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Cell Biology
The World of Cells
Cillia and Flagella
Cilia and Flagella
Cilia ("eyelashes") and flagella ("whips") are motile extensions protruding from the cell surface. Under the electron microscope both these organelles can be seen to have the same structure: an internal framework of nine pairs of microtubules arranged in a circle with a single pair of microtubules running the full length down the center.

Individual microtubules are composed of protein subunits arranged in the form of a hollow tube. Each cilium or flagellum is covered by the cell membrane and originates in the cytoplasm near a basal body, sometimes called a kinetosome. By using energy, the outer tubules move past each other, causing the organelle to bend.

mechanism of action This bending forces the external liquid over the cell surface, which clears stale liquid and debris away from the cell, brings new food and oxygen to the cell, or moves the cell into a new environment.

Cilia are present on single-celled organisms such as paramecium, a tiny, free-living protist that can be found in fresh water ponds. Usually about 2-10 µm long and 0.5 µm wide, cilia cover the surface of the paramecium and move the organism through the water in search of food and away from danger.

Cilia are also found, in modified form, in tissues such as the kidney and pituitary gland.

Flagella are often longer than cilia, about 50-100 µm in length, and there are rarely more than two per cell. they provide movement by an undulatory motion and are typically found as the motile organelle of animal sperm and some plant male gametes.

Figure legend: Eukaryotic flagella have a complex internal arrangement of microtubules that slide past one another. Energy is used in this process.

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© 2001, Professor John Blamire