What is the difference between cilia and flagella?

Cilia and flagella are both hair-like structures found in cells, but they differ in size, number, and function.

Cilia are small, numerous, and hair-like structures present on the surface of all mammalian cells. They are primarily responsible for locomotion and play a crucial role in moving fluids over a cell's surface. Cilia are typically shorter than flagella and are often present in large numbers on a single cell. For instance, the cells lining our respiratory tract have cilia that move in coordinated waves to push mucus and trapped particles out of our lungs.

On the other hand, flagella are longer, whip-like structures that protrude from the cell body. They are usually fewer in number, often one or two per cell. Flagella are responsible for the movement of the entire cell, like the sperm cells in animals. They move in a wave-like motion, propelling the cell forward. In some organisms, such as bacteria, flagella also serve as sensory organelles that respond to changes in temperature and chemical composition of the surrounding environment.

Both cilia and flagella share a common structural arrangement known as the '9+2' structure. This refers to a ring of nine double microtubules surrounding a central pair of single microtubules. However, despite this shared structure, their movement patterns are different. Cilia move in a coordinated, back-and-forth motion, while flagella move in a wave-like, undulating motion.

In terms of their role in cell signalling, cilia are known to play a significant role. They are involved in mechanosensation, chemosensation, and thermosensation. Flagella, on the other hand, are less involved in cell signalling and more focused on cell movement.

In summary, while cilia and flagella are similar in their basic structure, they differ significantly in their size, number, function, and movement. Cilia are smaller, more numerous, and involved in moving fluids over the cell's surface and in cell signalling. Flagella are longer, fewer, and primarily involved in propelling the cell through its environment.