Flagella are tiny, hair-like appendages on the surface of many cell types that provide either the motile force for movement or that serve as sensory receptors for information about the cell's external world. There are two very different kinds of flagella in the biological world. Bacterial flagella are very thin, solid rods made up of a chain of globular molecules of a single protein called flagellin. These rods, which have a helical shape, move only when spun by molecular motors in the cell membrane. They allow bacterial cells to swim through a liquid medium.
In contrast, the flagella of eukaryotic cells (those with a true nucleus) are much larger and more complex than those of bacteria. Typically about 0.82 ft (0.25 m) in diameter and 66-164.05 ft (20-50 m) in length, eukaryotic flagella usually are very similar in structure and function to cilia. The most obvious differences between cilia and flagella is that the former are relatively short and numerous, while the later are typically longer and more sparse. A typical flagellated cell usually only has two to eight flagella.
The internal structures of both cilia and flagella are generally the same. Each has an axoneme (a hollow bundle of microtubules and associated proteins) composed, generally, of a central pair of single tubules surrounded by a protein sheath, and a peripheral set of nine doublet microtubules. Each of the external doublets has a radial spoke linking it to the central pair. It also has two curving arms made of a protein called dynein. This is a molecular motor that "walks" along an adjacent doublet pair. Because the doublets are firmly anchored to the cell wall by a centriole, sliding of adjacent tubules causes a bend to propagate along the length of the flagellum. The three-dimensional bending causes flagella to move in a continuous helical spiral that propels a cell through a fluid medium. In contrast, cilia move in a pattern that resembles the arms of a person doing the breast stroke, with a rigid power stroke followed by a flexible recovery. The energy source for both ciliary and flagellar motion is adenosine triphosphate (ATP), which powers the "walking" motion of the dynein arms in both organelles.
Most motile, single-celled organisms, such as algae and protozoa, have either cilia or flagella. In multicellular animals, sperm are among the most commonly flagellated cells. Interestingly, the flagella of insect sperm show many variations from the typical nine plus two microtubular arrangement of most cilia and flagella. In certain insect species the ratio can be nine plus zero, nine plus one, nine plus five or seven, nine plus dozens, nine plus nine plus two, nine plus nine plus hundreds, or even no organization at all other than a random bundle of doublets. Amazingly, all these variations appear to be motile.
In many sensory organs, the actual sensory receptors are highly modified, non-motile flagella. Some examples are the outer segments of the rods and cones in the retina of the eye, the hair cells of the cochlea in the inner ear, and the olfactory cells of the nose. Apparently, the sensitive cellular membrane covering flagella makes a good receptor for a wide variety of information from the world around us.
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