Structure and Function of Cilia and Flagella

Many cells move using structures that extend from and are encased by their cell membranes. This cell motility can involve the movement of individual structures or the entire cell from one location to another. Cellular movement can be accomplished using components of the cytoskeleton, or network of filaments that give cells their shape, and special proteins (called motor proteins). Additionally, some eukaryotic cells—those with a defined nuclear membrane—have an arrangement of microtubules, or tubular protein structures that make up the cytoskeleton of cells, that extends from the cell membrane and beats back and forth, propelling the cell through its environment. These modified microtubules are called cilia and flagella.

A cilium (plural, cilia) is a small, hairlike projection from cells that can be used for motility. Cilia often cover the majority of a cell's surface, especially in the case of single-celled microscopic organisms, such as Paramecium. In this case, the cilia wave back and forth, like oars rowing a boat. The synchronized motion of each cilium pushes the cell forward using a power stroke, the movement toward one side, and then the cilia return to their original position in a recovery stroke. During the power stroke, cilia are extended upward, while during the recovery stroke, cilia tend to bend closer to the cell membrane. This motion can also be reversed, enabling the cell to move backward. The cilia of a Paramecium move like many tiny oars, propelling the organism through water at a rate that is four times its body length per second. Cilia can also act as antennae, receiving signals from the environment or other parts of the body. Cilia with this function wave back and forth but do not cause the organism to move. For example, many cilia can be found in the trachea, where they trap debris and keep it from entering the bronchi and alveoli of the lungs by moving the debris in the direction of the mouth. Cilia lining the mucus membranes of the nose have a similar function to those found in the lungs. Many vertebrate cells have at least one nonmotile cilium, which may collect signals and trigger changes to a cell's normal activities. An example of specialized types of nonmotile cilia are those that exist in olfactory, or smell-sensing organs, of animals. Nonmotile cilia in the olfactory organ serve as odorant, or smell, receptors.The human eye also has nonmotile cilia that serve an important role through their connections with the light-sensing photoreceptor cells of the retina.
A flagellum (plural, flagella) is a threadlike tail that allows some cells to move. Unlike cilia that almost cover the entire cell membrane, commonly fewer flagella (one or two) are found on cells. As a flagellum waves back and forth, it moves liquid over its surface. This undulation has a snakelike motion, moving the cell in the same direction as the flagellum is moving. Sperm are an example of a type of cell that uses flagellar motion to move.
While they differ in function, cilia and flagella have a common structure. Both are composed of structures called microtubules. A microtubule is a hollow tube made of protein that makes up part of the cytoskeleton. In this structure, the plasma membrane extends out around a group of microtubules arranged in a $9+2$ pattern. This means there are nine pairs (or doublets) of microtubules arranged in a ring, surrounding two individual microtubules in the center. The microtubules are attached to the cell surface within a basal body, a structure that acts as a point of attachment. The doublets and the inner pair are held together by specialized proteins that are involved with the bending motion of the structure. In order to get the cilia and flagella to bend instead of sliding past one another, each of these structures uses a dynein, which is a motor protein that moves along the microtubule found in a flagellum or cilium. One side of the dynein protein maintains contact with one half of the doublet and the other side moves along the other microtubule. This is what allows the microtubules to bend. Cilia that do not move typically have a $9+0$ arrangement in which they lack the inner microtubules.