Flagella (singular=flagellum), are extensions of the cell commonly found in many Bacteria and Archaea, and even in some Eukaryotic cells. Cells that have flagella often use their flagella as their main method of motility. Most bacterial cells use their flagella to travel randomly, a process called random walk, unless there is a chemical signal present, in which the cell would travel to the attractant (or away from a repellant) through chemotaxis.
Types of Flagella
There are three major types of flagella found among cells
Peritrichous flagella are a series of flagella that appear around the cell in all directions. Some examples of cells that have peritrichous cells are E.coli cells and salmonella cells.
Lophotrichious flagella are a group of flagella that are bundled together on one side of the cell.
Monotrichous flagella have only one flagellum attached to one side of the cell
Form and Function
Bacteria/Archaea cells: The primary function of flagella is to allow for cell motility. The flagella are comprised of protein monomers called flagellin. The flagella are able to perform this by spinning in an either clockwise or counterclockwise direction. When the flagella (or flagellum, for monotrichous cells) are spinning in the counterclockwise direction, the flagella is driving the cell forward towards the attractant or away from the deterring signal. When the flagella is rotating clockwise, the entire cell stops the forward movement in order to change direction. The flagella is able to rotate in either direction by generating torque in the basal body. Within the basal body is a specialized protein called the mot protein, which generates torque to propel the circular flagellar motion. The basal body also acts as an anchoring system for the flagella, consisting of a set of rings known as the L, P, MS, and C rings. The flagella is able to switch between the clockwise and counterclockwise directions through Fli proteins, which senses where the attractant/deterrent is, and switches the direction of rotation for the flagella in response to the chemical signal (therefore performing chemotaxis).
Eukaryotic cells: Eukaryotic cell flagella differ from bacterial cells in terms of how the flagella move in order to propel the cell forward. Because eukaryotic cells are much bigger than bacterial cells, instead of a clockwise/counterclockwise direction, most eukaryotic cell flagella move in a “whip-like” motion which is fueled by ATP hydrolysis in the cell. Eukaryotic cells with flagella also have many more microtubules in the membrane to help aid in support during cell chemotaxis.
Slonczewski, Joan, and John Watkins. Foster. Microbiology: An Evolving Science. New York: W.W. Norton &, 2009. Print.