What is a key characteristic of cilia and flagella in terms of their structure?

Cilia and flagella are important cellular structures that play pivotal roles in cell motility and movement of fluids across cell surfaces. A key characteristic of cilia and flagella is that they are filled with microtubules, which are structural components that form the underlying framework of these organelles.

The architecture of cilia and flagella is typically organized into a "9+2" arrangement of microtubules: nine pairs of microtubules form a ring around two central microtubules. This specific arrangement allows them to maintain their shape and provides the flexibility necessary for movement. The coordinated sliding of these microtubules, driven by motor proteins like dynein, enables cilia and flagella to effectively propel cells or move substances over the cell surface.

The other options do not accurately represent the structural characteristics of cilia and flagella. While lipids are an essential component of the cell membranes and various organelles, they do not constitute the primary structural elements of cilia or flagella. Energy production is an essential cellular function but is not a structural characteristic of these organelles. Lastly, while cilia can play a role in protecting cells by clearing mucus and pathogens, serving as a protective barrier is not related to their