Cilia and Flagella
- Cilia and flagella are complex filamentous cytoplasmic structures protruding through a cell wall.
- They are tiny, especially the differentiated appendages of the cell.
- Flagella (singular = flagellum) are long, hair-like structures that extend from the plasma membrane and are used to displace an entire cell.
- Cilia (singular = cilium) are short, hair-like structures that are used to move entire cells (such as paramecia) or substances along the outer surface of the cell (eg, the cilia of cells lining the fallopian tubes which move the egg to the uterus, or cilia lining the cells in the airways which trap the particles and move them to the nostrils).
- The terms cilium (meaning an eyelash) and flagellum (meaning a whip) are often used arbitrarily.
- In general, the cilia are shorter than the flagella (<10 μm vs.> 40 μm).
- Cilia are present on the surface of the cell in much greater numbers (hair cells often have hundreds of cilia, but flagellated cells usually have a single flagellum).
- The real difference, however, is in the nature of their movement. The eyelashes row like oars. The movement is two-phase, consisting of an effective stroke in which the eyelash is kept rigid and only bends at its base and a recovery stroke in which the curvature formed at the base passes to the end.
- The flagella squirm like eels. They generate waves that pass along their length, generally from base to tip at a constant amplitude.
- Thus, the movement of water by a flagellum is parallel to its axis while an eyelash moves water perpendicular to its axis and, therefore, perpendicular to the surface of the cell.
Structure of cilia and flagella
Despite their different beat pattern, cilia and flagella are structurally indistinguishable.
All cilia and flagella are built on a common fundamental plan:
- A bundle of microtubules called an axoneme (1 to 2 nm in length and 0.2 μm in diameter) is surrounded by a membrane that is part of the plasma membrane.
- The axoneme is connected to the basal body which is an intracellular granule located in the cell cortex and which comes from the centrioles.
- Each axoneme is filled with ciliary matrix, in which are embedded two central singlet microtubules, each with the 13 protofilaments and nine outer pairs of microtubules, called doublets. This recurring pattern is known as the 9 + 2 table.
- Each doublet contains a complete microtubule, called sub-fiber A, containing all 13 protofilaments. A B sub-fiber is attached to each A sub-fiber with 10 protofilaments.
- Sub-fiber A has two dynein arms which are oriented in a clockwise direction. The doublets are linked together by nexin links.
- Dynein is an ATPase that converts the energy released by the hydrolysis of ATP into the mechanical work of ciliary and flagellar beats.
- Each sub-fiber A is also connected to the central microtubules by radial rays ending in fork-shaped structures, called buttons or ray heads…
This regular arrangement of microtubules and associated proteins with the nine-lane motif is also observed in centrioles. But unlike centrioles, cilia and flagella have a central pair of microtubules, so the overall structure is called the 9 + 2 axoneme.
Note: Eukaryotic flagella diverge from prokaryotes in their composition. Flagella in eukaryotes contain much more protein and show some similarity to motile cilia, with the same general patterns of movement and control.
Working of Cilia and Flagella
Using the ATP produced by the mitochondria near the base of the cilium or flagellum as fuel, the dynein arms push onto the adjacent outer doublets, forcing a sliding motion to occur between the adjacent outer doublets. Because the arms are activated in a strict sequence both around and along the axoneme, and because the amount of slip is limited by the radial radii and inter-doublet bonds, the slip is converted to flexion.
Bacterial flagella use a fundamentally different mechanism. Like the propeller of a boat, the movement of the bacterial flagellum is entirely driven by the rotary motor at its base. The bacterial flagellum itself is a specialized piece of the extracellular cell wall, made up of a protein (flagellin) that has no similarity to tubulin or dynein. The cilia and flagella are full of the cytosol to their tips and use the ATP in this cytosol to generate force along their entire length.
Functions of Cilia
- Cilia are used for locomotion in isolated cells, such as some protozoa (eg, Paramecium).
- Movable eyelashes use their rhythmic ripple to sweep away substances, such as cleaning up dirt, dust, microorganisms, and mucus, to prevent disease.
- Cilia play a role in the cell cycle as well as in animal development, such as in the heart.
- Cilia selectively allow certain proteins to function properly.
- Cilia also play a role in cellular communication and molecular trafficking.
- Non-motile cilia serve as a sensory device for cells, detecting signals. They play a crucial role in sensory neurons. Non-motile cilia can be found in the kidneys to detect urine flow, as well as in the eyes of the retinal photoreceptors.
- They also provide habitats or recruiting areas for symbiotic microbiomes in animals.
- Eyelashes have also been discovered to participate in the vesicular secretion of exosomes.
Functions of Flagella
- Flagella are generally used for the locomotion of cells, such as sperm and Euglena (protozoan).
- Flagella play an active role in cell nutrition and eukaryotic reproduction.
- In prokaryotes such as bacteria, flagella serve as propulsive mechanisms; they are the main way for bacteria to swim in fluids.
- It also provides a mechanism for pathogenic bacteria to help colonize hosts and thus transmit disease.
- Flagella also function as bridges or scaffolds for adhesion to host tissue.