that enable most microorganisms and animals to move, not higher plants. Climas in multicellular organisms cause cell or group movement and the transportation of fluids or materials through them. Cilia protect the lungs from dust, pollution, and possibly hazardous germs. Cilia also create water currents that transport food and oxygen to calm gills and snail digestive systems. Flagella are present on gametes, sponges, and coelenterates, where they generate water currents for respiration and circulation. Cilia and flagella are essential for the movement of single-celled eukaryotic organisms. Ciliophora protozoa have cilia on their surfaces, whereas Mastigophora protozoa have flagella.