Some animal cells possess flagella, though the majority do not. These specialized structures are whip-like appendages that extend from the cell surface, generally associated with cell movement.
What Flagella Are and How They Function
Flagella are microscopic, hair-like structures that protrude from the cell membrane. The term “flagellum” originates from the Latin word for “whip,” describing its typical motion. Their function involves propelling cells through liquid environments or moving fluids around the cell.
Eukaryotic flagella are composed of microtubules, protein filaments arranged in a “9+2” pattern, consisting of nine fused pairs of microtubules surrounding two central single microtubules. This internal structure, known as the axoneme, is anchored to the cell by a basal body. Movement is generated by motor proteins called dyneins, which attach to the microtubules and use energy to cause a bending or wave-like motion. This coordinated sliding of microtubules creates the necessary force for propulsion.
Key Examples in Animal Cells
The mammalian sperm cell is a prominent example of an animal cell with a flagellum. The flagellum, often called the sperm tail, enables the sperm to swim through the female reproductive tract. This movement is necessary for fertilization, allowing the sperm to reach and penetrate the egg.
The sperm flagellum is a highly organized structure, with its internal microtubule arrangement supporting its propulsion. Without an effective flagellum, a sperm cell would be unable to navigate the reproductive tract, leading to fertility issues. This specialized adaptation shows how the flagellum’s function is directly tied to the biological role of the cell it inhabits.
Distinguishing Flagella from Cilia
Both flagella and cilia are hair-like appendages extending from the cell surface, but they differ in length, number, and movement patterns. Flagella are typically long and few in number per cell. Their movement is generally a whip-like or wave-like motion designed to propel the entire cell forward.
Cilia, in contrast, are shorter and more numerous, often covering the entire cell surface. Their movement is typically a coordinated, oar-like beating pattern, which moves fluids or particles across the cell’s surface. For example, cilia lining the human respiratory tract clear mucus and trapped debris from the lungs, acting as a defense mechanism. Both structures share a similar microtubule-based internal organization, but their functional specialization dictates their differing roles.