Cilia and Flagella: Videos & Practice Problems

Cilia and flagella are essential organelles that facilitate the movement of cells and the surrounding fluids. Cilia are typically found in groups on the plasma membrane and move in a coordinated back-and-forth motion. This movement not only propels the cell through its environment but also helps to move fluids in the extracellular space, similar to how a swimmer pushes water aside. In contrast, flagella are usually singular and exhibit a spinning motion, akin to a propeller or rope, which allows for the propulsion of cells like sperm or certain protozoa.

Both cilia and flagella share a similar structural framework, characterized by the arrangement of microtubules in a formation known as the 9 + 2 axoneme. This structure consists of nine outer doublets of microtubules surrounding a central pair. Each doublet is made up of two types of tubulin proteins: the 'A' tubule, which is a complete circle containing 13 protofilaments, and the 'B' tubule, which is incomplete and typically contains 10 or 11 protofilaments. The outer doublets are interconnected by proteins, with nexin being a crucial component that links adjacent doublets together, ensuring structural integrity and coordinated movement.

At the base of cilia and flagella lies the basal body, which serves as the growth site for microtubules. The basal body is structured differently, consisting of nine triplets of microtubules arranged in a circular pattern. This foundational structure is vital for the assembly and function of the cilia and flagella, allowing them to perform their roles effectively in cellular movement.

Cilia and flagella are essential organelles that facilitate movement in various organisms. Their motion is primarily explained by the sliding microtubule model, which highlights the role of the motor protein dynein. Dynein is crucial for the movement of these structures as it attaches to microtubules and "walks" along them, utilizing ATP as an energy source.

In the sliding microtubule model, dynein binds to the B-tubules of microtubules with its head and moves toward the minus end. This movement causes the adjacent microtubules to slide against each other, resulting in a bending motion of the cilia or flagella. The bending is what enables the organelles to move back and forth or spin, creating the necessary propulsion.

Additionally, there is a process known as intraflagellar transport, which is distinct from the sliding microtubule model. This process involves the transport of molecules within the flagella, moving them from one region to another, either from the base to the tip or vice versa. However, the sliding microtubule model focuses on the overall movement of the cilia and flagella as a whole.

As dynein moves along the microtubules, it encounters barrier proteins that prevent the microtubules from sliding past each other. Instead of sliding, the microtubules bend, which is the fundamental mechanism behind the movement of cilia and flagella. This bending motion propagates throughout the entire structure, allowing for effective locomotion.