Many aquatic microorganisms possess external appendages for locomotion, to obtain food and for other purposes. One means is the use of multiple, short, hairlike cilia. Another is the much longer flagellum, or -- if there are two or more -- flagella. Cilia and flagella function in different ways. For that reason, their locations differ. Cilia are generally at the front of or all around the organism, while flagella -- more complicated structures -- are located at the rear.

Cilia Structure

Cilia are short hairlike structures. One cilium does not serve a useful function alone, but in quantity serves a variety of purposes. The structure of an individual cilium includes microtubules for support. When microtubules slide in relation to each other, the cilia are made to bend in a particular direction. The microtubules also provide a support matrix for certain proteins. The individual cilium is encased in a plasma membrane and connected to the organism by means of a basal body.

Cilia Functions and Locations

Cilia propel a microorganism by means of a pull stroke plus a recovery stroke, much like the movements of a swimmer. As a swimmer has arms and legs all around his body, so cilia are located all around the exterior of many microorganisms when their primary function is transportation.

Cilia are sometimes used to capture food. One example is the rotifer, whose two cilia-lined mouth parts draw food into its stomach. The hairs accomplish this because they wave in consecutive order, generating a controlled suction. When feeding is the primary purpose, the cilia are located at the front of the organism.

Larger organisms also feature cilia in their bodies, such as in the trachea of humans to remove dirt from the lungs, and in the fallopian tubes of females, to transport the ovum to the uterus.

Flagella Structure

The flagellum is a whip-like structure that consists of three main parts. It has an ion-driven motor part, a long filament consisting of the protein flagellin and a hook, which serves as a kind of universal joint joining the motor to the filament. There is a basal body with rings that act as bearings for the structure.

Flagella Functions and Location

The filament of a flagellum rotates in a kind of sinusoidal motion, resembling a ship’s propeller or a corkscrew. Since the flagellum pushes instead of pulls, it is located at the rear of the microorganism. If there is more than one, they may act as a bundle. The direction of rotation determines the path of the microorganism. Flagella also serve as sensors, particularly for the detection of moisture.