What describes the structure of cilia and flagella?

The structure of cilia and flagella is characterized by the presence of a "9+2" arrangement of microtubules. This means that they consist of nine pairs of microtubules (doublets) that are arranged in a circular pattern around two central microtubules. This cylindrical structure allows for the flexibility and movement necessary for the functioning of cilia and flagella in various biological contexts, such as locomotion in protozoa or the movement of substances across epithelial surfaces in animals.

This assembly of microtubules is critical because it provides the basis for the bending and beating motions that cilia and flagella perform. The interaction of the dynein arms (which are motor proteins) with the microtubules facilitates this movement by causing the doublets to slide against one another, leading to the characteristic whip-like motion of flagella and the coordinated beating of cilia.

The other options contain inaccuracies regarding microtubule arrangement or constituent proteins. For example, single microtubules or triplet microtubules do not correctly reflect the structural organization of cilia and flagella, and actin filaments are not the primary structural component of these organelles, as they are mainly composed of tubulin